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

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