Science Advisory Report 2019/021
Assessment of Atlantic Cod (Gadus morhua) in the southern Gulf of St. Lawrence (NAFO Div. 4T-4Vn (Nov. – April)) to 2018
- The cod-directed fishery has been closed since 2009, with a 300 t TAC in place to cover bycatch in other groundfish fisheries, Indigenous food, social and ceremonial fisheries, a limited recreational fishery, and scientific purposes. Preliminary landings for 2017 and 2018 are about 60 t.
- Since 2009, the exploitation rate has averaged 0.2% for ages 5-8 and 0.7% for ages 9 plus. These low levels have a negligible impact on the population trajectory.
- The biomass index for commercial-sized cod (≥ 42 cm) from the annual DFO research vessel survey was at the lowest level observed in the 48-year record in 2017 and 2018. The 2017 and 2018 indices averaged 8% of the already low values in 1995-2002, and 2.5% of the average biomass in the 1980s. Similar trends were observed for both sentinel survey indices.
- Estimated spawning stock biomass (SSB) declined steadily between 1997 and 2018.SSB at the beginning of 2018 was estimated at 13,900 t, the lowest level in the 69-year record, at 4% of the high biomass levels in the 1980s.
- The SSB in 2018 is estimated to be 17% of the Limit Reference Point (LRP), with no chance that the stock is at or above the LRP.
- Recruit abundance has been declining since the mid-1980s due to declining SSB, despite above average recruitment rates for most recent year-classes.
- Under current productivity conditions, there is a 90% (0-100 t annual catch) or 99% (300 t) probability that SSB will decline further between 2018 and 2023, with an expected decline of 32% from the 2018 SSB.
- Extremely high natural mortality of cod 5 years and older is the reason for the lack of recovery of this stock. Natural mortality of adults has increased over the past 40 years and is now estimated to be 55 - 57% annually (M = 0.81 – 0.85), much higher than what was estimated in the 1970s (18%, M = 0.2). At this high level of natural mortality, this stock is expected to continue to decline with high probability, even with no fishing.
- This stock is experiencing an “Allee effect”, in which the per capita rate of population growth decreases as population size decreases. This is opposite to the usual behaviour of populations at low abundance. Additionally, since 2000, this stock has been experiencing a production deficit, averaging negative 7,000 t per year, indicative of a “strong Allee effect”. If a strong Allee effect persists, it will drive a population to extinction.
- Predation by grey seals is concluded to be the main cause of the elevated natural mortality of this cod stock over the past 20 years, and thus of the Allee effect.
- Over the past 20 years, cod have progressively moved out of traditional foraging areas in inshore waters and into deeper offshore waters during their feeding season in the southern Gulf of St. Lawrence. This appears to result from the high and increasing risk of predation by grey seals in inshore waters in summer. This change in distribution is expected to incur costs (e.g., reduced foraging success) and has been coincident with declines in the condition of cod.
- At the current abundance of grey seals in this ecosystem, recovery of this cod population does not appear to be possible, and its extinction (SSB < 1,000 t) is highly probable.
This Science Advisory Report is from the February 20-21, 2019 meeting on the Assessment of Atlantic Cod (Gadus morhua) of the southern Gulf of St. Lawrence (NAFO Div. 4T-4Vn (Nov. – April)) to 2018 with advice for fishing periods May 2019 to May 2023. 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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