Assessing Larval American Shad Growth and Survival with in situ mesocosm experiments in three differing habitats within a coastal estuary Donna Marie Bilkovic*, David Stanhope and Carl Hershner *Virginia Institute of Marine Science PO Box 1346, Gloucester Point, VA 23062 Contact Number: 804-684-7331 December 2002 INTRODUCTION Habitat can be defined as the place where the organism lives including all its physical, chemical and biological dimensions (Odum 1971; Hoss and Thayer 1993). These dimensions include water quality, physical structure, flow regime and biotic interaction. Essential fish habitat (EFH) is further defined as “those waters and substrate necessary to fish for spawning, breeding, feeding, or growth to maturity” (Magnuson-Stevens Act, 16 U.S.C. 1801 et seq.) With new mandates to identify and protect EFH for all species managed under fisheries management plans, evaluation of fish habitat has become a priority. The methods used to identify and define essential fish habitat have ranged from intensive microscale sampling to coarse macroscale delineations. Historically, assessment of fish habitat occurred on small scales and addressed water quality, physical structure and prey/predator interactions. With increases in geographic information systems (GIS) capabilities, large-scale depictions of fish distributions have been completed, however, these surveys often lack the detail necessary to describe the processes driving distribution. Research at both scales is necessary to accurately define and describe essential fish habitat. Macroscale assessments of fish distribution, linked with process-oriented experiments will elucidate the driving forces behind distribution and allow for a more complete identification of essential habitat. The coastal plain estuaries on the East Coast, including the Chesapeake Bay, provide essential spawning and nursery habitat for numerous commercially and recreationally important fish species. The determination of essential larval and juvenile fish habitat in this region offers a sizeable challenge. Without the advantages that historic landings data may allow with adult stages, additional surveys are necessary to first delineate distributions of the early life stages of targeted fish species. Next linkages between specific habitats and early life stage survival and growth must be developed. Simulated ecosystem studies (mesocosms) are useful methods to evaluate the impacts of habitat (including water quality, prey availability, and habitat structure) on the early life stages of fish. Numerous types of mesocosms, including enclosed containers or mesh bags, in both drifting and stationary deployments, have been utilized to examine larval fish survival and growth (Lafontaine and Leggett 1987a). Within the Chesapeake Bay, mesocosm experiments conducted have typically been closed systems that encapsulate a column of water from the natural system, but do not incorporate varying environmental conditions present throughout the experiment (Cowan, Jr. and Houde 1990; Cowan, Jr.,