Research Document - 2015/051

Atlantic Halibut on the Scotian Shelf and Southern Grand Banks: Data review and assessment model update

By den Heyer, C.E., Hubley, B., Themelis, D., Smith, S.C., Wilson, S., and Wilson, G.

Abstract

Atlantic halibut (Hippoglossus hippoglossus) is currently the most valuable groundfish fishery in Atlantic Canada. Between 2001 and 2013, 77% of the Canadian longline fish were caught in Northwest Atlantic Fisheries Organization (NAFO) Divisions 4VWX5ZY. The Atlantic halibut fishery was unregulated until 1988, at which time a total allowable catch (TAC) was implemented for the Scotian Shelf and Southern Grand Banks management unit (3NOPs4VWX5Zc). In 1994, a minimum legal size limit of 81 centimetres was adopted. Changes in the groundfish fishery and the introduction of the minimum legal size for Atlantic halibut have changed the length and age composition of the catch. In 1998, the Industry-DFO Halibut Survey was initiated to provide a fishery-independent index of exploitable biomass throughout the management unit. Prior to 2010, science advice was provided based on the research vessel (RV) abundance indices, catch per unit effort and length composition of catch. The DFO summer RV survey in 4VWX and DFO Spring survey in 3NOPs provide fisheries-independent indices of abundance. A length-based, age-structured assessment model was accepted in 2010 and stock reference points were adopted in 2011. With support of industry, a tagging program began in 2006. An updated multiyear mark-recapture model estimates natural mortality (M) at 0.14 and fishing mortality (F), from 2007 to 2013, at 0.13, 0.19, 0.13, 0.11, 0.07, 0.12 and 0.07, respectively. The data preparation for the assessment includes a new methodology for estimating the length-weight relationship and improved data handling for the DFO-Industry Halibut Survey and Commercial Index. The DFO 4VWX summer RV survey stratified mean number of halibut per tow peaked in 2011, but remains well above the long-term mean. The DFO 3NOPs Spring RV survey stratified mean number of halibut per tow has been increasing over last 10 years, and the Industry-DFO Halibut Survey biomass index is also increasing, with 2013 being the highest in the time series. The updated assessment model shows high abundance and recruitment in the 1970s. Biomass increased as fish grew and survival was good, but recruitment dropped and was relatively low in the 1980s and 1990s. Biomass has increased steadily since 1992 and reached 20,280 metric tonnes (mt) in 2013. Recruitment has been above average since 2003 and peaked in 2008. Exploitation rates were about 0.1 for the longline and otter trawl fisheries in 1970, but rapidly increased to 0.3 or greater in the late-1980s and early-1990s. Current fishing mortality is about 0.1 for the longline fishery. In recent years, fishing pressure on females appears to have been higher than for males.

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