Sample Size Requirements for Detecting Length Differences of White Shrimp

Subsampling to obtain length measurements is often necessary when large numbers of organisms are captured with bag seines in a resource monitoring program. Catches of white shrimp were used to examine the representativeness of the subsamples and to construct sample size selection curves for determining the number of length measurements required to detect a given percentage difference between samples. A wide size range of white shrimp (Penaeus setiferus) is often captured in the same sample. Subsampling to measure 19 white shrimp from Texas Parks and Wildlife Department (TPWD) bag seine collections to estimate mean lengths was not always completely random but systematic discrepancies were not evident. A subsample of 19 shrimp can be used to detect a 15% difference in mean lengths among collections 80% of the time (a = 0.01) with a CY of 13.3%, and in the worst case (CY = 37.9%), can detect a 50% difference in mean length 80% of the time (u = 0.01). There was no significant correlation between shrimp mean length and variance, nor between number caught and variance (P ≥ 0.05)

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