Evaluating the Effectiveness of Grassbed Treatments as Habitat for Juvenile Black Bass in a Drawdown Reservoir

-Many reservoirs in arid regions experience highly variable water levels caused by seasonal inflow fluctuations and designated outflow requirements. At Shasta Lake, California, managers plant cerealgrain grass beds on exposed drawdown shorelines to increase juvenile fish habitat, localize productivity, and increase invertebrate fish prey. To determine the efficacy of these plantings, the abundance of juvenile black ba es Micropterus spp. (20-55 mm standard length) and the amount of periphyton and macroinvertebrate prey were compared among three treatment types: (I) planted grassbeds of cereal barley Hordeum vulgare; (2) artificial rope grassbeds, which eliminated physical deterioration and nutrient release; and (3) nonplanted control sites with predominately sand and gravel substrates. In comparison with control areas, juvenile black bass abundance averaged 54 times higher in planted grassbeds and 230 times higher in artificial grassbeds. Periphyton (chlorophyll a) and benthic invertebrate biomass did not differ significantly between planted grassbeds and control sites. In artificial grassbeds, periphyton was more than two times the control levels, and benthic invertebrate biomass was more than 12 times the control levels. We conclude that the long-term availability of physical structure, rather than nutrient release associated with decomposition of grassbed materials, drives use and effectiveness of grassbed treatments. Future management deci ions in drawdown reservoirs should emphasize increasing long-term availability and integrity of physical habitat for juvenile fishes in the linoral zone. lructurally complex lentic habitats can benefit juvenile fishes by providing refuges from predators and substrate for their prey organisms (Glass 1971 ; Savino and Stein 1982; Tabor and Wurtsbaugh 1991). Therefore, complex habitats are often used as nursery areas (Hall and Werner 1977), where survival of young fish may increase. In natural lakes, macrophyte beds and large woody debris from riparian areas often provide abundant areas of complex habitats (Mittelbach 1986; Schindler and ScheuereU 2002). However, in drawdown reservoirs , vegetation along the exposed shoreli ne is generally very limited because of highly variable fluctuations in water level, moderate to steep slopes, and relatively simple sediment composition. Summer reservoir drawdown may severely impact ageo fishes of many species by spatially reducing cover and feeding areas required by juvenile fish (Noble 1981). Turbidity from inflows and wave action on soils exposed by fluctuating water level often limit attached .. Corresponding author: [email protected] J Present add ress: U.S. Fish and Wildlife Service, Anadromous Fish Restoration Program, 4001 North Wilson Way, StOCkton, California 95205, USA Re eived April 22, 2008; accepted December 29, 2008 Published onl ine July 13, 2009 algae and aquatic macrophyte growth in reservoirs, thereby limiting areas where juvenile fish may ftnd cover and food (Kimmel and Groeger 1984). Artificial habitat has long been used as a fisheries management tool in areas where little natural habitat exists (Tugend et aJ. 2002). Artificial habitat structures have included conifer bundles (i.e., Christmas trees) , brush piles, automobile tire bundles, rock cribs or cages, dowel stake beds, and substrate modifiers, such as gravel (Brown 1986). The primary goal of artificial habitat management has been to concentrate fish, improve cover for target fish species, and increase angler catch rates (Brown 1986). Fishing conditions in certain areas often improve for a limited time after artificial habitats are added, but analyses of these processes rarely addre population-level growth. Another common goal of artificial habitat, which has been widely ignored in most habitat asse sments, is to increase fish survival by reducing predation or increasing food availability. Predation affects prey behavior and habitat selection (see reviews by Mittelbach and Chesson 1987; Lima and Dill 1990; Sih 1994). Similarly, predation by adult black basses Microplerus spp. is an important factor in determining survival and condition of juvenile black bass at Shasta Lake, California (J.Z., unpublished data).