FISHERY BULLETIN: VOL. 82, NO. 2 



vegetated samples compared with 41.8 cm (1 SD = 

 11.8, n = 81) in nonvegetated samples. Changes in 

 tide level were not large (about 30 cm) but were 

 important relative to sample depths. Since sam- 

 pling was executed at high tide, tide station mea- 

 surements were comparable between sampling 

 periods and useful for establishing variability in 

 high-water level. Mean high water during the 

 summer was 12 cm lower than in the spring reflect- 

 ing seasonally variable tidal inundation (Hicks et 

 al. 1983) and greater accessibility to vegetation 

 (Provost 1976) in the spring. 



A weak negative relationship between shrimp 

 density and temperature within a range of 17.0° to 

 34.0°C was apparent (r = - 0.34 in vegetation, P < 

 0.01, n = 57). Since temperature and oxygen levels 

 were inversely related, the trend, attributed to 

 temperature, also extended to an observed rela- 

 tionship between oxygen concentration and 

 shrimp density. However, oxygen levels were al- 

 ways near saturation (vegetated* = 8.2 ppm, 1 SD 

 = 1.4, n = 81; nonvegetated x = 8.1 ppm, 1 SD = 1.4, 

 n = 81) and unlikely to have influenced shrimp 

 distribution. Shrimp densities did not correlate 

 well with salinities (range of 19 to 35 ppt), tur- 

 bidities (range of 3.0 to 55 nephelometer turbidity 

 units), or water depths (overall range of 5.5 to 76 

 cm). In addition, temperature, salinity, oxygen, 

 and turbidity did not differ between habitats la- 

 test of 81 paired observations for each). 



Sampler Performance 



Test results suggested that shrimp recovery 

 from the drop sampler was more variable and 

 somewhat less effective in vegetation (x = 91% 

 recovery, 1 SD = 6.6%, n = 4) than in habitat 

 without vegetation (jc = 97.5% recovery, 1 SD = 

 2.5%, n = 4). However, a t-test between means by 

 habitat revealed no significant difference (P > 0.1) 

 and justified combining means (94%, 1 SD = 5.8%, 

 n = 8). 



Mean shrimp densities on nonvegetated bottom, 

 comparing our 1.8 m diameter drop sampler, a 5.5 

 m wide bag seine, and a 3.7 m wide otter trawl, 

 were 0.285/m 2 , 0.104/m 2 , and 0.054/m 2 , respec- 

 tively. Assuming 97.5% recovery and no avoidance 

 with the drop sampler, conservative estimates of 

 efficiency were 33% for the bag seine and 17% for 

 the otter trawl. Clearly, the data from the drop 

 sampler were more accurate (Table 6). 



DISCUSSION 

 Habitat Selection 



Significant differences in habitat-related 

 shrimp densities from a Galveston salt marsh (Ta- 

 ble 2, Fig. 4) demonstrate that P. aztecus may 

 select for S. alterniflora habitat. In support, 

 laboratory data of Giles and Zamora (1973) 

 suggest that P. aztecus and P. setiferus each prefer 

 S. alterniflora as opposed to barren substrate. In 

 addition, marsh grass transplanted on a dredge 

 spoil in Galveston Bay increased shrimp numbers 

 (Trent et al. 1969) and elimination of marsh 

 habitat to create waterfront housing diminished 

 shrimp abundance (Mock 1966; Gilmore and Trent 

 1974; Trent et al. 1976). In other instances, P. az- 

 tecus has been associated with vegetation includ- 

 ing Ruppia and Vallisneria in Mobile Bay (Loesch 

 1965), seagrasses in the Laguna Madre (Stokes 

 1974), and Juncus, Spartina, and seagrasses in 

 Mississippi Sound (Christmas et al. 1976). The 

 latter reported movement of postlarvae into 

 marsh vegetation during tidal inundation. 



The determinants of selection may have less to 

 do with S. alterniflora per se than with other 

 characteristics of vegetated habitat. For example, 

 in our case, shrimp numbers were not related to 

 the density or biomass of marsh grass (Table 5) but 

 simply to its presence or absence. Also, attraction 

 to vegetation differed between outer and inner 

 marsh (Table 2). Other studies have shown that 



TABLE 6. — Comparative gear efficiencies for sampling Penaeus aztecus in a Galves- 

 ton West Bay salt marsh. Area sampled and number of replicates for each device are 

 as follows: Drop sampler 2.8 m 2 (n = 22); beam trawl 3.0 m 2 (n = 12); bag seine 109 

 m 2 (/! = 10); otter trawl 72 m 2 (n = 10). 



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