FISHERY BULLETIN: VOL. 82, NO. 2 



comparable, since sampling was executed during 

 unknown variable tidal stages and the degree of 

 flooding in intertidal vegetation appears to 

 greatly influence shrimp densities on nearby non- 

 vegetated subtidal bottom. Perhaps the only 

 meaningful density estimates are those taken dur- 

 ing low tide in nonvegetated habitat or those 

 taken in vegetated habitat at flood tide. In any 

 case, tide stage must be uniform for data to be 

 comparable. 



Sampling Integrity 



The sampling approach in our investigation 

 provided more realistic density estimates than 

 traditional methods for sampling shrimp in es- 

 tuaries (Table 6). We agree with Loesch et al. 

 (1976) in concluding that techniques such as the 

 area-swept method using an otter trawl are among 

 the poorest for quantifying P. aztecus. Past recog- 

 nition of this problem stimulated development of 

 the push net (Allen and Inglis 1958), small beam 

 trawl (Renfro 1963; Loesch 1965), and marsh net 

 (Pullen et al. 1968). These samplers improved ac- 

 curacy on nonvegetated bottom, but were ineffec- 

 tive when vegetation was present and did not solve 

 avoidance problems. Further improvement came 

 for sampling in seagrasses, but not salt marshes, 

 with the invention of a sled-mounted suction sam- 

 pler (Allen and Hudson 1970) and modification of a 

 drop net technique (Hoese and Jones 1963; Gil- 

 more et al. 1976). Our methodology has been de- 

 signed to minimize escape, improve recovery from 

 the area sampled (including burrowed shrimp), 

 and to operate in salt marsh habitats. The drop- 

 sampler method proved to be nearly as effective 

 among vegetation as on nonvegetated bottom. 



CONCLUSION 



We contend that differences in P. aztecus den- 

 sities between vegetated and nonvegetated marsh 

 bottom were due to habitat selection. In support, 

 we refer to Loesch (1965), Trent et al. (1969), and 

 Stokes (1974) who have associated brown shrimp 

 distributions with estuarine vegetation, and a 

 laboratory experiment by Giles and Zamora ( 1973 ) 

 demonstrating P. aztecus prefer S. alterniflora in- 

 stead of barren substrate. Finally, our fish gut 

 examinations indicate that immediate effects of 

 predation did not account for the density differ- 

 ential. 



Since S. alterniflora is characteristically inter- 

 tidal, and not continuously available to shrimp, 



the adjacent subtidal zone provided an important 

 alternate habitat during low tide. We propose that 

 the amount of edge between habitats facilitated 

 shrimp movement, and the reticulated nature of 

 the salt marsh was an important feature for in- 

 creasing the amount of edge. In addition, intertid- 

 al vegetation was more accessible and its potential 

 for utilization greater during spring and fall high 

 tides. This interaction may in part account for 

 seasonal peaks in P. aztecus populations. In our 

 investigation, recruitment began abruptly with 

 equinox tides. The shrimp population during the 

 spring and early summer was dominated entirely 

 by P. aztecus. 



Our shrimp densities from vegetated habitat 

 were higher than any previously reported includ- 

 ing those from seagrass and mangrove systems. 

 The high densities in vegetation were possibly 

 governed by the amount of total marsh, ratio of 

 vegetated to nonvegetated habitat, and size of re- 

 cruitment. The densities on nonvegetated marsh 

 bottom were probably controlled by the relative 

 accessibility of nearby vegetated habitat. In any 

 case, the observed density differential strongly 

 implies that marsh vegetation provides a vital 

 function for juvenile brown shrimp. 



ACKNOWLEDGMENTS 



Edward F. Klima and the staff of the Southeast 

 Fisheries Center Galveston Laboratory, National 

 Marine Fisheries Service, are especially acknowl- 

 edged for their support of this investigation. In 

 particular, C. Albrecht, S. Dent, D. Gleason, K. 

 Griffith, E. Martinez, and J. Wellborn are recog- 

 nized for field and laboratory assistance. Equip- 

 ment and important logistical support were 

 kindly provided by K. Baxter and Z. Zein-Eldin. 

 The manuscript was reviewed by E. Klima, S. Ray, 

 G. Thayer, and Z. Zein-Eldin, and final prepara- 

 tion was assisted by D. Patlan, J. Doherty, and B. 

 Richardson. 



LITERATURE CITED 



ALDRICH, D. V, C. E. WOOD, AND K. N. BAXTER. 



1968. An ecological interpretation of low temperature re- 

 sponses in Penaeus aztecus and P. setiferus postlar- 

 vae. Bull. Mar. Sci. 18:61-71. 

 ALLEN, D. M., AND J. H. HUDSON. 



1970. A sled-mounted suction sampler for benthic or- 

 ganisms. U.S. Fish Wildl. Serv., Spec. Sci. Rep. Fish. 614, 

 5 p. 

 ALLEN, D. M., AND A. INGLIS. 



1958. A pushnet for quantitative sampling of shrimp in 

 shallow estuaries. Limnol. Oceanogr. 3:239-241. 



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