FISHERY BULLETIN: VOL. 82, NO. 2 



TABLE 2. — Mean number of Penaeus aztecus per m 2 by 

 zone in vegetated and nonvegetated salt marsh habitats 

 from Galveston West Bay 29 March through 23 July 

 1982. 



'Veg = Spartina alterniflora habitat; Non = Nonvegetated habitat. 



difference within brackets not significant between vegetated and 

 nonvegetated pairs; for all others, the difference was highly significant 

 (P < 0.001 . r-test, paired observations). 



Penaeus aztecus densities for each 20 mm size 

 interval were more abundant in Spartina habitat 

 than adjacent nonvegetated bottom (Fig. 5). Vege- 

 tated habitat contained 89 to 96% of all shrimp in 

 size classes under 50 mm and 75 to 78% of larger 

 size classes (Table 3). Those under 30 mm in length 

 comprised 77,% of all shrimp and those under 60 

 mm made up 98% of the total (Table 3). Size class 

 distributions differed between habitats (Kol- 

 mogorov-Smirnov test, P = 0.02; Fig. 5), but the 

 very small sample size from nonvegetated habi- 

 tat decreased the strength of this observation. 



The highest P. aztecus densities in vegetation 

 and the lowest on nonvegetated bottom were 

 characteristic of the innermost zone (Table 1). The 

 degree of vegetated-nonvegetated differences 

 suggested an apparent selection for vegetated 



habitat and greater selection in the inner zone 

 compared with the outer zone. The increase in 

 vegetated to nonvegetated shrimp densities coin- 

 cided with an increase in S. alterniflora coverage 

 between the outer and inner marsh (Fig. 6). Areal 

 coverage of vegetation, determined from aerial 

 photographs (Fig. 2), differed by a factor of 3 be- 

 tween the outer and inner marsh, and selection, as 

 measured by the ratio of shrimp density in vege- 

 tated habitat to density in nonvegetated habitat, 

 differed by a factor of 9 from outer to inner zones 

 (Fig. 6). In addition, the ratio differed between the 

 middle and inner zone, but shrimp densities 

 within vegetation between those zones (Table 2) 

 did not change significantly (ANOVA, Duncan's 

 multiple range test, 0.05 level). Due to the inter- 

 tidal nature of vegetated habitat, shrimp were 

 forced into subtidal areas at low tide and redis- 



TABLE 3. — Percent abundance among size classes for Penaeus 

 aztecus in a Galveston West Bay salt marsh. 29 March through 23 

 July 1982. n = number of shrimp per size interval; N = total 

 number of shrimp collected. 



MAR 29. APR I APR 13-15 APR 2628 MAY 11.14 MAY 26 28 



R- 



2 lOh 



JUN 22 24 JUL 21-23 



, Cb_ P 



NONVEGETATED HABITAT 



n n~^ «=i-R- 



1 n -i 



. n^ 



SPARTINA VEGETATED HABITAT 



H- 



TT„ 



*=T- 



fckfi 



30 5 °70 90 ,0 30 5 °70 90 ">30 5 °70 90 '°30 5 ° 9 ° '°30 50 70 9 ° '° 



SIZE CLASSES PENAEUS AZTECUS 



1 



50 90 10 



1 



50 90 



30 70 30 70 



FIGURE 5. — Densities of Penaeus aztecus by size class in adjacent vegetated and nonvege- 

 tated habitats from Galveston West Bay during 1982. Size class distributions differed 

 between habitats (Kolmogorov-Smirnov test, P = 0.02). 



330 



