Research Document - 2012/122
Assessment of witch flounder (Glyptocephalus cynoglossus) in the Gulf of St. Lawrence (NAFO Divisions 4RST), February 2012
By D.P. Swain, L. Savoie, and É. Aubry
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 again declined recently. In the 2011-2012 fishing season, the total allowable catch remained at 1,000 t. Landings in 2011 were 442 t (318 t in 4R and 124 t in 4T). The proportion of large fish (40 cm or greater in length) in the landings declined from 67-80% in the 1970s to 10% in 2011. 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 level since 2001. 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 and was at its lowest level in 2011. The proportion of large fish in the RV survey catches has declined sharply. In catches by the September RV survey, the proportion of fish 40 cm or longer declined from 86% in the late 1970s to 8% in 2006-2011. Recruitment has been relatively strong in the 1990s and 2000s but has not resulted in improved abundance of 40+ cm fish. RV survey catches indicate that a strong year-class is again approaching 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 (BMSY), is estimated to be 10,700 t of commercial-sized fish (30 cm or longer). The estimate of the biomass of fish 30 cm and longer in 2011 is 5,000 t, less than 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. Projections of the population model indicate that 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.
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