Research Document - 2012/016
Preliminary investigations into standardizing the 4T snow crab (Chionoecetes opilio) bottom-trawl survey index
By Cadigan, N.
The 4T snow crab survey has expanded since it was initiated, and total biomass estimates may not be comparable over the entire time-series because of changes in the area surveyed. In this paper a simple model is proposed to standardize the abundance and biomass time-series by extrapolating crab densities in areas not fished in some years. The model involves multiplicative mixed-effects, where year and spatial location are fixed effects, and year×spatial location are random effects. Trawl catches of crab are assumed to have a Negative Binomial distribution. The multiplicative mixed-effects model will be more appropriate when the spatial distribution of crab only changes randomly from year to year. The results indicate that 4T snow crab stock size is currently at a low level compared to most other years since 1988. The 2010 biomass estimate is the 4th lowest in the time-series, but improved compared to 2009 which was the 2nd lowest biomass estimate in the time-series.
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