The effects of minimum size limit regulations on exploited fish populations off the Atlantic coast of the southeastern United States.

摘要

The commercial snapper-grouper fishery off the Atlantic coast of the southeastern United States contains many exploited species that are managed with species-specific regulations, though primarily managed with minimum size limits. Minimum size limits vary by species and are intended to protect small fish in order to restrict harvest and prevent growth overfishing. Disadvantages of minimum size limits may include sublegal-size bycatch of co-occurring species, release mortality for undersize fish, and differential size-selective mortality that removes faster growing fish. Effects of size limits were investigated utilizing catches from a commercial fisherman and an individual-based model. The entire catch of a commercial fisherman was characterized by species, length and release condition for fishing trips between June and November 2004. While overall discard percentages were low (13%), release mortality was high (61%). For three exploited species (red porgy, scamp, and red snapper), the high discard proportions in association with high release mortality indicated that current regulations may not be protecting these stocks as expected. From July 2005 through September 2007, both legal and sublegal portions of a commercial fisherman's catch for three species with different exploitation levels (vermilion snapper, red porgy and scamp) were retained for life history analyses. Length, age, sex and reproductive state were compared for the legal and sublegal portions of the population. Indications of differential size-selective mortality were apparent in the vermilion snapper and red porgy stock. For these species, the commercial catch included many young, large fish whereas discarded fish had similar age ranges and mean ages, but smaller sizes at age than the landed fish. For scamp, there were no signs of differential mortality due to size-selective fishing. The legal-size catch consisted only of old, large fish. The sublegal-size fish had little overlap in age range, significantly different mean age, and only small differences in size-at-age when compared to legal-size fish. An individual-based model was constructed in C++ to investigate the effect of two management regulations (size limits and quotas) on the life history parameters of the exploited vermilion snapper stock. The model ran simulations under 5 different fishing pressures for eight minimum size limits and six quota limits and compared population size, yield, von Bertalanffy growth parameters K and L-infinity, age at first maturation and probability of 50% maturation at length. Additional model simulations investigated the effects of changes in discard mortality, inheritance of growth and maturation traits, and variation in growth parameters. The final model simulation investigated the ability of a stock to recover to previous parameter values after the cessation of fishing. The results of these studies show more information on catch composition, discard proportion, release mortality and the effects of size limits on population parameters would assist fishery managers in effectively managing exploited stocks. Many of these results have already being incorporated in recent stock assessments. For many species, size limits may not be the most effective management tool. Species specific or mixed-species quotas may be a more appropriate alternative.