Measurements of estuarine shrimp densities applied to catch predictions.
Zamora, G., Jr.
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New methodology was used to determine densities of brown shrimp (Penaeus aztecus), white shrimp (P. setiferus) and other species in Galveston Bay salt marsh. These density measurements permitted comparisons of abundances between habitats and realistic estimates for projections of standing stocks. The data were obtained using a cylindrical drop sampler that encloses 2.8 m of marsh bottom. The method was compared against other techniques traditionally employed in estimating shrimp abundances. In side-by-side shallow water sampling on bottoms without vegetation, a 1-m wide beam trawl, 5.5-m wide bag seine and 3.7-m wide otter trawl were 82, 33 and 17% as efficient, respectively, as the drop sampler in catching shrimp. With marsh vegetation present, the 1-m wide beam trawl was 23% as efficient as the drop sampler. Standing stock estimates extrapolated from drop sampler density measurements to 90 acres were similar to mark-and-recapture estimates covering the same area. In a small pond (172 m) with 1,200 white shrimp, the stock estimate using six drop samples was 1,166. The major disadvantage for estimating stocks is that drop sampler data reflect natural patchiness in distributions, which often results in higher variances than methods that integrate patches.