DID TRAWLING ON THE BROOD STOCK CONTRIBUTE TO THE COLLAPSE OF ALASKA'S KING CRAB?

Abstract

The 1976 U.S. Magnuson‐Stevens Fishery Conservation and Management Act effectively eliminated the no‐trawl zone known as the Bristol Bay Pot Sanctuary, located in the southeastern Bering Sea, Alaska. Implemented by the Japanese in 1959, the boundaries of the Pot Sanctuary closely matched the well‐defined distribution of the red king crab (Paralithodes camtschaticus) population's mature‐female brood stock, thus affording a measure of protection to the reproductive potential of the stock. In 1980, the point at which the commercial harvest of Bristol Bay legal‐male red king crab reached an all‐time high after a decade‐long increase, domestic bottom trawling in the brood‐stock sanctuary began in earnest with the advent of a U.S.–Soviet, joint‐venture, yellowfin sole fishery. In the first year of trawling in the Pot Sanctuary, the Bering Sea/Aleutian Islands (BSAI) red king crab bycatch increased by 371% over the 1977–1979 average; in 1981 the BSAI bycatch increased another 235% over that in 1980, most of which were mature females. As the number of unmonitored domestic trawls in the brood‐stock area increased rapidly after 1979 and anecdotal reports of “red bags” (trawl cod‐ends plugged with red king crab) began to circulate, the proportion of males in the mature population (0.25 in 1981 and 0.16 in 1982) jumped to 0.54 in 1985 and 0.65 in 1986. It is unlikely that normal demographics caused this sudden reversal in sex ratio. Our hypothesis is that sequential, sex‐specific sources of fishing mortality were at work. Initially there were ten years (1970–1980) of increasing, male‐only exploitation in the directed pot fishery, followed by a drastic reduction in the male harvest after 1980 (to zero in 1983). Then, beginning around 1980, there was an increase in bottom trawling among the highly aggregated, sexually mature female brood stock concentrated near the western end of the Alaska Peninsula, an area documented by previous investigators to be the most productive spawning, incubation, and hatching ground for Bristol Bay red king crab. There has been considerable discussion about possible natural causes (e.g., meteorological regime shifts, increased groundfish predation, epizootic diseases) of the abrupt collapse of the Bristol Bay red king crab population in the early 1980s. The purpose of our study was to conduct a rigorous examination of existing data in order to evaluate the relative likelihood that the collapse was caused by human fishing instead of natural mortality. Our discussion focuses on the association between record harvests of male crab in the directed fishery, the onset of large‐scale commercial trawling within the population's primary reproductive refuge, and the population's collapse.