Research Document 2024/053

Scientific Requirements for the Rebuilding Plan of Southern Gulf of St. Lawrence (NAFO Division 4T and 4Vn November to April) Atlantic Cod (Gadus morhua)

By Turcotte, F., McDermid, J.L., Ricard, D.

Abstract

The Atlantic Cod (Gadus morhua) stock in the southern Gulf of St. Lawrence (NAFO Division 4T-4Vn (November-April)) is below its limit reference point (LRP) and in the Critical Zone of the Precautionary Approach. The new Fish Stocks Provisions and the amended Fisheries Act legally require Fisheries and Oceans Canada (DFO) to develop a rebuilding plan for this stock. A rebuilding plan comprises several elements that require DFO Science sector advice including: (i) stock status, (ii) causes of stock decline, (iii) rebuilding target and timeline, (iv) additional measurable objectives, (v) likelihood of management measures meeting rebuilding objectives, (vi) how to track rebuilding progress, and (vii) frequency of the periodic review of the rebuilding plan.

Southern Gulf of St. Lawrence (sGSL) Cod stock size going back to 1917 was estimated using a surplus production model. Cod biomass exceeded B_MSY until the late 1940s. Biomass relative to B_MSY started to decline in the 1950s as catch started increasing from stable values. The sGSL Cod biomass in 2018 was 2.4% of the biomass in 1917. The source of serious harm and cause of decline for sGSL Cod is fishing mortality higher than F_MSY in 1955 and onwards. Other sources of serious harm, and likely consequences of the overfishing, include a lasting state of low production and low biomass, recruitment overfishing, high natural mortality and a predation-driven Allee effect, low growth and body condition, and a decrease in age-at-maturity.

A review of the biomass reference points generated a new LRP using the statistical catch-at-age model; initial 0.25B₀. Its value is estimated at 210,000 t of SSB. With this new LRP, the stock is now estimated to have declined into the Critical Zone in 1990.

In addition to the stock having a 75% probability of being at or above the LRP, the rebuilding target should include that the stock must be at or above this level for 4 consecutive years, and population projections must show the stock is likely to continue its positive trajectory under harvest for 4 years after the rebuilt state has been achieved.

Projections showed that the stock is unlikely to rebuild to the rebuilding target under prevailing conditions, even in the absence of fishing mortality, and that the environmental conditions that would allow to reverse the decline in Cod biomass are unlikely to occur. Projections showed that at 300 t of bycatch, the population SSB in 10 years would be reduced by 10%. At 500 t of bycatch, the population SSB in 10 years would be reduced by 16%. Additional measurable objectives for the rebuilding plan should include recovering the truncated age structure, increase size and condition at age, recover the spatial distribution in shallow waters, and promote recruitment by protecting spawning grounds.

Rebuilding progress will be tracked using the interim indicator survey and stock assessment models. The periodic review of the rebuilding plan should be set to the 4-year stock assessment cycle with an interim update at the halfway point.

Research Document 2024/053 (PDF, 2,467 KB)

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