Growth of juvenile red king crab (Paralithodes camtschaticus) in Bristol Bay (Alaska) elucidated from field sampling and analysis of trawl survey data

Abstract

An analysis of in situ growth rate was conducted for juvenile red king crab (Paralithodes camtschaticus) in Bristol Bay, Alaska.  Growth of early juveniles (~2-40 mm carapace length (CL); ages 0-3 yr) was determined by fitting seasonalized Gompertz growth models to length-frequency data.  The parameters of the growth model and resulting size-at-age estimates were compared with those from studies conducted at Unalaska and Kodiak Islands by fitting the same growth model to published length-frequency data from separate sources.  Growth of late juvenile and early reproductive crabs, ~30-100 mm CL, was examined by analyzing length-frequency data from the National Marine Fisheries Service annual Bering Sea trawl survey from 1975 through 1999.  Mean CL associated with strong size modes of crabs in Bristol Bay length-frequency distributions was resolved using the FiSAT software package (FAO-ICLARM Stock Assessment Tools) to track the modal size progression of strong year classes and assign mean size-at-age characteristics to the stock

Growth of early juvenile crabs was slower in Bristol Bay than that observed by other researchers at Unalaska or Kodiak.  Sizes at 1, 2 and 3 years after settlement were estimated to be ~9 mm, 23 mm, and 47 mm CL in Bristol Bay compared with ~16 mm, 38 mm, and 66 mm at Unalaska; at Kodiak, estimated sizes of 12 mm and 42 mm were obtained for age-1 and age-2 crabs, respectively.  Within the Bristol Bay trawl survey data, a total of 24 modes were identified for both males and females <~100 mm CL, which included the modal progression of two year classes that presumably settled in 1976 and 1990.  The 1976 year class grew slowly and would not have recruited to the reproductive stock until ~9 years after settlement, whereas the 1990 year class appeared to recruit at ~8 years after settlement.  Both estimates indicate that Bristol Bay red king crabs are older at reproductive maturity than the ~6 years after settlement presently assumed.  An attempt to resolve discrete mean size-at-age from length-frequency data met with little success because variability in growth between year classes markedly obscured size-at-age characteristics in the stock.