Selection of Estuarine Habitats by Juvenile Gags in Experimental Mesocosms

—The degradation and destruction of estuarine habitats threaten the organisms that depend on these habitats for food and shelter. Gags Mycteroperca microlepis reside on rocky reefs for most of their lives but initially settle and rear in estuarine habitats before moving to offshore reefs. Gag populations have declined to the point where some consider them vulnerable to extinction, and the recovery of the species requires an understanding of what habitats these fish use and why they use them. We examined the habitat selection of juvenile gags in North Carolina using experimental mesocosms. We manipulated the shelter characteristics of habitats and compared the foraging rates of gags to determine the specific attributes of habitats that influence habitat selection. Gags selected structured seagrass or oyster reefs over sand and shell hash habitats. While gags did not show a preference between eelgrass Zostera marina and oyster reefs, they did choose oyster reefs over shoal grass Halodule wrightii and selected seagrass habitats with high shoot densities over those with lower densities. The addition of a small shelter to the habitat that the gags did not choose dramatically increased their use of that habitat. Finally, when we provided pinfish Lagodon rhomboides as prey, gag foraging rate did not vary among seagrass habitats with different shoot densities. However, gags consumed penaeid shrimp at much lower rates in seagrass habitats of high shoot density. Our results agree with those of other studies suggesting that seagrass habitats are important to gags. However, our results also emphasize that gags select specific attributes within seagrass landscapes and suggest that oyster reefs may be important habitats for them. Worldwide declines of exploited fish species (e.g., Music et al. 2000) are widely publicized examples of crises in resource management. Recognition of the drawbacks of traditional fishery management schemes, which focused on assessing stock size and controlling fishing mortality, was codified in the Magnuson–Stevens Fisheries Conservation and Management Act of 1996. This act requires fisheries agencies to protect, restore, and enhance all ‘‘essential fish habitats,’’ thereby making the conservation of fish habitats an integral part of fishery management. The vegetated habitats typical of many estuaries are complex habitats with a diversity of microhabitats that may contribute to the growth and survival of fish that depend on estuaries (Orth et al. 1984; Irlandi and Crawford 1997; Levin et al. 1997; Minello 1999). While seagrass meadows and the emergent vege* Corresponding author: [email protected] Received March 11, 2002; accepted June 25, 2002 tation of marshes and mangroves are considered critical habitats for most estuarine-dependent species, specific habitat types essential to maintaining the productivity of most such species have not been identified (Minello 1999). Because microhabitat quality can be an important determinant of recruitment success (Levin 1991; Carr 1994), there is a clear need to determine which microhabitats within estuaries are essential to fish productivity. We examined habitat selection by juvenile gags Mycteroperca microlepis. Historically, gags were one of the most important components of recreational and commercial fisheries in the South Atlantic Bight (Harris and Collins 2000). Indeed, in the United States gags accounted for 27% of recent recreational catches by weight (Levin and Grimes 2002). Gags have been overfished to the point where some consider them vulnerable to extinction (Music et al. 2000). The dire status of gag stocks clearly makes understanding the factors affecting the production of new recruits of fundamental importance. 77 SELECTION OF ESTUARINE HABITATS Postlarval gags settle in estuarine waters in spring and early summer (Keener et al. 1988). Once settled, juvenile gags show strong site fidelity (Heinisch and Fable 1999) and suffer low rates of mortality (Koenig and Coleman 1998). Newly settled gags occur in structurally complex habitats and are frequently reported as members of seagrass communities (Summerson and Peterson 1984; Ross and Moser 1995; Koenig and Coleman 1998). After a 4–6 month period during which body length increases by about 700%, gags leave the estuary for offshore reef habitats, where they reside for the rest of their lives (Adams 1976; Keener et al. 1988). Although seagrass meadows are typically considered gag nursery habitat (Ross and Moser 1995; Koenig and Coleman 1998), seagrass beds are complex habitats with a diversity of microhabitats that may or may not contribute to the growth and survival of gags. For instance, in Texas, nearly four times more red drum Sciaenops ocellatus were captured in seagrass beds dominated by shoal grass Halodule wrightii as in those dominated by turtle grass Thalassia testudinum (Rooker and Holt 1998). While it is important to determine which microhabitats are essential for successful recruitment of juvenile gags, low densities of fish make this difficult. To overcome this obstacle, we examined the habitat selection of gags using outdoor experimental mesocosms. Specifically, we asked whether juvenile gags choose among typical estuarine habitats (seagrass, oyster reefs, shell hash, and sand) that vary structurally. We then manipulated the shelter characteristics of these habitats and compared the foraging rates of gags to determine the specific attributes of habitats that influence habitat selection.