Research Document 2019/038
Assessment of the southern Gulf of St. Lawrence Atlantic Cod (Gadus morhua) stock of NAFO Div. 4T and 4Vn (November to April), March 2019
By Swain, D.P., Ricard, D., Rolland, N., and Aubry, É.
Abstract
The stock of Atlantic Cod (Gadus morhua) in the southern Gulf of St. Lawrence (sGSL) (NAFODivision 4T and Subdivision 4Vn (November to April)) supported landings averaging 30,000 tonnes (t) annually between 1917 and 1940 and 56,000 t annually between 1941 and 1992. The stock collapsed in the early 1990s due to high mortality. Moratoria on directed fishing forsouthern Gulf cod were put in place in 1994-1997, 2003 and since 2009. Reported by-catch in fisheries for other groundfish have averaged 110 t annually since 2009, with catches near 60 t in 2017 and 2018. Estimated fishing mortality since 2009 has averaged 0.2% annually for cod aged5-8 years and 0.7% for cod 9 years and older. Spawning stock biomass (SSB) and abundanceare at the lowest levels observed in the 69-year record and are declining. Estimated SSB at thestart of 2018 was 13,947 t, 12% of the already low value in 2000 and 4% of the 1985 value (380,000 t). SSB is estimated to be 17% of the limit reference point (LRP = 80,000 t) with no chance of recovering to the LRP over the next five years, even with no fishing mortality. The probability that SSB will decline between 2018 and 2023 is 90% with annual catches of 0 or 100 t and 99% with catches of 300 t. SSB is projected to decline to 9,400 t at the start of 2023 with no fishery catch and to 9,100 t with annual catches of 300 t. If current conditions were to persist, the population would be expected to decline to extinction by mid-century (SSB < 1,000 t was used asa proxy for local extinction). The ongoing decline of this population is due to the high natural mortality of adult cod (i.e., ages 5 years and older). Natural mortality (M) of about 18% annually (an instantaneous rate of M=0.2) is considered normal for adult cod. In this population, natural mortality of adults has increased over the past 35 years and is now estimated to be about 55% annually (M=0.81-0.85). Predation by grey seals appears to currently be the main cause of this mortality. Based on population models, this predation can account for all of the adult M above the normal level (0.2) since about the year 2000. This cod population appears to be experiencing a predation-driven Allee effect. An Allee effect occurs when the per capita rate of population growth decreases as abundance decreases. This is opposite to the expected increase in population productivity at low abundance due to reduced intraspecific competition. In addition, southern Gulf cod are experiencing a strong Allee effect with a production deficit since 2001. If they persist, strong Allee effects will drive a population to extinction. This Allee effect is caused bythe severely depleted abundance of southern Gulf cod and the high and increasing abundance of their grey seal predators. At the current high abundance of grey seals, this cod population is expected to continue to decline toward extinction.
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