Science Advisory Report 2017/036
Stock assessment of Witch Flounder (Glyptocephalus cynoglossus) in the Gulf of St. Lawrence (NAFO Divs. 4RST) to 2016
- During 2013 to 2016, the total allowable catch (TAC) of Witch Flounder in the Northwest Atlantic Fisheries Organization (NAFO) Div. 4RST was set at 300 t. Landings in 2013 to 2016 were 250 t to 296 t. Preliminary landings for 2016 were 263 t.
- The fishery for Witch Flounder is now primarily a directed fishery, with most of the catch taken by seines in southwest Newfoundland (NAFO Div. 4R) and northwestern Cape Breton Island (NAFO Div. 4T).
- There has been a contraction in the size composition of Witch Flounder in the landings. Fish 40 cm or longer made up 70% to 80% of the landings in the late 1970s but only 20% of the landings in 2006 to 2011. The proportion of these large fish has increased in recent years, accounting for up to 23% of the landings.
- Based on research vessel surveys, the abundance of Witch Flounder measuring 30 cm and greater has increased throughout the GSL where it was historically found, including the Estuary, western Newfoundland, and around Anticosti Island.
- Reference points for spawning stock biomass of Witch Flounder >= 30 cm total length were derived from a surplus production model. The Limit Reference Point (LRP), defined as 40% of biomass for maximum sustainable yield (Bmsy), is estimated at 10,480 t, the Upper Stock Reference default of 80% Bmsy at 20,960 t, and the maximum removal rate equivalent to Fmsy at 0.072.
- DFO (2012) indicated that a strong pulse of recruitment evident in the 2009 to 2011 survey data was approaching commercial sizes and that protecting this incoming fishery recruitment by keeping catches as low as possible for the following decade could promote rebuilding of the 40+ cm size group. The imposition of a TAC of 300 t for the 2013 to 2016 fishery years seems to have contributed to the increased biomass of the stock as anticipated.
- The 2016 median estimate of the spawning stock biomass (SSB) is 13,270 t, slightly above the LRP (10,480 t) with a 38% chance that the estimated biomass is at or below the LRP. The fishing removal rate was estimated at < 0.04, below the maximum removal rate.
- Projections of stock biomass after fishing for 2017 to 2021 indicate that the biomass is expected to increase at annual catch options to 500 t. The probability of the biomass being below the LRP in 2021 is 23% even in the absence of fishing and increases to 29% under a TAC of 500 t. The probability that the stock biomass will be in the healthy zone in 2021 is 46% with no fishery removals and 37% at an annual catch of 500 t.
- An interim year update will be provided mid-way in the five-year assessment cycle in early December 2019, to determine if the indicator signals that a re-assessment is warranted. The trigger for a re-assessment will be if the three-year running average of the biomass index from the combined RV surveys in NAFO Div. 4RST falls below the Limit Reference Point, rescaled to the biomass index.
This Science Advisory Report is from the March 2, 2017 regional peer review meeting on the Stock status and fishery advice for May 2017 to May 2022 for Witch Flounder (Glyptocephalus cynoglossus) from NAFO Divisions 4RST, Gulf of St. Lawrence. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.
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