Red King Crab Mating Success, Sex Ratio, Spatial Distribution, and Abundance Estimates as Artifacts of Survey Timing in Bristol Bay, Alaska

Abstract

AbstractThe Bristol Bay, Alaska, fishery for red king crab Paralithodes camtschaticus is a male‐only fishery. A safeguard against male overfishing is the requirement that at least 8.4 million successfully mated, newly fertilized females be present on the grounds each year; otherwise no harvest is permitted. Estimation of the number of mated females in the population is complicated by the timing of the Bering Sea trawl survey, which in most years moves through Bristol Bay before red king crab spawning is complete. From 1977 through 2000, an average of 22% (range, 0–87%) of the broodstock remained unmated at the time of the May–June survey. Eighty‐nine percent of the observed annual variation in the proportion of unmated crabs was explained by water temperature and the date of the survey. Thus, the degree of mating success shown by each year's survey, although influenced by male availability, is predominantly an artifact of temperature and survey timing. Also, red king crabs tend to spawn in untrawlable nearshore waters, increasing the difficulty of obtaining unbiased estimates of abundance from sampling an open population. Because the proportion of mature females that emigrate each year to the nearshore spawning grounds is greater than that of males, the sex ratio within the sampled region of a survey conducted during spawning is biased toward males. This bias masks one of the more obvious signs of male overharvest—a male‐depauperate sex ratio. Finally, a survey conducted during spawning cannot provide an accurate mapping of the spatial distribution of the postspawning broodstock returning from inshore spawning grounds to incubate their newly fertilized eggs during the next 10–12 months. A time–temperature model indicated that delaying the Bristol Bay red king crab survey until the end of June would ameliorate the problems caused by sampling an open population engaged in spawning.