Science Advisory Report  2012/017

Assessment of witch flounder (Glyptocephalus cynoglossus) in the Gulf of St. Lawrence (NAFO Div. 4RST)

Summary

• In 2011-2012, the total allowable catch (TAC) remained at 1,000 t. Landings in 2011 were 442 t (318 t in 4R and 124 t in 4T).
• The fishery for witch flounder is now primarily a directed fishery, with most of the catch taken by seines. Fishing effort by seines dropped sharply in 2010.
• Size distribution in the landings has contracted, the proportion of large fish (40+ cm) in the landings decreased from 67-80% in the 1970s to 10% in 2011.
• Witch flounder are slow growing and late maturing. Because of this low productivity, they are vulnerable to overexploitation.
• The 4RST research vessel (RV) survey index of commercial biomass (30+ cm), available since 1987, declined sharply in the early 1990s and remains at a relatively low level.
• The 4RST sentinel survey index of the biomass of fish 30 cm and longer, available since 2003, was at its lowest level in 2011.
• Size distribution in the survey catches has also contracted over time. The proportion of fish 40 cm and longer in the catches of the 4T RV survey, available since 1971, declined from 73% in the 1970s to 14% in the 2000s. Fish 40 cm and longer comprised 21% of the 4RST survey catches in 1987-1990 but only 4.5% of the catches in 2005-2011.
• Recruitment has been relatively strong in the 1990s and 2000s but has not resulted in improved abundance of 40+ cm fish, consistent with high mortality at commercial sizes.
• Population models indicate a 90% decline in commercial biomass since 1961.
• The limit reference point (LRP) for this stock, set at 40% of the biomass producing the maximum sustainable yield (B_MSY), is estimated to be 10,700 t of 30+ cm fish.
• The estimate of the biomass of fish 30 cm and longer in 2011 is 5,000 t, about half the LRP. Based on the uncertainties in the estimates of both the 2011 biomass and the LRP, the probability that biomass is below the LRP in 2011 is 93%.
• The maximum removal reference in the healthy zone (the exploitation rate at MSY) is estimated to be 0.07 for this stock. The estimated exploitation rate in 2011 was 0.09.
• Biomass is expected to increase with annual catches of 300 t and decrease with catches of 850 t (the average level in 1998-2008). However, even with no catch, there is a 62% probability that biomass will remain below the LRP in five years.
• A strong year-class is now approaching commercial sizes. Protecting this year-class by keeping catches as low as possible for the next decade may promote rebuilding of the 40+ cm size group.

This Science Advisory Report is from the regional peer review meeting of February 22, 2012 on the Assessment of stock status of witch flounder (Glyptocephalus cynoglossus) from the Gulf of St. Lawrence, NAFO Divisions 4RST. Additional publications from this process will be posted as they become available on the Fisheries and Oceans Canada Science Advisory Schedule.

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