Sample size requirements for estimating the mean length of penaeid shrimp within a trawl sample

The Texas Parks and Wildlife Department collected 34 samples along the central Texas coast during June and July 1982 as part of the Southeast Area Monitoring and Assessment Program (SEAMAP). An analysis was conducted on 24 of these samples with catches greater than 25/tow to determine the optimum number of penaeid shrimp to measure to determine differences in mean length with a known precision. Sample mean lengths ranged from 96 to 134 mm total length and coefficients of variation ranged from 8.3 to 22.3%. There were significant differences among the sample mean lengths (Fs = 167.38; P less than 0.001). There were no significant correlations between sample mean lengths and variances, or numbers caught and variances (P greater than or equal to 0.05). Sample sizes required to detect a 10% difference among means ranged from 24 to 52 with a CV of 12% and 75 to 140 measurements for a CV of 23.3%. These analyses should help define the variability associated with the 1982 SEAMAP shrimp data and suggests that measuring 50 shrimp should be adequate to determine the mean length of Penaeus aztecus in a sample off Texas during June- July.