Research Document - 2008/028

Recovery Potential Assessment of 4VWX Cusk (Brosme brosme): Population models

By T.D. Davies and I.D. Jonsen

Abstract

In May 2003, cusk (Brosme brosme) was designated as Threatened by the Committee on the Status of Endangered Wildlife in Canada. In this report, we use Bayesian state-space implementations of the Schaefer surplus production and stochastic exponential growth models to estimate population parameters of cusk in the 4X and combined 4VWX NAFO divisions. Models were fit using either single or multiple datasets simultaneously of the following: the groundfish longline fishery CPUE index spanning 1986 to 2007 in 4Xnopqu; the standardized research vessel summer trawl survey conducted by DFO from 1970 to 2007; two industry/science surveys, the 4VsW sentinel survey spanning 1995 to 2003 and yearly sampled stations in the 3NOPs4VWX halibut survey from 1998 to 2007; and the Northeast Fisheries Science Center autumn bottom trawl survey in the Gulf of Maine from 1963 to 2007.

All models identified significant reductions in cusk exploitable biomass, although uncertainty in historical exploitable biomass estimates was large. The 4Xnopqu longline CPUE index, which is thought to most closely meet the assumption of proportionality to overall abundance estimated a 62% decline since 1986, however, parameter estimates had large credible limits. The model using the summer RV survey index estimated a decline of 94% since 1970; however, there is concern that some hyper-depletion may be occurring in this index giving over-estimates of total decline. The GoM autumn trawl survey estimated a similar decline, 86% since 1964, but may have similar proportionality issues as the RV survey index. The sentinel and halibut surveys were of insufficient duration and contrast to estimate declines in a useful historical context.

Stochastic simulation using median parameter values derived from the model using the 4Xnopqu longline data estimated the required reductions in landings needed to give a 75% chance of meeting three different management outcomes: no further decline in biomass, at least a 50% increase in biomass, or, at least a 100% increase in biomass after 15 years in the 4X NAFO division. Assuming that unquantified fishing mortality remains constant, landings in 4X would need to be reduced to 625 t or 200 t to provide a 75% chance of the population either remaining stable or increasing 50% after 15 year, respectively.

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