Science Advisory Report 2015/012
2014 Assessment of Atlantic Halibut on the Scotian Shelf and Southern Grand Banks (NAFO Divisions 3NOPs4VWX5ZC)
Summary
- Catch rates in 4VWX DFO summer RV survey provide an index of abundance of small halibut (30-70 cm) that have not yet recruited to the fishery. The catch rates increased between 2000 and 2011. The catch rates between 2012 and 2014 remain well above the long-term (1970-2013) mean, indicating a period of high recruitment.
- The catch rate of the industry-DFO halibut longline survey provides an index of exploitable biomass throughout the management unit. The catch rate has increased since 2004, with the 4 highest catch rates in the time series observed between 2011 and 2014.
- A new statistical catch at length (SCAL) model was used to assess the stock status of Atlantic Halibut and the impact of the fishery on biomass/population trends. SCAL model estimates of spawning stock biomass (SSB) between 1970 and 2013 indicate that the halibut stock has increased from the depleted state of the early 1990s to the present. The spawning stock biomass in 2013 is estimated to be 6,668 (SE=234) mt; the highest in the time series.
- SCAL model estimates of the legal-sized (greater than 81 cm since 1994) exploitation rate suggest that there were short periods of intense exploitation in the 1970s and early-1990s; current exploitation rates are the lowest on record and are below the natural mortality rate (M=0.14) estimated from a multi-year mark-recapture model.
- Fishing mortality rates estimated from the multi-year mark-recapture model have also declined between 2007 and 2013, and have been lower than natural mortality since 2008.
- The stock-recruit relationship for halibut could not be well described by the more commonly used models; therefore, interim reference points were chosen. The limit reference point (B_lim) was defined as the minimum SSB in the time series (1982-2013) that produced 50% of the maximum recruitment and the upper stock reference point (B_upper) was defined as the highest SSB in the time series. Using the SCAL model, B_lim was estimated to be 2,600 mt and BC was estimated to be 6,668 mt.
- In general, fixed TAC strategies increased the probability of falling below the reference points by 2045 with the same level of catch. Forecasting model (HAL) simulations indicated that F0.1, F0.125, F0.14, F0.15, F0.2 harvest strategies have 0.99, 0.67, 0.37, 0.25, and 0.00 probability of being above BC, and 0.00, 0.00, 0.01, 0.04, and 0.72 probability of falling below B_lim by 2045. Higher F strategies (F0.14, F0.15 and F0.2) resulted in higher catches in the short term (2014-2024) before declining in the medium (2025-2035) and long term (2035-2045), whereas with the F0.1 strategy the short-term increase in catch is smaller, but the projected catch is higher in the medium and longer term.
- HAL model simulation of the release of live halibut >125 pounds (167 cm), assuming fecundity is proportional to biomass, did not improve stock performance with either constant F or constant TAC, and in some cases led to increased probability of falling below BC. There was also no indication that increasing the minimum legal size to 85 cm would impact stock performance as measured by the probability of falling below reference points or projected catch.
- Observer coverage is variable geographically and seasonally and is not well matched to the spatial and temporal distribution of the fishery. Estimates by quarter and NAFO area range from 0 to over 100% of landings (by weight) observed. Variable observer coverage contributes to uncertainty in the length composition of the fishery (a major input to the assessment model) and in the amount and species composition of the bycatch which is estimated from observed sets.
- Between 2009 and 2013, 74 species of fish, invertebrates and birds were observed in the catch of the halibut-directed longline fishery. The primary catch is Atlantic Halibut, followed by White Hake, Atlantic Cod, Cusk, and Barndoor, White and Thorny skate, all species of wolffish, and all species of dogfish. Some of these species are of commercial value in some NAFO areas and not others; and some are species of conservation concern in some areas but not others.
This Science Advisory Report is from the December 8-9, 2014, Assessment Framework for Scotian Shelf and Southern Grand Banks Atlantic Halibut (Div. 3NOPs4VWX5Zc) – Part 2: Assessment. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.
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