Selection of vegetated habitat by brown shrimp, Penaeus aztecus, in a Galveston Bay salt marsh
Zamora, G., Jr.
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Densities of the brown shrimp, Penaeus aztecus, in vegetated and nonvegetated habitats of a Galveston West Bay salt marsh were compared. Each of 81 sample pairs taken between 29 March and 23 July 1982 consisted of one sample from Spartina alterniflora habitat and another from nonvegetated habitat. Overall a mean density for shrimp of 11.7/m2 in vegetation was significantly greater than the mean density of 1.4/m2 in nonvegetated habitat (P<0.001, t-test, 81 paired observations). In addition, shrimp densities varied according to a pattern of lower numbers and less apparent attraction in the outer bayside part of the marsh to that of highest numbers and greatest attraction in the innermost marsh. Accordingly, respective means for the outer, middle, and inner marsh zones in vegetated/nonvegetated sample pairs were 7.5/2.3, 11.0/1.0, and 16.6/0.6. Simple presence or absence of S. alterniflora, area covered by vegetation, and location within the marsh were the primary observed correlates to shrimp density patterns. Mean high water in vegetation was 22.1 cm compared with 41.8 cm for adjacent nonvegetated habitat, making vegetated habitat less accessible during periods of low water. Mechanisms that may have enhanced utilization of vegetated habitat for P. aztecus were reticulation in salt marsh macrostructure, relatively low tidal range, and seasonal periods of high water. The nursery function of the salt marsh was confirmed by dominance of small shrimp, with 95% of all individuals being smaller than 50 mm in rostrum through telson length. During April, the maximum mean density of postlarvae under 30 mm was 16.4/m2. Recruitment of postlarvae continued throughout the summer. A 2.8 milimeters squared drop sampler, used to obtain the data, was found to be 2 to 5 times more effective for estimating densities of P. aztecus than trawls or seines. Consequently, our study improved the accuracy of estimates on estuarine shrimp densities, while also providing reliable evidence that P. aztecus may select for vegetated marsh habitat.