Research Document - 2011/002

A Framework for the Assessment of the Scotian Shelf and Southern Grand Banks Atlantic Halibut Stock

By M.K. Trzcinski, S.L. Armsworthy, S. Wilson, R.K. Mohn, and S.E. Campana

Abstract

The literature and data on Atlantic halibut was reviewed with the primary goal of supporting the first quantitative age-structured assessment of the Scotian Shelf and southern Grand Banks Atlantic halibut stock. The Canadian and foreign landings back to 1970 were used in the assessment. The catch at length was estimated using observer and port sampling data from 1988 for the longline fishery and from 1984 for the otter trawl fishery. The long-term average discard rate was used to estimate the total catch back to 1970. The total catch in the longline fishery was further divided into males and females based on observer data. A length-based, age-structured model was fitted to the length compositions in the catch and to the catch rate and length composition of halibut caught in the Scotia-Fundy groundfish Research Vessel survey (1970-2009) and halibut survey (1998-2009).

Simulation testing, model fit and model comparisons indicate that the catch at length (CAL) model was good for estimating stock size and the impact of the fishery. Model estimates indicate a high population biomass and recruitment in the 1970s, increased biomass, but poor recruitment in the 1980s, low biomass and recruitment in the 1990s, and increasing biomass and recruitment in the 2000s. The spawning stock biomass in 2009 was estimated at 6527t (2592t females only). Exploitation rates were about 0.2 for the longline and otter trawl fisheries in 1970, but rapidly increased to 0.4 or greater in the late 1980s and early 90s as the population decreased. Current fishing mortality is about 0.2 for the longline fishery, but in recent years there was increased pressure on females with fishing mortality of 0.30 compared to 0.14 for males in 2009. Fishing mortality from the otter trawl fishery has been low since the mid 1990s and in 2009 was estimated at 0.02. Candidate biological reference points were estimated from the catch at length (CAL) and virtual population analysis (VPA) models using the approach outline by Sissenwine-Shepherd (1987). Both assessment models gave similar results (BMSY=4900t, FMSY=0.36). Based on assessment model results, 3NOPs4VWX5Zc Atlantic halibut population is in a productive period due to high recruitment. The SSB is estimated to be in the healthy zone; above the BMSY. Current fishing mortality (0.2) is well below FMSY (0.36). Although catch projections were not made, recent high recruitment would be expected to result in higher SSB at the current fishing mortality rate in the near term. The proposed framework for assessing Atlantic halibut takes advantage of length frequency and catch rate data from multiple datasets and is both robust and effective in estimating stock status, the impact of the fishery, and the provision of management advice.

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