Clark et al.: A habitat-use model for juvenile Farfantepenaeus aztecus In Galveston Bay 



265 



29 5° N 



29° N 



95 W 



Figure 1 



Map of Galveston Bay, Texas. 



94.5° W 



parisons of brown shrimp densities among different bot- 

 tom types in Louisiana and Texas estuaries have been 

 conducted within limited temporal and spatial scales 

 (Peterson and Turner, 1994; Zimmerman et al., 1984; Zim- 

 merman et al., 1990b; Rozas and Minello, 1998; Minello, 

 1999). 



Our work expands upon these studies by developing a 

 multivariate bottom-type use and environmental model 

 incorporated into a geographic information system (GISi 

 that provides a spatial assessment of habitat use. In ad- 

 dition, the model is designed to be transferable to other 

 northern Gulf of Mexico estuaries and thus would allow 

 fishery managers to identify the relative importance of 

 habitat types for population maintenance and recruitment 

 into the fishery. 



Materials and methods 



Geographic setting 



The Galveston Bay complex (Fig. 1) encompasses approxi- 

 mately 2020 km 2 and is one of the largest estuaries in 

 the northern Gulf of Mexico (NOAA, 1989). Comprising 

 several major embayments, including Trinity, Galveston, 

 East, and West bays, the complex contains many smaller 

 interconnecting subbays, rivers, streams, tidal creeks, 

 wetlands, reefs, and tidal flats around its periphery. 



The bay bottom is mostly flat and shallow (mean depth 

 is approximately 2 m) and has slightly elevated oyster 

 reefs, elevated dredge material areas, river channels, and 

 deeper dredged navigation channels. 



Data collection 



Sixteen years (1982-97) of brown shrimp density data 

 were analyzed to quantify areas of potential EFH. A total 

 of 46,080 brown shrimp were captured during this time 

 period with a mean total length of 27.5 mm (Fig. 2). Data 

 from published studies by Czapla (1991), Minello et al. 

 ( 1991), Minello and Zimmerman ( 1992), Minello and Webb 

 ( 1997 ), Rozas and Minello ( 1998 ), Zimmerman et al. (1984, 

 1989, 1990a, 1990b), Zimmerman and Minello (1984), 

 and various unpublished sources from the Galveston 

 Laboratory of the National Marine Fisheries Service were 

 combined to comprise a comprehensive density database of 

 associated bottom-type and environmental data that would 

 support model development and GIS analyses. All samples 

 were collected by using a drop trap sampler, described in 

 Zimmerman et al. (1984), which employs large cylinders 

 ( 1.0 or 2.6 m 2 area) released from a boom affixed to a boat 

 to entrap organisms. This quantitative technique samples 

 fishes and macro-invertebrates in highly structured shal- 

 low-water habitats such as salt marshes, seagrass beds, 

 and oyster reefs where the efficiency of conventional trawl 

 and bag-seine gear is diminished. 



