270 



Fishery Bulletin 102(2) 



SNB 



1.2 



1 



0.8 

 0.6 

 0.4 

 0.2 

 



li ifll 



spring summer fall winter 



Salinity zone 



|a 0-0.5 D 0.5-5 □ 5-15 G15-25B> 25 | 



Figure 6 



Seasonal density predictions for brown shrimp (F. aztecus) by bottom type and 

 salinity zone. ME = marsh edge; SAV = submerged aquatic vegetation; SNB = 

 shallow nonvegetated bottom. 



values from drop sample data collected in Galveston Bay. 

 Regression analysis from this plot exhibited a strong posi- 

 tive relationship (r 2 =0.83, P<0.0001) between predicted 

 and observed density data (Fig. 8). This analysis was per- 

 formed to verify how the model represented the observed 

 density data. 



Model performance and transferability were assessed 

 by regressing predicted mean density values from the 

 Galveston Bay model on observed mean density values 

 from drop sample data collected in Matagorda, San 

 Antonio, and Aransas bays (Fig. 9). Regression analy- 

 sis produced a positive relationship for the entire drop 

 sample data from these bays combined (r 2 =0.56) and in- 

 dividually: Matagorda — r 2 =0.54; San Antonio — r 2 =0.57; 

 and Aransas — r 2 = 0.56. In Aransas and San Antonio 

 bays, brown shrimp densities were greatest during the 

 spring within the SAV bottom type and within salinities 

 >15 ppt. In Matagorda Bay, brown shrimp densities were 

 greatest in the spring within ME bottom types in waters 

 >15 ppt. No SAV samples were taken in this estuarine 

 system. 



Use of bottom types 



Results from spring (1985) and fall (1984) drop samples 

 within Christmas and West Bay (in lower Galveston Bay) 

 bottom types revealed significantly greater brown shrimp 

 densities in Christmas Bay SAV than adjacent ME and 

 SNB (P<0.0001). Brown shrimp densities in West Bay 

 ME were not significantly different from Christmas Bay 

 SAV but were significantly greater than densities within 

 adjacent SNB and Christmas Bay ME and SNB bottom 

 types (Fig. 10). 



The model results were also used to roughly estimate 

 an overall population of approximately 1.3 billion juve- 

 nile brown shrimp in Galveston Bay during the spring 

 season, by multiplying predicted densities by bottom-type 

 area (Table 3). Total area of bottom types in Galveston 

 Bay were as follows: 4.5 km 2 (SAV); 84.9 km 2 of marsh 

 edge (ME); and 1627.2 km 2 of nonvegetated bottom (29% 

 [476.2 km 2 ] of the latter area was considered SNB). On 

 the basis of predicted densities in different salinity regimes, 

 we estimated that there would be 51.0 million shrimp 



