SELECTION OF VEGETATED HABITAT BY BROWN SHRIMP, 

 PENAEUS AZTECUS, IN A GALVESTON BAY SALT MARSH 



Roger J. Zimmerman, 1 Thomas J. Minello, 2 and Gilbert Zamora, Jr. 3 



ABSTRACT 



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 altermflora habitat and another from nonvegetated habitat. 

 Overall a mean density for shrimp of 11.7/m 2 in vegetation was significantly greater than the mean 

 density of 1.4/m 2 in nonvegetated habitat (P < 0.001, /-test, 81 paired observations). In addition, shrimp 

 densities varied according to a pattern of lower numbers and less apparent attraction to vegetation 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. 

 alter ni flora . 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/m 2 . Recruitment of postlarvae continued 

 throughout the summer. 



A 2.8m 2 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. 



Estuaries have long been cited in their role as 

 nurseries for penaeid shrimp (Anderson et al. 

 1949; Kutkuhn 1966; Thayer et al. 1978; Weinstein 

 1979). Growth and production of penaeids in estu- 

 aries have been associated with temperature (St. 

 Amant et al. 1966; Zein-Eldin and Griffith 1966; 

 Aldrich et al. 1968; Pullen and Trent 1969), salin- 

 ity (Hildebrand and Gunter 1952; Gunter 1961; 

 Barrett and Gillespie 1973; Browder and Moore 

 1981), and vegetation (Turner 1977; Faller 

 1979). 



In salt marshes, vegetation may function vari- 

 ably to provide food, substrate, and protection for 

 young penaeids. It is well known that Spartina 

 alterniflora contributes to a detritus-based food 



•Southeast Fisheries Center Galveston Laboratory, National 

 Marine Fisheries Service, NOAA, 4700 Avenue U, Galveston, TX 

 77550; presently on IPA assignment from Center for Energy and 

 Environment Research, University of Puerto Rico, Mayaguez. 



2 Southeast Fisheries Center Galveston Laboratory, National 

 Marine Fisheries Service, NOAA, 4700 Avenue U, Galveston, TX 

 77550; presently on IPA assignment from Texas A&M University 

 at Galveston. 



3 Southeast Fisheries Center Galveston Laboratory, National 

 Marine Fisheries Service, NOAA, 4700 Avenue U, Galveston. TX 

 77550. 



Manuscript accepted October 1983. 

 FISHERY BULLETIN: VOL. 82, NO. 2, 1984. 



web (Teal 1962; de la Cruz 1965) which at least 

 potentially includes shrimp (Jones 1973). Mi- 

 croalgae and epibenthic biota associated with 

 marshes may also serve in the food web (Haines 

 1977) and be used as food by foraging shrimp 

 (Trent et al. 1969; Jones 1973). Since dense aquatic 

 vegetation impedes certain predators ( Vince et al. 

 1976; Nelson 1979; Coen et al. 1981; Heck and 

 Thoman 1981), marsh grasses could also furnish 

 protective cover for postlarval and juvenile 

 penaeids. Unfortunately, our understanding of 

 shrimp relationships to vegetation has been im- 

 paired by the inherent difficulty of sampling in 

 marine vegetation. 



Our aim was to overcome the sampling problem 

 and to obtain accurate data on shrimp densities 

 that could reliably depict differences between es- 

 tuarine habitats. In the present study, Penaeus 

 aztecus densities were compared between adjacent 

 vegetated and nonvegetated habitats within a 

 Galveston West Bay salt marsh. Since our experi- 

 mental design incorporated paired sampling of 

 habitats and imples with actual as opposed to 

 relative numoers of shrimp, both the resolution 



325-33V 



