264 



Abstract— A density prediction model 

 for juvenile brown shrimp (Farfan- 

 tepenaeus aztecus) was developed by 

 using three bottom types, five salinity 

 zones, and four seasons to quantify pat- 

 terns of habitat use in Galveston Bay, 

 Texas. Sixteen years of quantitative 

 density data were used. Bottom types 

 were vegetated marsh edge, submerged 

 aquatic vegetation, and shallow non- 

 vegetated bottom. Multiple regression 

 was used to develop density estimates, 

 and the resultant formula was then 

 coupled with a geographical informa- 

 tion system (GIS) to provide a spatial 

 mosaic (map) of predicted habitat use. 

 Results indicated that juvenile brown 

 shrimp (<100 mm) selected vegetated 

 habitats in salinities of 15-25 ppt and 

 that seagrasses were selected over 

 marsh edge where they co-occurred. 

 Our results provide a spatially resolved 

 estimate of high-density areas that will 

 help designate essential fish habitat 

 (EFH) in Galveston Bay. In addition, 

 using this modeling technique, we were 

 able to provide an estimate of the over- 

 all population of juvenile brown shrimp 

 (<100 mm) in shallow water habitats 

 within the bay of approximately 1.3 

 billion. Furthermore, the geographic 

 range of the model was assessed by 

 plotting observed (actual) versus 

 expected (model) brown shrimp densi- 

 ties in three other Texas bays. Similar 

 habitat-use patterns were observed 

 in all three bays — each having a coef- 

 ficient of determination >0.50. These 

 results indicate that this model may 

 have a broader geographic application 

 and is a plausible approach in refining 

 current EFH designations for all Gulf 

 of Mexico estuaries with similar geo- 

 morphological and hydrological char- 

 acteristics. 



A habitat-use model to determine essential 

 fish habitat for juvenile brown shrimp 

 (Farfantepenaeus aztecus) in Galveston Bay, Texas 



Randall D. Clark 



John D. Christensen 



Mark E. Monaco 



Biogeography Program 



Center for Coastal Monitoring and Assessment 



National Center for Coastal Ocean Science 



National Ocean Service. NOAA 



Silver Spring, Maryland 20910 



E-mail address (For R. D Clark) Randy Clarkfflnoaa gov 



Philip A. Caldwell 



Geoffrey A. Matthews 



Thomas J. Minello 



Fishery Ecology Branch 



Galveston Southeast Fisheries Science Center Laboratory 



National Marine Fisheries Service, NOAA 



Galveston, Texas 77550 



Manuscript approved for publication 

 22 December 2003 by Scientific Editor. 



Manuscript received 20 January 2004 

 at NMFS Scientific Publications Office. 



Fish. Bull. 102:264-277 (200 1 1 



Shallow estuarine habitats, whose com- 

 plexity promotes survival and growth, 

 are used by many young fish and macro- 

 invertebrate species (Boesch and 

 Turner, 1984). A complete understand- 

 ing of how these habitats sustain spe- 

 cies productivity is unknown and has 

 become a focal point of federal fishery 

 management programs. The National 

 Marine Fisheries Service (NMFS) 

 has developed guidelines to identify 

 essential fish habitat (EFH) for all 

 federally managed species based on 

 four levels of available information 

 that encompass the ecological linkages 

 between habitats and fishery produc- 

 tion. Examination of habitat-use pat- 

 terns (habitat-related densities) are 

 needed to determine which habitats 

 are likely to be most essential. These 

 patterns are measurable and can be 

 reasonable indicators of habitat value. 

 Relative habitat values have been esti- 

 mated by comparing animal densities 

 under the assumption that high densi- 

 ties reflect greater habitat quality and 

 preferred habitat (Pearcy and Myers, 

 1974; USFWS, 1981; Zimmerman and 

 Minello, 1984; Sogard and Able, 1991; 

 Baltzetal., 1993). 



Considerable bottom-type variation 

 exists in northern Gulf of Mexico estu- 

 aries, including intertidal marsh, sub- 

 merged aquatic vegetation, oyster reef, 

 mangroves, tidal mudflats, and sub- 

 tidal bay bottom. Within each of these 

 habitats, environmental and structural 

 gradients may affect the functional 

 role or importance of these habitats 

 for particular species. To understand 

 these relationships, fisheries indepen- 

 dent monitoring (FIM) data are needed 

 to determine species-habitat affini- 

 ties that provide evidence that not all 

 habitats are of equal importance for the 

 maintenance of a population (Monaco 

 et al., 1998; Minello 1999; Beck et al., 

 2001). Habitat affinities may change 

 with spatial and temporal fluctuations 

 of environmental variables, such as sa- 

 linity and temperature (Copeland and 

 Bechtel, 1974; Baltz et al., 1998). 



In this study we developed predictive 

 models that estimate brown shrimp 

 {Farfantepenaeus aztecus, formerly 

 Penaeus aztecus [see Perez-Farfante 

 and Kensley, 1997]) habitat-use pat- 

 terns and interactions as a function 

 of density-independent processes in 

 Galveston Bay, Texas. Previous com- 



