Minimum Sizes for Red Grouper: Consequences of Considering Variable Size at Age

Abstract

The influence of variation in length at age on estimates of yield per recruit (YPR) for alternative minimum sizes was evaluated for the U.S. Gulf of Mexico stock of red grouper Epinephelus morio. The YPR and biological reference points, F _0.1 (the value of F at which the slope of the yield-per-recruit curve is 10% of the slope at the origin) and F _MAX (fishing mortality that maximizes yield per recruit) were influenced by both the minimum size and the treatment of variability in length at age. The importance of including variation in length at age increased with increasing minimum size. Despite this result, maximum YPR for the existing growth model occurred with a minimum size of 50 cm total length regardless of whether variable growth was included or not. However, large minimum sizes cause the fishery to harvest the faster-growing members of each year-class, and this effect was most profound for the 50-cm minimum size. To the extent that growth is heritable, this process can induce strong genetic selection for slow growth that may significantly reduce future productivity of the stock.