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



269 



Table 2 



Variable coefficients (log +1) derived from brown shrimp multivariate regression model. ME 

 aquatic vegetation; SNB = shallow nonvegetated bottom. 



marsh edge; SAV = submerged 



y-intercept 



Bottom type 



Season 



0.335 



0.113 (ME) 



0.043 (SAV) 



-0.156 (SNB) 



0.239 (spring) 



0.165 (summer) 



-0.045 (fall) 



-0.359 (winter) 



Salinity zone 



Bottom type x salinity zone 



-0.525 



-0.147 



0.079 



0.286 



0.307 



(0-0.5) 



(0.5-5) 



(5-15) 



(15-25) 



(>25) 



-0.104 

 -0.055 



0.159 



0.273 

 -0.396 



0.123 

 -0.030 



0.049 

 -0.018 

 -0.119 



0.288 

 -0.168 

 -0.018 



0.114 

 -0.096 



(ME/0-0.5) 



(SAV/0-0.5) 



(SNB/0-0.5 



(ME/0.5-5) 



(SAV/0.5-5) 



(SNB/0.5-5) 



(ME/5-15) 



(SAV/5-15) 



(SNB/5-15) 



(ME/15-25) 



(SAV/15-25) 



(SNB/15-25) 



(ME/>25) 



(SAV/>25) 



(SNB/>25) 



the data well (r 2 =0.73, n=47). Overall, density predictions 

 were highest in the spring, declined through summer and 

 fall, and reached the lowest values during winter (Fig. 6). 

 SNB density predictions were highest in the >25 ppt salin- 

 ity zone and declined as salinity declined in the estuary. 

 ME density predictions exhibited similar density predic- 

 tion trends; however, a smaller peak was observed in the 

 0.5-5 ppt salinity zone. This result may be an artifact of 

 two fall samples that exhibited high density within this 

 salinity zone. Density predictions within SAV were near 

 zero in the lower two salinity zones, peaked in the 15-25 

 ppt salinity zone, and slightly decreased in the >25 ppt 

 salinity zone. 



Model prediction maps 



For all seasons, highest density predictions corresponded 

 with ME and SAV bottom types within the region of the bay 

 with highest salinity — Christmas and West bays (Fig. 7). 

 Density predictions decreased within all bottom types as 

 salinity declined in the middle and upper regions of the 

 bay. Spring density predictions were the highest; maxi- 

 mum values were predicted within ME (6.14/m 2 ) and SAV 

 (14.49/m 2 ) located in Christmas and West bays (Fig. 7). 

 Density predictions steadily declined through the middle 

 bay and declined to 1/m 2 or less within SAV and SNB in 

 the upper region of the bay (Trinity Bay) where salinities 

 were less than 5 ppt. Density predictions during summer, 

 fall, and winter were lower than those observed during the 

 spring but exhibited similar spatial trends — higher pre- 

 dictions within the high salinity vegetated bottom types, 

 and decreasing with decreasing salinity. 



Model performance 



Spatial patterns were assessed by plotting predicted mean 

 density values from the model and observed mean density 



