Framework for the assessment of Atlantic Halibut stocks on Scotian shelf and Southern Grand Banks / S.P. Cox, A. Benson, and C.E. den Heyer.: Fs70-5/2016-001E-PDF

"Atlantic halibut is the most valuable groundfish species per unit weight landed on the Atlantic coast. In recent years it has become one of the most valuable groundfish fisheries in Atlantic Canada. The 2014 framework for Atlantic halibut on the Scotian Shelf and Southern Grand Banks developed new methods for monitoring stock size and productivity, forecasting long-term performance of a range of fishing mortality (F) strategies, and evaluating performance of alternative management strategies. In this document, the authors describe: (i) a comprehensive statistical catch-at-length assessment model (SCAL) to be used in the stock assessment years, (ii) a proposed interim procedure for deriving annual total allowable catch (TAC) advice between stock assessments, and (iii) an operating model (HAL) for evaluating performance of alternative TAC interim procedures (i.e., for testing the proposed interim procedure). A Fisheries and Oceans Canada (DFO) Canadian Science Advisory Secretariat science peer review process provided opportunity for scientific peer review of data inputs, model structure, reference points, interim procedure, harvest strategies and objectives for the fishery. The 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 SSB in 2013 is estimated to be 6,668 mt (Standard Error=234 mt). Legal-sized (greater than 81 cm since 1994) exploitation rate suggests 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 (M) rate of M=0.14 estimated from a multi-year mark-recapture model. 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 (BLIM) 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 (BUPPER) was defined as the highest SSB in the time series. The assessment and operating models currently depict similar dynamics of halibut sex-specific growth, mortality, and at-sea discarding of under-sized fish in the four main fisheries. The interim procedure uses an empirical harvest control rule based on a 3-year running average of the DFO-Industry Halibut Survey and the DFO 4VWX Research Vessel survey index of recruitment. Based on HAL model simulation, fixed TAC strategies increased the probability of falling below the reference points by 2045 with the same level of catch. The higher F strategies (i.e., F=0.14, F=0.15 and F=0.2) 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 F=0.1 strategy the short-term increase in catch is smaller, but the projected catch is higher in the medium and longer term. Release of live halibut greater than 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 BUPPER. There was 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."--Abstract, p. iv.

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