Stock-recruitment relationships for three major Alaskan crab stocks

Stock-recruitment (S-R) relationships have important implications for harvest strategies but are difficult to develop for crab stocks because crab lack retainable hard body parts to age them and lack of sufficient knowledge about complex crab reproductive biology also complicates estimating effective spawning biomass. To evaluate harvest strategies, we developed S-R relationships for three major crab stocks in Alaska: Bristol Bay red king crab, Paralithodes camtschaticus, Bristol Bay Tanner crab. Chionoecetes bairdi and eastern Bering Sea snow crab, Chionoecetes opilio. We used abundance and recruitment estimates from length-based models, based on growth data for recruitment age and estimated effective spawning biomass from male and female abundance, male fertilization capability, sex ratio, size and shell condition of males, molting period duration of the female population and duration of male attendance during mating. Results varied with species. For red king crab, weak recruitment was associated with extremely small spawning biomass and strong recruitment was associated with intermediate spawning biomass, suggesting possible density-dependent effects. However, the king crab recruitment trends were also consistent with patterns of decadal climate shifts. Results were equivocal and a general Ricker model fit the king crab data slightly better than an autocorrelated Ricker model. For Tanner crab, the autocorrelated Ricker model fit the data much better than the general model and most of the variability of Tanner crab recruitment can be explained by a cycle with a period of 13-14 years. For snow crab, the autocorrelated Ricker model generally fit the data much better than the general model and S-R observations formed a circular pattern. Despite the weak evidence for S-R relationships, harvest strategies must be precautionary to reduce the risk that crab stocks fall to levels so low that reproductive output is insufficient to produce large year classes when environmental conditions become favorable. (C) 2003 Elsevier B.V. All rights reserved.