Science Advisory Report 2021/022

Stock Assessment of Yellowtail Flounder (Limanda ferruginea) of the southern Gulf of St. Lawrence (NAFO Div. 4T) to 2020

Summary

• Yellowtail Flounder is currently caught in a relatively small directed fishery concentrated around the Magdalen Islands with landings averaging 120 tonnes (t) over the past 10 years.
• Yellowtail Flounder ⋝ 25 cm (minimum size limit for the fishery) comprised 80% of the observed commercial catch throughout the 1970s and 1980s declining to 20.5% since 2011.
• Based on the research vessel (RV) survey, the abundance of fish < 25 cm in length increased 10-fold from 1985 to 2013 while the abundance of larger fish declined by 94% from 1981 to 2011 and has remained at a very low level. This suggests that mortality is high for larger fish and low for smaller fish.
• Based on a population model of the 4T stock, natural mortality of larger and older Yellowtail Flounder increased from 21% annually in 1985-1990 to 86% or more annually since 2009. In contrast, estimated natural mortality of small and young Yellowtail Flounder has remained below 53% annually from 1985 to 2020.
• Similar changes in natural mortality have occurred in many fish species in the southern Gulf of St. Lawrence (sGSL). There is strong evidence that predation by grey seals is an important cause of the exceptionally high natural mortality experienced by larger and older individuals of these species.
• Estimated spawning stock biomass (SSB) has declined by 50% from its peak observed value in the early 2000s. In addition, the composition of the SSB has changed. Fish aged 7 years and older have declined from 30% of the SSB to less than 0.1%. Fish 4 years and younger now comprise 90% of the SSB.
• Fishing mortality (F) is estimated to be very low for ages 6, and younger. The estimates of fishing mortality correspond to fishing rates of less than 1% for ages 2 and 4 and less than 6% for age 6. For age 8+, fishing rates were less than 12%.
• A limit reference point (LRP) was derived based on the estimated September biomass of large Yellowtail Flounder (⋝ 25 cm). The LRP was estimated to be 5,710 t. The stock is considered to have been in the critical zone since 2009, and the index in 2020 was 39% of the LRP.
• 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.
• The population was projected forward 10 years assuming recent productivity conditions would persist. The probability that the stock would remain below the LRP was estimated to be 100% in all years of the projection and at all three catch levels examined (0, 100 and 300 t). The estimated biomass at the end of the projection was 22% (0-100 t) or 20% (300 t) of the LRP.
• At current levels of productivity, 10-year projections indicate SSB declines of 43.7%, 44.0% or 45% with catches of 0 t, 100 t or 300 t. Fishing mortality is estimated to be such a small portion of total mortality that differences in projection trajectories are negligible between these levels of catch.

This Science Advisory Report is from the February 16, 2021 science peer review meeting on the stock assessment of Yellowtail Flounder (Limanda ferruginea) of the southern Gulf of St. Lawrence (NAFO Div. 4T). Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2021/022 (PDF, 6,441 KB)

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