ZIMMERMAN ET AL.: SELECTION OF HABITAT BY PENAEUS AZTECUS 



oxygen (YSI oxygen meter, Model 51 B) and 

 maximum and minimum depth were recorded. 

 Water samples (500 ml) were also procured in 

 order to measure turbidity (HF Instruments, 

 Model DRT-15). In vegetated samples, emergent 

 plant material was cut and removed to measure 

 plant biomass and to facilitate capturing the 

 macrofauna. Tide level was recorded from a per- 

 manent station at the beginning and end of each 

 sampling operation. All field work was done dur- 

 ing daylight within about 2 h before and after high 

 tide. 



In the laboratory, shrimp were identified, 

 sorted, and measured to the nearest millimeter 

 from rostrum tip to end of telson. Shrimp numbers 

 for each millimeter size interval were recorded for 

 each sample. Associated macrofauna from each 

 sample, including fish, crabs, and other shrimp, 

 were identified, measured, and counted. Gut con- 

 tents of the fish were examined for penaeid shrimp 

 as well as other identifiable material. Plant 

 biomass from each sample was dried in sunlight 

 until weight change was negligible. Sediments 

 and epiphytes were allowed to fall away as the 

 material dried. The resulting dry weight was 

 taken using a Mettler K-7 toploading balance and 

 reported as grams above-ground dry plant 

 biomass. Stem density was calculated by weighing 

 a subsample (about 20*7 of the total) and counting 

 the number of culms. 



Sampler Effectiveness 



Since the experimental design assumed no sam- 

 pling bias, the method was tested for recovery 

 efficiency both in vegetated and nonvegetated 

 habitats. Fifty shrimp, in the size range of 23 to 91 

 mm, were marked by clipping a uropod and placed 

 into deployed samplers. After a 30-min adjust- 

 ment period, the usual sampling procedure was 

 followed and recovery was recorded. 



Since our density data were compared with 

 other surveys, it was useful to test the effective- 

 ness of the drop sampler in relation to other col- 

 lecting devices. These included aim beam trawl, a 

 5.5 m bag seine, and a 3.7 m otter trawl. During 

 the initial test, eight replicate vegetated- 

 nonvegetated sample pairs were taken using the 1 

 m beam trawl (3.0 m 2 ) and the drop sampler (2.8 

 m 2 ). Later, 10 nonvegetated sample replicates 

 were obtained for each of the following: the drop 

 sampler, a 5.5 m bag seine (110 m 2 ), and a 3.7 m 

 otter trawl (75 m 2 ). The data were reported as 

 mean and standard deviation of shrimp density 



(per m 2 ) for each sampler. The efficiency for each 

 device was calculated relative to the drop sampler. 



RESULTS 



A total of 3,277 penaeid shrimp (97% P. aztecus) 

 were collected in 81 paired samples taken between 

 29 March and 23 July 1982. Shrimp densities in 

 the marsh were significantly higher in S. alter- 

 niflora habitat than adjacent nonvegetated 

 habitat (P < 0.001, t-test, 81 paired observations). 

 The magnitude and integrity of the relationship 

 between shrimp density and habitat type held 

 consistently throughout all sampling dates (Table 

 1, Fig. 4) and zones within the marsh, except for 

 the outer zone during March and April (Table 2). 

 Comparison of marsh zones (Table 2) revealed 

 highest P. aztecus densities and greater selection 

 for vegetated habitat in the innermost marsh di- 

 minishing toward the outer zone. Shrimp densi- 

 ties in nonvegetated habitat were highest in the 

 outer zone and diminished significantly toward 

 the inner zone (ANOVA, P < 0.001). 



TABLE 1. — Percent of Penaeus aztecus in 

 vegetated iSpartina alterniflora) and non- 

 vegetated habitats of a Galveston West Bay 

 salt marsh, 29 March through 23 July 1982. 



Habitat 



S 20 



- 



5 



| 10- 



o 

 z 



"r- 



I 



H 



MAR 29 APR 13 APR 26 MAY 11 MAY 26 JUN 22 JUL 21 



FIGURE 4. — Mean densities of Penaeus aztecus compared be- 

 tween vegetated Spartina alterniflora habitat and adjacent non- 

 vegetated habitat. 



329 



