obtained by mark recapture and an alternative drop 

 sampler method, and discuss previously unreported 

 results of 1970-71 studies (Welker and Baxter^). 



Methods 



Sydnor Bayou is a shallow coastal tidal pond in 

 Galveston Bay (Fig. 1). The site was chosen because 

 the single narrow entrance could be blocked easily 

 with netting, thus preventing immigration and 

 emigration of shrimp during the experiment, and 

 because Sydnor Bayou was the site of a similar study 

 in 1970. 



The pond covers 32.4 to 36.4 ha, depending on the 

 tide, is about 0.9 km long and 0.2 km at its widest 

 point, narrowing to 6 m at the mouth. Maximum 

 depth is about 1.5 m at high tide, with a 0.25 m tidal 

 difference Average salinity during the marking was 

 20.5 ppt and mean surface temperature was 28°C. 



Weekly sampling of Sydnor Bayou with a 3 m otter 

 trawl (25 mm stretched mesh) began 25 April 1983 

 to monitor the size of the juvenile shrimp. By 23 May 

 1983, most shrimp caught in the trawl were larger 

 than the 40 mm TL (total length) minimum needed 

 for tagging, and we decided to begin the mark- 

 recapture experiment the next week. 



Sydnor Bayou was blocked at dawn on 31 May 1983 

 across bridge B-1 (Fig. 1) with a 45.7 m net having 

 a 6 mm mesh. The net was anchored to the bottom 

 and remained in place for the duration of the 

 experiment. 



A 1.8 m diameter, 0.8 m deep round tank with con- 

 tinuous water flow and two 147 L aerated ice chests 

 were set up on shore to hold shrimp during the mark- 

 ing process. Shrimp were caught in 49 5-min trawl 

 hauls and transported to the marking site in aerated 

 45 L ice chests. lb minimize marking mortality, only 

 shrimp 40 mm TL and larger were marked and held 

 in the large tank. Marking was accomplished by in- 

 jection with pink fluorescent pigment as described 

 by Klima (1965). Representative length -frequency, 

 species-composition, and sex ratio information was 

 obtained from shrimp captured in one trawl haul. 



Marked shrimp were released within the hour 

 after the target number (4,100) had been marked. 

 Shrimp were scattered along the shallow grassy 

 shoreline from moving skiffs. No dead or moribund 

 shrimp were released, and release operations ceased 

 whenever shore birds were attracted. 



Four 61 cm x 61 cm x 20 cm wire cages, each 



SYDNOR BAYOU 

 36.4 HA 



CORPUS 

 CHRIS 



COW TRAP 1 AND 2 

 27.5 AND 2.8 HA 



MUD LAKE 

 6.4 HA 



Figure 1.— Tfexas ponds selected for brown shrimp mark-recapture 

 studies: Sydnor Bayou (1970 and 1983); Cowtrap, Mushroom, Caran- 

 cahua, and Mud Lake (1971). 



containing 25 marked and 25 unmarked shrimp, 

 were set out in the pond and remained submerged 

 through all tidal stages. After 24 h, cages were raised 

 and all shrimp, dead and alive, were counted, 

 measured, and recorded for an estimate of marking 

 mortality. 



Recapture trawling began 18 h later, allowing 

 marked shrimp time to distribute themselves in the 

 unmarked population. For three consecutive days, 

 all trawlable bottom was sampled by 5-min trawl 

 hauls. Shrimp were returned to the laboratory where 

 marked shrimp were identified under ultraviolet 

 light. All marked and up to 100 unmarked recoveries 

 were measured per tow. Length-frequency distribu- 

 tions for releases, marked recoveries, and unmark- 

 ed recoveries are shown in Figure 2. 



We estimated an initial population of juvenile 

 brown shrimp using Bailey's (1951) modification of 

 the Petersen formula 



^Welker, W., and K. N. Baxter. Juvenile brown shrimp popula- 

 tion estimates in Itexas tidal marsh ponds. Unpubl. manuscr., 8 p. 

 Southeast Fisheries Center Galveston Laboratory, National Marine 



Fisheries Service, NOAA, 4700 Avenue U, Galveston, TX 77550. 



678 



