Chapter 10 : Artificial Reefs and Structures

10.1  Introduction
10.2  Expected Benefits
10.2.1  Increased Catch Rates
10.2.2  Increased Cover for Spawners and Juveniles
10.2.3  Increased Food Availability
10.2.4  Increased Production
10.3  Possible Drawbacks
10.3.1  Overfishing
10.3.2  Navigation Hazards
10.3.3  Leachates
10.4  Management Practices
10.4.1  Justification and Evaluation
10.4.2  Reef Site Selection
10.4.3  Reef Design
10.4.4  Reef Area
10.4.5  Permits and Regulations
10.4.6  Reef Construction
10.4.7  Reef Installation
10.4.8  Reef Markers
10.4.9  Maintenance and Monitoring
10.4.10  Brochures and Electronic Media
10.4.11  Volunteer Assistance

10.1 Introduction

Submerged structures are often lacking in reservoirs because of removal during construction, decomposition over time, lack of recruitment potential from the riparian zone, or little structural material in the landscape prior to impoundment. Deficiency of submerged structures can have negative effects on fish abundance, ecological diversity, and fisheries (Bolding et al. 2004; Wills et al. 2004). The lack of structure has been identified as a major habitat degradation in U.S. reservoirs, although its preponderance varies across geographic regions (section 1).

Installing reefs and spawning structures in reservoirs has been a common practice. The overarching goal of these installations has been to enhance the naturalness of the artificial aquatic environment created by the impoundment and to aggregate fish to facilitate predator–prey interactions, including fishing. Pragmatic goals of artificial reefs include creating new fishing sites, improving angling efficiency, providing more food for fish, increasing growth rates, improving reproductive success, improving juvenile survival, providing protection from predators, and in general improving fish production. The addition of structural habitat may increase carrying capacity and biomass, at least in the areas where the structures are placed (Bortone et al. 1994; Polovina 1994; McCann et al. 1998). Enhancements with reefs may increase species diversity, complexity of trophic interactions, and ecosystem stability (McCann et al. 1998; Neutel 2002).

Installing supplementary habitat is a common habitat management activity amongst freshwater fisheries management agencies, with 80% of state agencies in the USA having installed some type of supplementary habitat enhancements (Tugend et al. 2002). However, habitat addition is far from a ubiquitous management strategy, as less than 20% of regulated lakes and reservoirs have received habitat enhancements (Tugend et al. 2002). Common types of artificial structures include woody materials (e.g., trees, brush, lumber), stone materials (e.g., gravel, riprap, boulders), and synthetic materials (e.g., plastics). The effectiveness of supplementary habitat varies greatly depending on objectives, materials, structure size and morphometry, target species, and existing habitat.
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10.2 Expected Benefits

Fish habitat programs are a popular and effective way to increase catch rates. However, attraction is not a big concern where fishing effort is low, and attraction can promote overharvest where fishing effort is high. Moreover, whereas fish attractors can concentrate a fish population, too many fish attractors could again dilute the local density of target species. Other benefits of artificial reefs and structures may include increased production through reduced mortality and increased growth, although direct links to increased production are difficult to document.
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10.2.1 Increased Catch Rates

In Lake Havasu, Arizona, angler success more than doubled after an extensive program of reef installation (Jacobson and Koch 2008). Studies have demonstrated that habitat enhancement structures concentrate fish (Prince and Maughan 1979; Brown 1986; Moring et al. 1989; Rogers and Bergersen 1999). However, the extent of concentration varies for several reasons, including species-specific characteristics (Hubbs and Eschmeyer 1938; Rodeheffer 1939, 1945), diel fluctuations in fish distribution (Moring and Nicholson 1994), age of the reef (Moring and Nicholson 1994), and the reef’s physical attributes. In general, the average number of fish and species attracted increases with the structural complexity, which is achieved by increasing the volume and surface area of the reef, although the benefits are likely asymptotic (Wickham et al. 1973; Rountree 1989). Further, fish abundance, life stage, and species composition vary with structure interstice size (the space within structures) in a complex fashion (Wege and Anderson 1979; Johnson et al. 1988; Lynch and Johnson 1989; Walters et al. 1991).

The morphometric characteristics of reservoirs also influence the effectiveness of attraction. Enhancement structures may be less effective in systems with bathymetrically complex bottoms (Pardue and Nielsen 1979) or with adequate natural habitat (Madejczyk et al. 1998; Rogers and Bergersen 1999). Depth at which habitat enhancement structures are placed controls variables such as temperature, dissolved oxygen, and light availability (section 10.4.2.4) and can influence the effect of the habitat structure (Walters et al. 1991; Johnson and Lynch 1992).
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10.2.2 Increased Cover for Spawners and Juveniles

Habitat enhancement structures have been associated with increased recruitment by means of providing cover for nest spawners (Vogele and Rainwater 1975; Hoff 1991; Hunt et al. 2002), thereby increasing nest density. Nest success also may increase if structures provide habitat that allows adults to protect their young more effectively (Hoff 1991). By increasing cover, structures also can offer juveniles refuge from predation (Bohnsack and Sutherland 1985; Johnson et al. 1988; Moring and Nicholson 1994), provide shade that serves as cover (Helfman 1979, 1981; Johnson and Lynch 1992; Raines and Miranda 2016), and provide sites for orientation and schooling (Klima and Wickham 1971; Bohnsack and Sutherland 1985).
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10.2.3 Increased Food Availability

Figure 10.1. Submersed reefs provide substrate for periphyton and associated biota. Photo credit: A. Norris, Department of Agriculture and Fisheries, Agri-Science, Queensland, Australia.


Prey abundance in and around the reef may be enhanced (Wege and Anderson 1979; Moring et al. 1989), in turn increasing the feeding efficiency and growth of predators (Wege and Anderson 1979; Bohnsack 1989). Many studies have reported observations of fishes feeding within artificial reefs. The added substrate provided by reefs provides additional periphyton and associated biota (Figure 10.1) (Van Dam et al. 2002), although there is a debate about how much new fish biomass subsequently is produced and whether the added biomass is a significant contribution to a population or assemblage. Attraction of prey and predator fish to the reef facilitates predator–prey interactions. Improved feeding efficiency implies faster growth rates in artificial reefs, but this has not been demonstrated on a general basis.
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10.2.4 Increased Production

An underlying rationale for structure deployment is the production hypothesis, i.e., reefs provide additional critical habitat that increases the environmental carrying capacity and eventually the abundance and biomass of fish in the entire system. Thus, barren, unproductive substrate may be transformed into highly productive environments through the addition of artificial reefs (Stone et al. 1979). Mechanisms suggested for this transformation include (1) providing additional food; (2) increasing feeding efficiency; (3) providing shelter from predation; (4) providing recruitment habitat for individuals that would otherwise have been lost; and (5) attracting fish, whereby reefs help vacate space elsewhere, space that is eventually colonized with new biomass (Randall 1963; Ogawa 1973; Stone et al. 1979; Matthews 1985).

Although the production hypothesis has been recognized for a long time, progress toward resolving its propositions has been slow. Attraction and production are not mutually exclusive and can be considered opposite extremes along a gradient. While artificial reefs may merely attract and concentrate some fishes, they may promote the production of others. Most fishes probably lie somewhere between these two extremes. Demonstrating attracting mechanisms does not refute the possibility of increased production. Attraction behavior in fish presumably evolved because of some selective advantage such as faster growth and increased survival, both of which promote production. Nevertheless, a better understanding of the relative importance of attraction and production is critical for wise fisheries management and the effective construction and deployment of artificial reefs.
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10.3 Possible Drawbacks

10.3.1 Overfishing

Artificial reefs can be used to increase public access to fish by making it easier for anglers to locate fish and also to increase catch rates by concentrating fish (Bohnsack 1989). Under heavy fishing pressure, structures that attract fish may promote overfishing by increasing fish catchability. Fishes normally dispersed over a wide area would instead be concentrated in a smaller area around reef structures and possibly be depleted more rapidly by fishing. In waters where stocks are relatively low, the addition of structure may improve catch rates but intensify problems associated with overharvest. These concerns may not be applicable if the target species draws primarily anglers that practice catch and release, such as many black bass fisheries.

Additional concerns come from the possibility of removing top-level predators that may concentrate in artificial reefs. Removal of these predators may influence predator–prey dynamics and shift fisheries toward less desirable conditions. Alternatively, overharvest of a population could cause a shift in fishing efforts toward more susceptible species. Ironically, the lowered catch rates caused by overfishing are often cited as the primary reason for creation of artificial reefs (Polovina 1991); in such cases, reefs would be detrimental.
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10.3.2 Navigational Hazards

Structures installed in reservoirs to enhance fish habitat can become hazards to commercial navigation or recreational boating. Trees, brush piles, buoys, or other structures can shift positions because of wind and wave action or float to the surface if not properly anchored. Artificial structures also can become a navigation problem in reservoirs where water levels fluctuate, bringing near the surface structures that might have been installed well under the surface of the normal pool elevation.
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10.3.3 Leachates

Materials used to construct reefs may produce harmful leachates that create water-quality and aquatic health concerns, either immediately after deployment or as the structures age. Some of these materials include plastics and treated wood.
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10.4 Management Practices

Responsibilities related to installing reefs and other structures in reservoirs are not limited to deployment activities. Project managers may need to establish that there is a need for these structures, identify the management goals for installing the structures, select appropriate sites, identify suitable materials, notify permitting agencies, and conduct post-deployment evaluations. This section addresses such considerations and tasks.
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10.4.1 Justification and Evaluation

Structures are often added to reservoirs without clear objectives or realistic expectations about benefits (Bolding et al. 2004). Lacking justification, it is difficult to evaluate the effectiveness of a program.
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10.4.1.1 Justification of needs

Justification of the program’s goals is the foundation for artificial reef projects. A written plan can have various benefits and can be viewed as the cornerstone of the program. Some of the benefits of a written plan include providing continuity to the program regardless of personnel changes; establishing strategies for reaching goals; serving as a handbook for the program; supporting and simplifying the decision-making process; providing a basis for adaptive management; and providing leverage for funding. Moreover, many agencies with jurisdiction over a reservoir may require that the objectives of the structure are defined clearly before any permits are granted.

Artificial reefs are worth considering only after physical and biological surveys of the reservoir have been conducted by trained personnel. These surveys can establish the extent of structure availability and whether the scarcity of structures and bottom reliefs is potentially limiting fishing opportunities, fish population characteristics, or fish assemblage structure. Although a detailed evaluation of structures as a limiting factor may be difficult, or prohibitively expensive, the general availability of structures in a body of water usually can be determined by visual examination of the littoral, particularly during low water. Alternatively, various side-scan sonar devices are available to conduct underwater surveys (Kaeser et al. 2013; TPWD 2016). The side-scan features on newer sounders can capture images of structure in wide swaths on both sides of the boat, and associated software can link these images to map the underwater topography.
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10.4.1.2 Clearly stated goals

Goals and objectives are the driving forces behind structure enhancement programs (section 12.2). Examples of goals may include the enhancement of recreational fishing, conservation of fish populations, and restoration of diversity. Structures can enhance recreational fishing by increasing catch rates where exploitation is not high. Conservation of fish populations may be achieved if the structures reduce juvenile mortality or increase growth rates. Structures may increase habitat diversity and at the same time promote species diversity (Kovalenko et al. 2012). These goals are appropriate in reservoirs where lack of structures clearly can be identified as a factor limiting fish assemblages, where this habitat has been lost because of aging processes or anthropogenic disturbances, or where it is necessary to relieve life-history bottlenecks, such as increasing survival of juvenile fishes. In some cases, adding supplementary habitat may be a more efficient method for increasing juvenile recruitment than is stocking. If habitat is limiting, stocking is likely to be ineffective as stocked fish will not have suitable habitat. Inappropriate goals include (1) aiming to increase attraction without proper protection from overharvest, and (2) aiming to increase production of certain fish species when there is no evidence that the scarcity of structure is a limiting factor.
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10.4.1.3 Evaluation of performance

It is crucial to evaluate whether the program is achieving the stated goal(s). An evaluation may require monitoring of whether the reef is influencing fishery harvest, enhancing production of selected species, or creating economic benefits. The extent of monitoring and variables monitored is guided by the project objectives, available resources, and apparent knowledge gaps. A preconstruction baseline evaluation may include existing fish assemblage and angler use data and form the basis against which a program’s success is measured. Some or all of these data already may be available from routine monitoring in previous years. Preconstruction estimates of economic effects can be extremely powerful in attracting funding for the venture. Postconstruction evaluations may monitor fish assemblages and angler use to evaluate any changes in the fishery, fish population dynamics, or species assemblage composition. A performance evaluation also can detect whether the reef is having any unexpected negative consequences as well as provide insight into the need for future modifications. Without this step, or without a suitable study design, a learning opportunity would have been missed and future reef construction efforts may be wasted.
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10.4.2 Reef Site Selection

10.4.2.1 Incompatible Sites

Conflicting uses need to be evaluated when planning where to deploy structures. Possible conflicts include presence of power lines; oil, gas, or sewer pipelines; alternative energy projects; restricted areas for civilian or military activities and other rights of way; and shoreline real estate developments. Exclusion of known commercial fishing areas and navigation lanes also need to be considered. If the purpose is to provide recreational fishing opportunities, the reef might not be used to its fullest potential if fishers have to travel a long way to get to it or environmental conditions at the site (e.g., wind, wave action) are such that preclude a pleasant experience. Alternatively, if the structures are established to enhance spawning and recruitment, distance from access sites may not be a concern (unless cost of deployment is high) or sites may need to be placed far from popular fishing areas. Attraction of fish to an extent that attraction interferes with access to natural spawning habitats such as tributaries, floodplains, and adjacent wetlands may need to be prevented. Last, reefs may need to maintain a reasonable distance from the dam and associated water intakes, discharges, or energy-production facilities.
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10.4.2.2 Compatible sites

Compatibility depends on the goal of the reef. For example, habitat enhancement may target a section of a reservoir where littoral fish habitat is composed primarily of barren mudflats, with the goal of increasing fish densities in these areas. Installing reefs would attract and potentially retain fish that would otherwise not stay long and move on to other areas with more desirable habitats. The reef provides the fish with a domain that includes the reef and the surrounding waterscape. The reef serves as a place to rest in cover, wait in ambush, or feed on periphyton or other small prey or from where to launch feeding or spawning forays into neighboring areas. Without the reef some of these basic life-history necessities may not be available, and therefore the fish would not remain in the area for a prolonged period of time. Thus, in this case the reef actually may create new biomass and enhance production. At the same time, anglers may benefit by having new areas to fish and an improved chance at catching fish in an area that in the past had been mostly unrewarding.

Alternatively, reefs may be installed in areas known to attract large numbers of fish with the goal of attracting even more fish, retaining fish at the site for longer, or both. Areas such as submerged rocky ledges, points, creek beds, and roads often tend to attract fish. These preferred fish staging areas potentially can get a boost when a reef is installed.

If the goal is to encourage shoreline-based angling, then it may be beneficial to locate some reef habitat under or adjacent to fishing piers, close to them, or within casting distance of shoreline areas that can be accessible to shore-based anglers.
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10.4.2.3 Substrate

The characteristics of the sediment need to be known or evaluated prior to deployment of the structure. Appropriate substrate conditions are required to prevent the reef or spawning bed from sinking beneath the sediment surface. Inappropriate placement can cause significant or total loss of exposed material and greatly reduce the utility of the structure. Although some settling of deployed material is expected in unconsolidated sediment, and can actually assist stabilizing a reef, conditions of the reservoir floor have to support deployed materials sufficiently to allow long-term success of the structure.

In general, areas with soft sediment such as soft clay, fine silt, or loosely packed sand may need to be avoided because they increase the likelihood of the structure sinking or subsiding. These areas are more common in the headwaters of the reservoir where major tributaries deposit their sediment loads but may also occur in major embayments where smaller tributaries enter the reservoir (section 3). If sedimentation is a major issue in the headwaters, reefs quickly may become covered with sediment; instead they could be installed in areas farther downlake where sedimentation may be less severe. In contrast, deployment on a hard substrate may increase the structure’s susceptibility to slide during storm events. The ideal substrate for structure placement would be a thin layer of soft sediment over a harder layer of soil.
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10.4.2.4 Depth

Reefs can create navigational hazards. The depth of the reef needs to be sufficient to allow for safe navigation over the reef. The required clearance (i.e., minimal water depth above the reef) depends on the location and anticipated type of traffic that would traverse the area. Generally, the depth clearance of a permitted artificial reef should not exceed the shallowest depth of surrounding natural features, and the top may be installed at least 3 ft below the water surface at mean annual low water. Installation in remote coves of the reservoir usually limits navigation risks. On a case-by- case basis, specific buoys or other markers may be needed to designate the reef area.

Figure 10.2. Brush (upper panel) and stone (lower panel) linear reefs designed to cope with fluctuating water levels. Photo credit: North Carolina Wildlife Resources Commission, Raleigh, and Missouri Department of Wildlife Conservation, Columbia.

The depth of the structure is also a consideration based on the goal of the structure. For example, if the goal is to provide habitat for a given species, the preferred depth range of that species can be factored into where the structure is placed. Physicochemical variables may influence use of structures, particularly in deeper water where temperature, dissolved oxygen concentration, and light intensity are reduced. Deeper reefs may have less periphyton and associated invertebrate communities. Water temperatures below about 10°C can cause centrarchids to leave structures in shallow littoral areas (Prince and Maughan 1979). Similarly, structures placed in cold, deeper water in summer may not attract many species. Prince et al. (1985) reported that some reservoir reefs at depths of 20 ft or less were virtually devoid of fish in the winter. For optimal value, the structure is placed above the summertime thermocline, particularly if the hypolimnion goes anoxic (section 6). High light levels often result in increased fish use of structure that produces shade (Helfman 1979). Fish may find diversity in light levels by using deeper structures, provided that temperature and oxygen needs are met in deeper water.

Figure 10.3. Fish attractor sign posted on a tree near the shoreline. Photo credit: Arkansas Game and Fish Commission, Little Rock.

Siting reefs in reservoirs with large water-level fluctuations can be problematic. Ideally, reefs are installed below the lowest pool elevation to avoid endangering recreational boaters. Nevertheless, at such low water levels the structure may not be available to most fish during periods of high water, which is usually a time of year when fish are most active. Where boating traffic is not high, speeds are low, and reefs can be marked, linear reefs may be appropriate. A linear reef may run from low to high water perpendicular or at an angle to shore (Figure 10.2). Such an arrangement is possible with any reef  material (section 10.4.3). A  linear reef provides access to cover at multiple water levels and enables fish to select a preferred depth during high water, which is often in summer when water-quality conditions differ most over depths. The wetting and drying cycles in these linear reefs are likely to deteriorate some reef materials more quickly. A marker buoy (section 10.4.8) can be positioned at the deepwater end of the reef to alert boaters, and a sign at the shallow end would tip off anglers as to how the reef is positioned so they can fish it effectively (Figure 10.3).
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10.4.2.5 Floating reefs

Floating reefs can provide structure that will rise and fall with the water level, eliminating the problem of the structures being too deep, too shallow, or out of the water for large parts of the year (Brouha and von Geldern 1979). When adequately buoyed and securely anchored, floating reefs offer year-long utility as well as possibly providing wave attenuation between the structure and the reservoir shoreline. Floating reefs may be combined with other existing floating structures (e.g., fishing piers, breakwaters, floating docks, buoys) or they may be constructed independently. However, floating reefs pose a risk to navigation. Their use may need to be limited to areas of the reservoir where (1) boating is primarily for fishing and (2) boating speeds are generally lower. Reduced speed limits could be put in place in areas with floating fish attractors to reduce collision risks further.
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10.4.2.6 Waves

Areas of consistently high wave energy may not be suitable for reef installations. High wave energy will decrease the durability and stability of a reef because of constant exposure to wave surge. The wave energy also may limit the settlement potential of periphyton if water is too turbulent and may exclude some fish species that would otherwise be attracted. High wave energy zones often are close to shore. Placing structures in these areas also may affect longshore sand transport, which may not be desirable and may need to be evaluated thoroughly. Analyses of seasonal weather patterns and wave fetch can be used to assist in the selection of most appropriate sites.
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10.4.3 Reef Designs

The morphology and complexity of habitat can be one of the more important factors influencing the effectiveness of structure as fish habitat. Relationships between habitat and species associations may best be summarized by the habitat diversity hypothesis, which states that species diversity increases with increasing availability of habitat types (Kovalenko et al. 2012). Fish abundance, richness, and diversity along shorelines of reservoirs are generally directly related to the structural complexity of available habitats (Barwick 2004; Newbrey et al. 2005). Higher species diversity can result in more complex food webs and longer trophic loops. Increased complexity of food webs creates a more stable fish assemblage that is less susceptible to chaotic dynamics and is made up of resilient interspecific interactions.

Interstitial space affects the size of fish attracted. Prey fish prefer small- or medium-size interstices when in the presence of predators (Crook and Robertson 1999). Early life stages of bluegill occupied small interstices within habitat, as close to body size as possible (Johnson et al. 1988), reducing danger from predation. However, even at younger life stages, largemouth bass were more likely to choose medium interstices rather than small ones. Potentially, this choice provides a balance between avoiding predation and having sufficient opportunity to ambush prey. Thus, diversity of interstitial space may best promote diverse fish assemblages within artificial reefs.

Besides complexity, the effect of structure on populations may depend on size of the reef. Rountree (1989) reported that fish abundance and diversity were related positively to structure volume and surface area. Bohnsack et al. (1994) reported that large reefs may have higher biomass densities than do small reefs but are oftentimes composed of fewer but larger individuals. Daugherty et al. (2014) reported that largemouth bass exhibited greater percentage occupancy in large structures but higher densities in small structures. Thus, depending on size, some artificial reefs may support fewer and larger fish, whereas others may support more and smaller individuals.

Figure 10.4. Different reef arrangement designs often have different attraction rates and attract different species. Figure reproduced from Bryant (1992).

Reef arrangement may also be important. The responses of largemouth bass and bluegills to reef arrangement in a Texas reservoir suggested that cluster-shaped reefs (roughly circular to minimize the amount of edge and maximize the amount of interior cover) provided greater protection from predation than did a linear design (trees organized in a line to maximize amount of edge; Daugherty et al. 2014). Greater protection from predation likely is related to the increased interior space provided by the clustered design, which reduces visual encounters with predators and excludes predators from the reef interior. The percent of reefs occupied and catch rate of bluegills was highest in cluster-shaped structures, but size of bluegills was smaller, suggesting that the clustered design  was  used  as  protective cover (Daugherty et al. 2014). In Ruth Reservoir, California, Bryant (1992) organized brush into three arrangements (Figure 10.4). The discrete open-center structure was the most used by juvenile and adult largemouth bass and smallmouth bass. However, both the continuous open-center and dense design structures were used by largemouth bass and smallmouth bass more than shoreline areas with no woody structures.

Size, morphology, and complexity of reefs influence the species and life stages attracted. Thus, as a general strategy to benefit as much of the fish assemblage as possible, a broad diversity of reef sizes, morphologies, and complexities may optimize the value of an artificial reef program. In some cases, it may be much easier to replicate a single standard design. Nevertheless, size, morphology, and complexity of the reefs may need to be linked to the goal(s) of the reef program.
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10.4.4 Reef Area

The amount of structure needed depends on the goals of the program. If the goal is fish attraction to improve catch rates in an underutilized fishery, this can be accomplished with a limited number of reefs distributed strategically near fishing areas or in areas targeted by managers for increased effort. Alternatively, if the goal is to alter population dynamics noticeably by possibly shifting rates of recruitment, growth, and mortality, and even maybe change community structure through large changes in habitat composition, then the amount of structure needed may be extensive and possibly unrealistic. A definition of “extensive” has been researched by a few investigators, but the research has not been sufficient to pin it down.

Crowder and Cooper (1979) offered a conceptual model of predator–prey interactions relative to structural complexity. They suggest that prey-capture rates per prey available decline with increasing structural complexity. However, prey density (diversity and abundance) is positively correlated with structural complexity. These counter currents can lead to maximal feeding rates at intermediate structure levels. Thus, at low levels of structure availability, fish can feed most efficiently, but few reefs are available, and thus overall utility of reefs is relatively low. At high levels of structure availability, despite relatively high prey densities, prey capture rates are low because of reduced feeding efficiency. Thus, high levels of structure availability are also a low-utility habitat. At intermediate structure levels, feeding efficiency is optimized as prey is relatively more available than in either high or low levels of structure availability. These authors also hypothesized that the actual level of structure that maximizes feeding rate is a function of fish size. A large predator would reach maximum feeding rates at a lower level of structure than would a smaller predator.

Field surveys linking aquatic vegetation density to characteristics of largemouth bass populations have suggested that densities in the 20%–50% range optimize some population characteristics (Durocher et al. 1984; Wiley et al. 1984; Miranda and Pugh 1997). Aquatic plants may have greater effects on water quality (Miranda and Hodges 2000) than do artificial reefs, but this target range could be used as a guideline for coverages of artificial structure needed. Clearly, 20%–50% coverages would be practicable in only small reservoirs or embayments of large reservoirs. In large reservoirs these levels of cover may be attempted through introduced structure combined with plant growth (section 11).
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10.4.5 Permits and Regulations

The permitting process, if any, for installing artificial reefs and other structures varies on a lake-by-lake basis depending on the agency that manages the facility. It is the responsibility of the organization installing the structures to obtain the necessary permits. For permitting, some agencies may require information such as location of the structure, including latitude and longitude; purpose and need for the structure; description of type, quantity, and composition of material to be placed in the water; and provisions for installation, monitoring, and managing the life of the structure.

Depending on the scope of the project, a permit may be needed from the agency managing water storage. For example, the U.S. Army Corps of Engineers (USACE) holds authority under Section 10 of the Rivers and Harbors Act and under Section 404 of the Clean Water Act to regulate structures and placement of materials into the waters of the USA (the U.S. Environmental Protection Agency [USEPA] has delegated the “404 process” to the USACE). Most USACE districts do not require special permits for adding small-structure projects in reservoirs, but it is always good to check with the local office with jurisdiction over the reservoir.

The USACE may coordinate with other federal agencies such as USEPA and U.S. Fish and Wildlife Service through the public notice process. For example, endangered species surveys may be requested from the reviewing agencies in order to note the presence and prevent damage or destruction to hard-bottom or endangered species. Surveys also may be requested to identify historical sites or artefacts to avoid. Applicable authorizations include the National Environmental Policy Act, which provides a mandate and framework for federal agencies to consider all reasonably foreseeable environmental effects of proposed actions and to involve and inform the public in the decision-making process by considering environmental effects and reasonable alternatives. It requires federal agencies to conduct an Environmental Assessment or Environmental Impact Statement for each project. The National Historic Preservation Act provides for evaluation of direct and indirect effects of the project on historic resources in the area. The Endangered Species Act provides a consultation requirement for any federal action (e.g., USACE issuing a permit) that may affect a listed species to minimize the effects of the action.
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10.4.6 Reef Construction

A diversity of reefs and spawning structures has been constructed and installed in reservoirs. These can be classified into three general types: (1) tree, brush, and lumber structures; (2) structures constructed from stone materials; and (3) structures constructed from synthetic materials such as plastics. Reefs concentrate fish to increase angling catch rates by anglers, promote predator–prey interactions, or provide cover for various species or life stages. Spawning structures provide spawning substrate for various species whose spawning habitat is limited in reservoir environments or has been degraded by long-term environmental changes.
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10.4.6.1 Tree, brush, and lumber reefs

Wood piles provide excellent habitat for a broad range of species and life stages. Wood piles include brush, shrubs, tree limbs, and entire trees (Figure 10.5). Common sources of brush have been conifers and hardwoods. Conifers tend to have smaller interstitial spaces but decompose quicker than hardwoods; stumps may last for decades. There are many advantages to using brush piles and woody debris, beginning with the wide range of interstitial spaces that provides diverse microhabitats for fish of various sizes. Perhaps the major allure of brush and woody debris is their naturalness and that, in time, they biodegrade and leave no trace. Brush jams form naturally in streams, so many fish species instinctively are attracted to brush piles and often colonize them quickly after installation. Brush and other tree remnants remain the most frequently used materials to build reefs because in most geographical areas they are abundant, inexpensive, are relatively easy to install with volunteers, and closely resemble the natural log and brush jams and root wads experienced by fish in their native environments.

Figure 10.5. Examples of tree and brush reefs. Photo credits: (Left column, top to bottom) U.S. Army Corps of Engineers (top two), Aquatic Environmental Services, and Billings Gazette; (Right column, top to bottom) Missouri Department of Conservation (top two), Denver Post, and Virginia Department of Game and Inland Fisheries.

Brush can be organized in various configurations to conform to the local waterscape or to optimize perceived fish habitat requirements. The size of the brush structure may depend on how many individual units are included in a cluster and how units are placed in relation to each other. Diversity of size, shape, proximity to each other, and depth is probably best to meet the various needs of diverse species and fish sizes. A range of depth contours may be targeted if water-level fluctuations are a concern (section 10.4.2.4). A quick way to introduce brush into shoreline habitats is by felling trees along the shoreline (sections 5.8.2.12 and 8.9.5).

Brush is often available locally, precluding the expense and logistical problems associated with long-distance transportation. Installing brush and woody debris lends itself well to recycling resources that otherwise would go to waste and to including volunteer groups in the gathering and installation processes (section 10.4.11). If the brush pile has to be sunk, “green” recently cut brush sinks easier. Brush that has been stockpiled for 1–2 weeks loses weight through desiccation and requires more weights to sink and secure. Depending on source and size, brush piles and woody debris may be longer lasting than Christmas trees (Bolding et al. 2004). Conifers lose much of their interstitial space (i.e., small branches) within 2–7 years, although the thicker branches remain. Most hardwoods lose their interstitial space in 10–12 years, but again thicker branches remain. Large trees and stumps may last 15–25 years or longer, but large trees may be hard to install (section 10.4.7). As interstitial space is lost to decay, or if larger trees or stumps are installed, the fish assemblage occupying the reef probably shifts toward larger individuals. Because of the hodgepodge nature of brush reefs, they need to be tied together and anchored to the bottom (section 10.4.7).

Reefs also may be constructed from commercially available lumber, scrap lumber left over at sawmills, or repurposed lumber such as wooden pallets (Figure 10.6). Reefs made with lumber can be costlier, but unlike brush piles they can be assembled off-site. Treated wood increases durability of reefs constructed with lumber. Commercially treated wood most commonly is preserved with creosote or copper products. Creosote is a distillate of coal tar and is a variable mixture of 200–250 compounds, with over a dozen inventoried in the USEPA’s List of Priority Pollutants (NOAA 2009). Exposure to creosote reportedly produces reproductive anomalies and immune dysfunction and impairs growth and development in fish exposed to sufficiently high concentrations over long periods of time (reviewed by NOAA 2009). Treatment with copper includes zinc, chromium, and arsenic, but copper is the focal point because it leaches from treated wood at rates that can affect aquatic resources. However, use of treated wood products is unlikely to cause detectable effects in aquatic environments unless used in excessively large quantities (NOAA 2009). To minimize risk, copper-treated wood is preferred over creosote-treated wood, and maximum prefabrication can be done before the structure is placed in the water to lessen the releases of treated debris associated with cutting and drilling (NOAA 2009).

Figure 10.6. Examples of lumber reefs. Photo credits: (Left column, top to bottom) Pond Boss Magazine, Long Lake Fishing Club, Kids And Mentors Outdoors Northwood Chapter, and Pennsylvania Fish and Boat Commission; (Right column, top to bottom) U.S. Army Corps of Engineers (top two), Kentucky Department of Fish and Wildlife Resources, and Pennsylvania Fish and Boat Commission.

Various studies have evaluated aspects of constructing and installing brush reefs (reviewed by Bassett 1994; Bolding et al. 2004). Results have differed depending on brush type, reef size, configuration, depth, local conditions, and many other variables. These studies suggest that brush reefs benefit fish in multiple ways, and providing diverse brush reefs is likely to optimize benefits.

Half-logs or spawning benches reportedly attract various substrate-spawning species. These structures provide overhead cover in areas that already have favorable spawning substrate for nest spawners such as centrarchids and percids (Bassett 1994). The structures consist of a log sawed longitudinally in half, or hardwood slabs, fastened with the flat surface down on two to three concrete blocks. These structures are 8–12 ft long, 10–15 in wide, and 5–10 in thick. Half-logs generally are placed on firm substrate, preferably gravel, at depths of 3–10 ft.
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10.4.6.2 Stone Materials

Stone materials including boulders, riprap, and gravel are natural, durable, and familiar to many species inhabiting reservoir environments. These materials may exist already in tributaries and within the reservoir basin but may need to be supplemented because the reservoir might have submerged them at depths where they are no longer available to many species or might have been blanketed by sediment. Rocks may be placed singly (if large boulders), in piles to form patches, in long reefs along shore, in wood cribs, or in other containment structures (Grove et al. 1991; Kelch et al. 1999; Houser 2007; Figure 10.7). Riprap is commonly applied on reservoir shorelines to control erosion and wave action (section 5.8.2) but can also be applied with the explicit purpose of providing fish habitat. Riprap additions also may be applicable in key areas where fish may concentrate for spawning, such as the mouth of tributaries. A mixture of stone sizes may work better than size-graded stones because this creates a diversity of interstitial voids among the stones that fish can exploit.

Figure 10.7. Examples of reefs constructed from stone materials. Photo credits: (Left column, top to bottom) FISHBIO, Missouri Department of Conservation, Texas Parks and Wildlife Department, and Nebraska Game and Parks Commission; (Right column, top to bottom) Pennsylvania Fish and Boat Commission (top two), Georgia Department of Natural Resources, and Pond Boss Magazine.

Another source of stone materials may be construction supplies and demolitions. This source may include concrete blocks, rock, brick rubble, fractured concrete and slabs, and ceramic and concrete pipes. Concrete blocks provide intermediate-size interstitial space, and after colonization by algae and sessile organisms they appear natural (Moring and Nicholson 1994). Large concrete slabs produce large interstitial spaces that tend to attract large predators. Variable rock sizes within a structure create greater habitat diversity through more diverse interstitial spaces. Oftentimes stone construction material can be installed in the form of jetties, serving as fish habitat and points of access to shoreline anglers. Hernandez et al. (2001) found that rock jetties act as a refuge area for larval and juvenile fish.

Gravel beds attract substrate spawners and likely improve spawning success (Irwin et al. 1997). Gravel often can be distributed along shoreline areas with a habitat barge or in smaller operations from a 4 × 8-ft plywood sheet mounted on the bow of a flat-bottom boat. Shores with hard clay substrate are preferred or gravel will sink or be covered with sediment rather quickly. Gravel attracts spawning centrarchids and other species and makes for good areas to fish. Gravel substrate also can be introduced in spawning boxes. These are square boxes approximately 3 ft wide by 1 ft deep. The top of the box is open, and the bottom consists of 0.5–0.75-in hardware cloth reinforced with wood braces. The box is filled about halfway with 1–3-in gravel and placed in 3– 10 ft of water on soft substrate. Centrarchids and other substrate spawners nest inside the box. Multiple boxes may be installed near brush or other installed reefs to increase the density of adults spawning within a concentrated area.
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10.4.6.3 Synthetic materials

Plastics including polyethylene and polyvinyl chloride (PVC) have been used when constructing artificial reefs. Plastics can be convenient because they are lightweight and easy to work with and handle. Some can be extremely durable, are inexpensive and readily available, and, unlike brush, some will not snag an angler’s lure. Various vendors may be found by searching online for “artificial fish habitat.”

Various plastics are available to construct reefs (Figure 10.8). Relatively inexpensive plastic crates, and plastic netting for snow and safety fencing, can be used as building blocks to assemble reefs. Although convenient to obtain and simple to assemble, the strength of reefs built from plastic crates would need to be ensured. Snow fencing has been used in various designs where it is wrapped around a frame; however, when it becomes untied from its frame it can float and become a navigational hazard or a hazard to wildlife. Pipes of PVC are readily available at hardware stores and are safe to use in water as they are used for residential plumbing. Their round shape will not snag most lures. Corrugated pipe is also readily available and can be easily shaped into diverse forms. Some of these pipes also can be filled with sand, gravel, or concrete to weigh them to the bottom, and some may sink if drilled with holes to allow water to infiltrate the structure. The larger-diameter pipes also may harbor small fish inside the pipe. In summary, these products are relatively cheap, readily available, and can be used safely to make simple, reproducible designs by people with limited construction skills.

Figure 10.8. Examples of reefs constructed from synthetic materials. Photo credits: (Left column, top to bottom) U.S. Bureau of Land Management, Georgia Department of Natural Resources, Arkansas Game and Fish Commission, and Mossback Fish Habitat; (Right column, top to bottom) Pond Boss Magazine, Fishiding Reclaimed Artificial Fish Habitat, Porcupine Fish Attractors, and Pond King Inc.

A variety of plastic artificial structures are manufactured commercially. They vary widely in size, design, cost, and materials. Common designs emulate aquatic plant material. Structures made of synthetic materials are advantageous because they have greater longevity than small-diameter brush, are light, are easy to transport and assemble on site for quick installation, and do not require special equipment for assembly. However, they do have disadvantages. Four issues that have been identified are (1) some lack complex structure and small interstitial spaces; (2) they can be expensive when compared with brush; (3) they have been found to attract fewer fish (Rold et al. 1996; Magnelia et al. 2008); and (4) they have had low satisfaction rates among state fisheries agencies when the objective is to increase angler catch rates (Tugend et al. 2002).

Structures made of synthetic materials seem to be less efficient than brush at attracting fish. However, they have been shown to concentrate largemouth bass successfully, with some designs having better attracting qualities than others (Rogers and Bergersen 1999). When natural materials are not readily available, as for example in reservoirs constructed in desert regions, reefs fabricated from commercial synthetic structures may be a good option.

Plastics can break down over time and be hazardous to fish (Rochman et al. 2013). Risks come from the material itself and from chemical pollutants that absorb into the materials. Some of these compounds are added during plastics manufacture, whereas others adsorb from the aquatic environment (Eerkes-Medrano et al. 2015). Polyethylene accumulates more organic contaminants than do other plastics such as polypropylene and PVC. Overall, the hazards associated with the complex mixture of plastic and accumulated pollutants are largely unknown (Free et al. 2014; Driedger et al. 2015).

Another limitation of plastic reefs is that they are not detected by some of the more inexpensive sonar devices available to recreational anglers. Some devices may detect them but not very clearly. Thus, plastic structures more than brush or stone may require identification with buoys for maximum benefit to recreational anglers. Alternatively, if the goal of the structure is something other than to concentrate fish to promote angler catch, unmarked, hard to find, plastic structures may be a good option.
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10.4.7 Reef Installation

Artificial reefs can be installed from boats, land if water levels fluctuate, the air if helicopters are available, or ice if the reservoir surface freezes substantially. Small brush piles and synthetic structures easily can be deployed from agency work boats or volunteers’ boats. Brush piles can be staged on a shore near the destination site or boat ramp, expediting loading and accelerating deployment. Large reefs may require a specially designed and equipped habitat barge (Figures 10.5, 10.7). Such barges typically are equipped with a winch, dump bed, or both to facilitate deployment of large or heavy wooden structures as well as stone materials. In addition to being able to install larger structures, barges can facilitate more rapid installations. Habitat barges can cost $50,000–75,000. However, some large trees may be hard to install even with a habitat barge. Large trees may have to be towed by the habitat barge or installed when the water level is low enough to access the site with heavy machinery.

In reservoirs where water levels fluctuate annually or over multiyear cycles, reefs may be built on-site or dragged to the site with all-terrain vehicles. Deployment during low water level allows for a faster, safer, and more precise installation. At latitudes where the reservoir surface freezes sufficiently to support vehicles, reefs may be constructed on the ice or off the ice and dragged to the desired position. Once positioned, the ice may be cut, or the reef left on the ice until the ice melts and the wooden or stone reef sinks into position. Occasionally agencies may have access to helicopters to hoist and transport reefs to a desired location (Figure 10.5).

Brush reefs and synthetic reefs generally need to be tied and permanently anchored to the bottom in such a way to prevent floating to the surface, floating of any of its member pieces, and excessive chafing of its lashings or anchor lines. Tying and anchoring can be accomplished with a combination of galvanized wire, polypropylene rope, nails, rods through drilled holes, stakes, concrete blocks, rock-filled mesh bags, sand bags, or other materials of suitable weight. In some synthetic reefs, such as those made from PVC or corrugated pipes, gravel may be incorporated into the pipes to ensure the structures remain on the bottom at the location intended. Heavy-weight cable ties are used (150-lb breaking strength or greater).

Strict safety guidelines are required to avoid overloading boats or snagging the reef on personnel or equipment as the reef is being deployed. Providing global positioning system (GPS) waypoints or installing markers prior to reef deployment or on-site construction is desirable to ensure the reef is sited at the exact location planned.
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10.4.8 Reef Markers

Figure 10.9. A marking buoy used to identify a reef. Photo credit: U.S. Army Corps of Engineers, Nashville District.

In navigable waters, artificial reefs need to be marked clearly with permanent buoys, as required by the U.S. Coast Guard (Figure 10.9). Nevertheless, even in non-navigable waters, reefs that are near the surface may pose potential navigational hazards to recreational boaters and may require buoys. Markers are also useful to inform fishers where the attractors are located. The buoy shape, color, or both are usually different from buoys with navigational significance. Buoys that are colored plastic all the way through are generally best as opposed to painted buoys because the paint will degrade from sun exposure and wave action. Nevertheless, the authority with jurisdiction over the reservoir may need to be consulted about the need for markers and the required marking and color system. Moreover, in some states if a boat strikes a buoy the possibility for litigation may be high, so consultation with agency counsel may be necessary. If markers are a risk, signage for reefs may be installed on shore in areas next to the reef.

Markers also can be placed to serve as moorings for recreational fishers to avoid anchor damage to the artificial reef. Mooring buoys typically have a metal ring protruding from the top that can be used as a tie-off point for boats. Mooring buoys are ready-made, commercially available spherical structures (about 18-in diameter) of polyethylene plastic. They are usually filled with polyurethane foam and treated with ultraviolet inhibitors so as to endure strain after continual exposure to sunlight.

Expenses include not only the buoys and anchors but also maintenance. Occasionally, a stray buoy has to be retrieved and redeployed and may create navigational problems. Anglers also may try to move buoys for various reasons. Buoy monitoring and maintenance programs can be oriented toward preventing failures in the field. Ultimately the use of buoys or other markers is at the discretion of the program manager, unless markers explicitly are required through the permitting process.
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10.4.9 Maintenance and Monitoring

Long-term stability and durability is an important consideration in siting an artificial reef and in selecting materials. The shallower the water on a high-energy shoreline, and the greater the water-level fluctuation, the more severe the physical conditions a shallow-water structure will experience. This severity may lessen the durability of the structure and may compel more frequent maintenance. Most wood materials are less stable and durable at shallower depths where they are more exposed to solar radiation, changes in weather conditions, wave action, and exposed to the atmosphere during low water levels. Reefs are designed to survive prevailing physical conditions to prevent large parts from breaking free or compromising overall structural integrity. Ideally, materials used need to be resistant to degradation due to air exposure and the chemical forces of the aquatic environment. Most commonly, some reefs degrade and collapse and may not provide the original benefits but continue to provide some habitat benefits.

The longevity of reefs will vary depending on multiple variables associated with architecture, construction materials, workmanship, siting within the reservoir, and local environmental and climatic conditions. Nevertheless, all reefs tend to deteriorate over time, and their effectiveness may change linearly, although some more rapidly than others. Thus, all reefs need to be monitored periodically to determine whether maintenance is needed to preserve or upgrade their effectiveness and safety or whether they need to be removed. Reefs built from natural materials such as wood and stone generally may remain in the reservoir as they degrade. However, reefs made from synthetic materials may need to be removed once their useful life has been exceeded.

Monitoring reefs may include a general evaluation of several aspects associated with the reef’s attraction or production capacity or whether the reef is still safe (e.g., has not degraded to the extent that it may affect navigation). Monitoring may be as simple as visual inspections during low water or more demanding inspections with divers or side-scan sonar technology. Annual surveys with side-scan sonar (TPWD 2016) may be sufficient to detect reef degradation.

Scoring of reef status may be achieved with a qualitative scale (e.g., functional, moderately functional, nonfunctional) or a semiquantitative scale (e.g., 0 to 10). A subjectively selected threshold may be used to initiate reef restoration or removal. In many cases, volunteers may be enlisted and trained to perform inspections (Halusky et al. 1994).
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10.4.10 Brochures and Electronic Media

Budgets and available resources dictate whether printed maps may be made available. A simple brochure can be created using any word-processing software and reproduced on the office copier. In fact, this avenue may be preferable to an expensive, quickly out-of-date, full-color brochure. Plain or extravagant, the main objective is to satisfy the public need for basic data on distribution of artificial reefs. Fishers mainly want to know the reef location and possibly the configuration and depth. A list of GPS coordinates will do, but a brochure can be greatly enhanced with a chart showing site descriptors. The information may also be made available online in agency websites, phone apps, and social media (Figure 10.10).

Figure 10.10. An example of how information about reefs may be made available online in agency websites, phone apps, and social media. Source: South Carolina Department of Natural Resources, Columbia.


This informational tool can increase the success rate of anglers who may not be familiar with a reservoir, like first-time anglers or anglers from outside the region. These anglers can use the information to find the areas where fish habitat improvements have been installed and have a more pleasant experience with the confidence they are fishing at a tactical location.
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10.4.11 Volunteer Assistance

Volunteers can expand greatly the manager’s ability to develop a reef program (Jacobson and Koch 2008). However, one significant hurdle in the use of volunteers is the availability of agency personnel to provide training and oversight of activities. Because volunteers are generally available only on weekends, the manager or coworkers need to also be available on weekends. This type of management is time intensive, and the manager needs to consider the full scope of the task before committing to relying on volunteer labor. The manager provides sufficient training to the volunteers and plans and organizes the activities. Most volunteers are well intentioned, but they have other demands on their time and may not be available to complete the activity, so turnover may be another challenge. Wilson et al. (1996) worked extensively with volunteers in developing habitat in Norris Lake, Tennessee, and provided useful advice for working with volunteers (Table 10.1).

Table 10.1. Suggestions for working with volunteers in habitat enhancement projects. Modified after Wilson et al. (1996).


Keys to successful volunteer program


  • Design projects that address real problems and that have a high probability of success. Projects should include activities that volunteers believe are worth their efforts.
  • Make sure that a project leader is present at all activities to answer questions and be an example.
  • Identify and support a leader among the volunteers who will take responsibility for recruiting and notifying other volunteers to fulfill work schedules.
  • Have all equipment and materials present at the site on time and in good working order. Start on time. The volunteers are there to work and will not want to waste time waiting.
  • Give clear instructions as to what will be done, how long everyone will work, the importance of safety, and agency policies.
  • Give the volunteers as much responsibility as allowable (e.g., driving boats, backing trailers, operating posthole diggers).
  • Make the project fun but have goals that require hard work. At the end of the day, everyone should feel that a significant amount of work was accomplished.
  • Be sensitive to when it is time to quit for the day. Avoid the tendency to work volunteers a little longer than was agreed.
  • Be sensitive to varying abilities and health of the volunteers and their tendency to work beyond their limits.
  • Let volunteers know they are appreciated. Keep an accurate account of time they work. Throughout the project, express gratitude for their efforts. After the project is completed, reward them with appropriate recognition and awards (e.g., a banquet, caps, personal letters).
  • Invite the media to feature volunteers at work. Encourage the media to emphasize the role of volunteers in the project.
  • Keep in touch with volunteers between activities. Let them know the results of other activities undertaken for the project.
  • Always speak positively about projects, agencies, and volunteers. Do not let projects become a forum for negativism toward user groups and agencies. Rather, let them be opportunities for building understanding and respect among the diverse user and management groups.

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