272 



Fishery Bulletin 102(2) 



(Christensen et al., 1997). In laboratory experiments, 

 Zein-Eldin and Aldrich (1965) concluded that higher sa- 

 linities are more favorable for brown shrimp. Salinities of 



ME + SAV x SNB 



0.2 0.4 0.6 0.( 



Observed log density 



Figure 8 



Relationship between predicted and observed densities of 

 brown shrimp (F. aztecus) in Aransas, Matagorda and San 

 Antonio bays and predicted densities from the Galveston Bay 

 model. ME = marsh edge; SAV = submerged aquatic vegetation; 

 SNB = shallow nonvegetated bottom. 



20 ppt or greater were considered optimum in data from 



Louisiana (Barrett and Gillespie, 1973). 



In the present study, brown shrimp were captured 

 throughout Galveston Bay, but highest densities 

 were observed in the lower bay where salinities were 

 greater than 15 ppt. This spatial trend was further 

 strengthened by greater abundance of vegetated 

 bottom types in the lower portions of the bay, where 

 nearly half of the total marsh edge and 90% of sea- 

 grass beds are located (Fig. 4). These bottom types 

 are regularly inundated and provide stable substrate 

 for brown shrimp prey (epiphytic algae and infauna), 

 whereas seasonal oligohaline marsh and SAV habitats 

 in the upper bay may not promote favorable condi- 

 tions for prey organisms (Zimmerman et al., 1990b). 

 Therefore, salinity effects and the greater availability 

 of vegetated habitats in the lower bay may work in a 

 complementary manner to provide nursery areas for 

 brown shrimp in Galveston Bay. 



Previous attempts to examine spatial patterns 

 of abundance and to determine linkages between 

 organisms and habitat included the development of 

 habitat suitability index (HSI) models. Early methods 

 were derived by the U.S. Fish and Wildlife Service 

 (USFWS) for freshwater species, where the HSI was 

 defined as a numerical index that represented the 

 capacity of a given habitat to support a selected spe- 

 cies. The scale of HSI values (0-1.0) reflects a linear 

 relationship between suitability and carrying capacity 



