Mesh size selectivity study of Penaeid shrimp trawled from Galveston Bay, Texas, May 1981
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The number and size of penaeid shrimp retained in trawls were found to be dependent upon the mesh size and twine size of the trawl. During the study brown shrimp (Penaeus aztecus) were smaller with mean sizes of 81.0-85.5 mm than white shrimp (P. setiferus) with mean sizes of 118.8-128.5 mm. There were no significant differences in the total number of white shrimp caught among the different trawl mesh sizes. The larger mesh sizes did result in a significantly larger white shrimp being caught. Trawls with larger mesh sizes caught significantly fewer brown shrimp than did trawls with smaller mesh sizes, however there was no significant differences among the mean sizes of shrimp caught. This suggested that when there was a wide range in the size of shrimp available the larger mesh trawl caught larger shrimp without a decrease in the total number caught (i.e. white shrimp). When the shrimp were fairly uniform in size, trawls with larger meshes resulted in smaller total catches with no differences among the mean sizes of shrimp caught (i.e. brown shrimp). Larger twines may change the effective mesh size (i.e. a trawl made with number 12 twine having a stretched mesh of 47.6 mm may catch the same number and size of shrimp as a trawl made with number 9 twine having a stretched mesh of 44.5 mm). Towing trawls for relatively short periods revealed a linear relationship between the catch (number) and tow time (a 45-min tow resulted in three times the number of shrimp caught as a 15-min tow). Tow times were not found to affect the size of the shrimp caught.