Research Document 2018/023

Assessment of Witch Flounder (Glyptocephalus cynoglossus) in the Gulf of St. Lawrence (NAFO Divisions 4RST), February 2017

By Ricard, D., and Swain, D.P.

Abstract

Witch Flounder (Glyptocephalus cynoglossus) are slow growing and late maturing. In the 1970s, the mean length of a 12-year-old female in the southern Gulf of St. Lawrence was only 41 cm, and the age at 50% maturity of females was 10 years. Because of these life-history characteristics, this species is particularly vulnerable to overexploitation. A fishery for Witch Flounder developed in the Gulf of St. Lawrence (NAFO Divisions 4RST) in the 1950s. Annual landings averaged over 3,500 t in the 1960s and 1970s, declining to an average of 1,800 t in the 1980s. Landings declined further in the early 1990s to a low of 320 t in 1995. Landings then increased to an average of 850 t annually in 1998-2008 but declined again in 2009-2010. While the total allowable catch remained at 1,000 t until 2011, landings in 2011 were 425 t. The total allowable catch was reduced to 500 t in 2012 and to 300 t in 2013 where it has remained since. Landings have closely matched the total allowable catch for the period 2013-2016. A research vessel (RV) survey index of commercial biomass (fish 30 cm or longer) was constructed for 4RST based on the August RV survey of the northern Gulf and the September RV survey of the southern Gulf. This index, available since 1987, declined sharply in the early 1990s, increased to an intermediate level in 1999 and 2000, but then declined again, fluctuating around 40% of the 1987-1990 for the period 2001-2010 and has shown an increasing trend since. A sentinel survey index of fish 30 cm or longer, based on the July sentinel survey of the northern Gulf and the August sentinel survey of the southern Gulf, is available since 2003. This index declined after 2006, was at its lowest level in 2011 and has shown an increasing trend since. A Bayesian surplus production model indicates a 90% decline in commercial biomass between 1961 and 2011 and an increasing trend since. The limit reference point (LRP) for this stock is estimated to be 10,480 t of commercial sized fish (30 cm and longer). The estimate of the biomass of fish 30 cm and longer in 2016 is 13,270 t. Based on the uncertainties in the estimates of both the 2016 biomass and the LRP, the probability that biomass is below the LRP in 2016 is 38%. Projections of the population model indicate that biomass is expected to increase, with probabilities of 74% that the biomass will be above the LRP in 2021 with annual catches of 300 t and 71% with annual catches of 500 t.

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