Science Advisory Report  2012/016

Assessment of winter flounder (Pseudopleuronectes americanus) in the southern Gulf of St. Lawrence (NAFO Div. 4T)

Summary

• Winter flounder annual landings in NAFO 4T reached over 3,000 tonnes in the 1960s, but varied widely due to unreliable catch statistics up to the mid-1990s. Landings declined since the early 1990s to less than 200 tonnes in 2007 and 2008. The decline in landings is partly attributed to reduced fishing effort, brought on by market conditions and the cost of fishing. A TAC of 1,000 tonnes has been in effect since 1996.
• Approximately 300 tonnes of winter flounder were caught in 2010 and 2011. Fishing effort and catches have concentrated around the Magdalen Islands where a lack of alternative sources of bait for the lobster fishery has led to a high demand for winter flounder.
• Catch rates in a survey conducted yearly since 1971 indicate that the number and biomass of 4T winter flounder were highest before the mid-1980s. The stock declined and over the past two decades, the abundance index has varied without a clear trend near the long-term average. The survey biomass index in the 2011 survey was the lowest on record.
• The size composition of the winter flounder stock is represented by progressively fewer large fish and a growing proportion of small winter flounder <20 cm in length. The shift towards smaller fish size has been accompanied by a reduction in the size and weight-at-age.
• Total mortality (Z) was estimated directly from survey data for winter flounder aged 7 years and over. Z was over 1.0 for most of the 1970s and 1980s and less than 1.0 since the 1990s. The last estimate of Z was 0.56, in the early 2000s.
• Catch rates in the mobile gear Sentinel survey of 4T indicate that the abundance and biomass of winter flounder have declined since 2003. A trawl survey of Northumberland Strait, conducted yearly from 2000 to 2009, provided no clear trend in winter flounder abundance or biomass, but suggests that the strait may be subject to periodic influxes of young winter flounder.
• An age-structured population model indicates that the spawning stock biomass (SSB) of 4T winter flounder has peaked in regular intervals, but with an overall decline since 1973. The age composition of the SSB became dominated by young, 3 to 5-year-old winter flounder after the early 1980s. Recruitment at age-3 increased sharply in the 1980s to a peak in the 1990s and early 2000s, but has been in a declining trend in the 2000s.
• Model estimates of fishing mortality (F) fluctuated due to the landings statistics. F was concentrated on winter flounder aged 6 years and over and increased continuously to a peak of 0.06 in the mid-1980s.
• Natural mortality (M) on 3 to 8-year-old winter flounder increased from the early 1970s to the early 1990s and has remained high (>1.0). M on fish age-9+ years was above 1.5 in the mid to late 1970s and then declined to levels near 0.2 in the early 2000s. While it is possible that estimates of M may be confounded with catch misreporting, particularly of older fish, grey seals are an important predator of winter flounder and may be a contributing factor to the increasing trend in M.
• Natural mortality is driving the dynamics of the 4T winter flounder stock. The stock has also undergone a loss of productivity due to changes in the size composition and declining size-at-age. These factors, combined with declining SSB and recruitment, paint a pessimistic outlook for an increase in stock abundance in the short term.

This Science Advisory Report is from the regional advisory meeting of February 23, 2012 on the Assessment of stock status of winter flounder (Pseudopleuronectes americanus) from the southern Gulf of St. Lawrence (NAFO Div. 4T). Additional publications from this process will be posted as they become available on the Fisheries and Oceans Canada Science Advisory Schedule.

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