Science Advisory Report  2010/006

Assessment of Atlantic Halibut on the Scotian Shelf and Southern Grand Banks (NAFO Divisions 3NOPs4VWX5Zc)

Summary

• Four catch rate analyses of the halibut survey show recent increases in the exploitable population of 3NOPs4VWX5Zc Atlantic halibut. Standardizing the catch rate with a general linear model (GLM) is considered the most credible analysis. When vessels effects are not accounted for, there is no trend in catch rates. When vessels effects are accounted for, there is a significant positive trend in catch rates over the past 12 years.
• Based on the catch rate analyses of the halibut survey, there appears to be stability or potential increase in the population of 3NOPs4VWX5Zc Atlantic halibut in the past 3 to 4 years.
• The catch rate in the commercial index in 4VWX is the highest since first being recorded in 1998. However, the commercial index catch rate does not show a linear trend over the survey time series.
• Recruitment has increased over the past five years in the halibut survey, has declined in the research vessel (RV) survey over the last two years, but remains above the long term mean. This recruitment is starting to show up as exploitable biomass, and the 2009 catch rates in the halibut survey are the highest on record.
• The 2008 exploitation rate of the exploitable biomass (>81cm) was estimated to be 15.0% (90% Confidence Interval: 13.3–16.8%) based on the tagging results, although this value is expected to increase as more tags are sent in. As noted in DFO (2009), this exploitation rate is higher than natural mortality (10%) and F0.1 (9%), and it is not known whether this rate is sustainable.
• The surplus production to catch ratio is expected to remain approximately the same as in 2008 (3:1), and the longer-term consequences of utilizing this ratio should be evaluated in the context of stock management objectives, reference points, and a risk management framework.
• Based on the abundance indices presented here, there is no basis to advise on a change in harvest level in 2010/2011.
• Atlantic halibut can move large distances creating some uncertainty in stock structure. Other sources of uncertainty including vessels, bait, and temperature effects on the halibut survey and commercial index, have not been fully analyzed. A lack of a population model and biological reference points make it impossible to know whether the stock is rebuilt or what is a precautionary harvest level.

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