ZIMMERMAN ET AL.: SELECTION OF HABITAT BY PENAEUS AZTECUS 



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INNER MARSH 



TABLE 4. — Within habitat densities of Penaeus aztecus from a 

 salt marsh in Galveston West Bay, 29 March through 23 July 

 1982. n = number of samples. 



MIDDLE MARSH 



10 



20 



30 



PERCENT SPARTINA COVERAGE 



FIGURE 6. — Selection by Penaeus aztecus for vegetated habitat 

 compared against percent coverage of Spartma alterniflora. 



tnbuted anew on each subsequent flood tide. 



Differential predation by fish did not account for 

 shrimp differences between habitats. Of four 

 species preying on shrimp, 328 were in vegetation 

 versus 48 on nonvegetated bottom. Among these, 

 18 from vegetated ( 5% ) and 3 from nonvegetated 

 (69c ) contained shrimp in gut contents. The pred- 

 ators, in order of vegetated/nonvegetated abun- 

 dance, were Lagodon rhomboides (pinfish 246/36), 

 Fundulus grandis (gulf killifish 45/0), Cynoscion 

 nebulosus (spotted seatrout 22/2), and 

 Paralichthys lethostigma (southern flounder 15/ 

 10). Only southern flounder contained shrimp in 

 gut contents (3 of 10) from nonvegetated habitat. 

 In vegetated habitat, 8 of 15 southern flounder, 10 

 of 22 spotted seatrout, 1 of 45 gulf killifish, and 3 of 

 246 pinfish contained shrimp. 



Mean density of P. aztecus in vegetation was 

 11.7/m 2 overall with a range of 0.7 to 43.2/m 2 (Ta- 

 ble 4). Densities were highest in the innermost 

 marsh (x = 16.6/m 2 ; range = 1.8 to 43.2/m 2 ) and 

 lowest in the outer marsh (x = 7.5/m 2 ; range = 0.7 

 to 28.2/m 2 ). The overall variance was less than the 

 overall mean. Among marsh zones, shrimp patch- 

 iness in vegetation decreased slightly from the 

 outer to inner marsh (Table 4). 



Density of P. aztecus in nonvegetated habitat 

 was 1.4/m 2 with a range of to 18.2/m 2 (Table 4). 

 Densities on nonvegetated bottom were highest in 

 the outer marsh (x = 2.3/m 2 ; range = to 18.2/m 2 ) 

 and lowest in the inner marsh (x = 0.6/m 2 ; range = 



to 2.1/m 2 ). Overall distribution on nonvegetated 

 bottom, as reflected by the variance to mean ratio 

 (coefficient of variation, Table 4), was patchier 

 (more clumped) than on vegetated bottom. Shrimp 

 distributions also were patchier in nonvegetated 

 outer and middle zones, than in the nonvegetated 

 inner zone. 



Stem density and above-ground biomass of S. 

 alterniflora were positively correlated (Table 5). 

 The overall range of values was 41 to 784 g m 2 for 

 biomass and 33 to 629 stems/m 2 with respective 

 means of 298 g/m 2 (1 SD = 175, n = 81) and 234 

 stems/m 2 ( 1 SD — 72, n =81). Between zones, plant 

 biomass from the outer to inner zone increased 

 from 258 to 348 g/m 2 . The weight per stem in- 

 creased (larger diameters) from outer to inner 

 marsh. Although the trend suggested a negative 

 relationship between shrimp density and vegeta- 

 tional density and biomass, correlation was not 

 significant over the range examined. 



Abiotic Relationships 



Water depth between vegetated and nonvege- 

 tated sample pairs was significantly different (P < 

 0.01, t-test of 81 paired observations). The mean 

 water depth was 22.1 cm (1 SD = 10.0, n = 81) in 



TABLE 5. — Density and biomass of Spartma alterniflora from a 

 salt marsh in Galveston West Bay, 29 March through 23 July 

 1982. n = number of samples. 



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