Science Advisory Report 2016/033

Stock assessment of Yellowtail flounder (Limanda ferruginea) of the southern Gulf of St. Lawrence (NAFO Div. 4T) to 2015

Summary

• Yellowtail flounder is currently caught in a relatively small directed fishery concentrated around the Magdalen Islands with landings less than 200 tonnes over the past 14 years.
• There has been a decrease in the size distribution of the Yellowtail flounder stock with percentages of fish ³ 25 cm (minimum size limit for the fishery) decreasing from an average of 90% before 1990 to 19% by 2011, with a slight increase to 41% in 2015.
• Based on a population model, natural mortality on larger and older Yellowtail is estimated to have increased from 22% annual mortality during 1985 to 1990 to 86% in 2009 to 2015. In contrast, natural mortality on small and young Yellowtail is estimated to have declined from 53% annually in 1985 to 1990 to 16% to 21% since 1997.
• Based on a population model, the spawning stock biomass (SSB) is estimated to be higher in the past decade than in the mid to late 1980s but the percentage of the SSB composed of larger and older (7+ years) fish has declined from 40% in 1985 to 1990 to less that 0.5% since 2013.
• Fishing mortality is estimated to be generally low and of such a small proportion of the estimated total mortality of Yellowtail flounder that there is no perceived difference in stock trends over the next five years at catch projections of 0 t, 100 t, and 300 t annually. SSB declined slightly over the period in all cases. At the scale of the southern Gulf of St. Lawrence, natural mortality appears to be the dominant factor affecting stock status.
• A limit reference point (B_lim = 1.06 kg per tow) was derived from the biomass index of large Yellowtail flounder (≥ 25 cm) from the September Research Vessel survey. The stock is considered to have been in the critical zone since 2006, and the index in 2015 was 61% of B_lim.
• From the population model, the SSB has not changed like the large Yellowtail index used to define the reference point. However, the SSB is now composed primarily of fish less than 25 cm (72 %). This is an important consideration as there is an assumed greater value to reproductive potential of larger animals in the population.
• The contraction in size structure of Yellowtail flounder, the large decline in the estimated size at 50% maturity, and the decline in abundance indices of the previously abundant commercial sized group are consistent with a stock experiencing very high levels of mortality.

This Science Advisory Report is from the February 29 and March 1, 2016 science peer review meeting on the Stock assessment of Yellowtail Flounder (Limanda ferruginea) of the southern Gulf of St. Lawrence (Northwest Atlantic Fisheries Organization (NAFO) Division 4T). 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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