Science Advisory Report 2017/022
Stock assessment of Winter Flounder (Pseudopleuronectes americanus) of the southern Gulf of St. Lawrence (NAFO Div. 4T) to 2016
- Winter Flounder is currently caught in a relatively small directed fishery concentrated mainly around the Magdalen Islands with landings ranging from 190 to 320 tonnes over the past 10 years. Preliminary landings in 2016 were 192 tonnes.
- In samples of catches in the September Research Vessel survey, there has been a decrease in the size distribution of the Winter Flounder stock with percentages of fish >= 25 cm (minimum size limit for the fishery) decreasing from an average of 85% in 1971 to 1975 to 30% during 2011 to 2015.
- Estimates of spawning stock biomass (SSB) were highest in the period from 1975 to 1994 at an average of 356,100 t (median). Over the recent period, the SSB estimate has declined to 235,700 t in 2003 and 76,270 t in 2016. The proportion of the SSB represented by older fish (5+) has also decreased over time, from 63% in the 1970s and early 1980s to 39% since 2010.
- Based on a population model, the estimates of natural mortality have recently increased for young fish (ages 2 to 4) and continually increased for older fish (aged 5+). The rates are high for ages 5+ and increasing from 0.68 at the start of the time series in 1973 to greater than 1 (63% annual mortality) recently.
- The instantaneous fishing mortality rate is estimated to be low to very low for all ages with maximum fishing rates over the series of 0.05 for ages 8+. Overall, the fishing mortality rate is very low compared to the estimated natural mortality rate.
- A limit reference point (LRP; 147.8 thousand tonnes), based on a proxy value for biomass at maximum sustainable yield (Bmsy), was defined as the average biomass over a productive period from 1973 to 1994. This period was chosen because the SSB was high and the SSB comprised an important number of older and larger fish. The stock is considered to have been below the LRP since 2006. In 2016, the estimated SSB was 54% of the LRP.
- Projections at catch levels of 0 t, 100 t, and 300 t show no perceivable difference in stock trends over the next five years. Fishing mortality is a very small proportion of the total mortality of Winter Flounder in the southern Gulf of St. Lawrence and natural mortality is estimated to be the dominant factor affecting abundance.
- The contraction in size structure of Winter Flounder, the 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 March 1, 2017 regional science peer review meeting on the Stock status and fishery advice for May 2017 to May 2022 for Winter Flounder (Pseudopleuronectes americanus) from NAFO Division 4T, southern 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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