Detecting early warnings of recruitment overfishing in male-only crab fisheries

J.M. ORENSANZ; B. ERNST; D. A. ARMSTRONG; A. M. PARMA

Anchorage, Alaska

Simposio; Assessment and Management of New and Developed Fisheries in Data-Limited Situations; 2004

Wakefield Symposium Series, University of Alaska

Male-only crab fisheries often are assumed to be relatively well protected from recruitment overfishing by virtue of male polygyny and femalesperm retention. These fisheries also offer the opportunity to directly assess the per capita female reproductive contribution. If the latter does not dwindle when a stock declines, then the decline cannot be attributedto overfishing. We explored expedient ways to assess per capita female reproductive contribution of snow crab (Chionoecetes opilio) and Tanner crab (C. bairdi) of the Eastern Bering Sea (EBS) by measuring clutch volume and estimating an objective clutch fullness index (CFI) based on modeling the maximum boundary line (MBL) of scatterplots of clutchvolume vs. body size. We investigated three estimation methods, selected quantile regression for further analysis, and examined the distribution of the CFI in extensive samples collected during trawl surveys conductedin 1992 and 1993. An objective protocol for the visual assessment of clutch size (based on an estimated MBL) was introduced in 1994, with a dramatic effect on estimated CFI. CFI distributions proved very useful for the identification of patterns in the data. Average CFI declined between1994 and 2000, then rebounded in 2001.