Research Document - 2012/052
Recovery Potential Assessment for the Laurentian South designatable unit of Atlantic Cod (Gadus morhua): the southern Gulf of St. Lawrence cod stock (NAFO Div. 4T-4Vn(Nov-Apr))
By D.P. Swain, L. Savoie, and É. Aubry
Abstract
In 2010, the Laurentian South designatable unit (DU) of Atlantic Cod (Gadus morhua) was assessed by the Committee on the Status of Endangered Wildlife in Canada as Endangered. The southern Gulf of St. Lawrence cod stock (management unit 4T – 4Vn(Nov-Apr)) is a component of this DU. The southern Gulf cod stock is at the lowest level observed in the 61-year record and is declining. Abundance of mature cod in 2008-2010 is estimated to average 37% of the average level in the mid to late 1990s and 10% of the average level in the mid 1980s. A limit reference point (LRP) has been established for southern Gulf cod based on the spawning stock biomass (SSB) below which the probability of poor recruitment is high. The LRP is estimated to be 80,000 t. Estimated SSB has been below the LRP since 2003. Estimated SSB in 2010 is 49% of the LRP. Southern Gulf cod are widely distributed over the Magdalen Shallows in summer, though the area occupied by adult sizes of cod in September declined from an average of about 64,500 km² in the 1980s to an average of about 47,000 km² in the 2000s. An index of geographic range, the minimum area containing 95% of cod, decreased by 50% between the 1980s and the 2000s for adult cod. Nonetheless, given the broad distribution of waters suitable for cod in the southern Gulf and the Cabot Strait, habitat is not considered to be limiting for this population. Southern Gulf cod do not have any known dwelling-place similar to a den or nest during any part of their life. Productivity of southern Gulf cod is currently unusually low. The instantaneous rate of natural mortality (M) of cod aged 5 years and older (5+ cod) increased to very high levels in the late 1980s and early 1990s, averaging 0.66 over the 1994-2009 period. Predation by grey seals is thought to be an important component of this high natural mortality. Other components of productivity are also currently low. Weight-at-age has been at a low level since the early 1980s. Recruitment rate (recruits per unit of SSB) was exceptionally high in the mid to late 1970s but declined to a low level in the 1990s and 2000s, comparable to the rates observed in the 1950s and 1960s. If these conditions persist in the future, this stock is expected to continue to decline, even with no fishing. Under these conditions, the probability of reaching the LRP would be zero. With no fishing, at current levels of other components of productivity, 5+ M would need to be reduced to 0.5 to halt the decline in SSB and to 0.4 in order to have a high probability (90%) of exceeding the LRP in 20 years. During the small directed fishery for southern Gulf cod in 2007 and 2008, fishing mortality is estimated to have been 0.106 for fully-recruited ages, a small fraction of natural mortality, but still unsustainable given current stock productivity. With the closure of the directed fishery in 2009, fully-recruited fishing mortality dropped to 0.014, a negligible level with no detectable effect on population projections. The only additional action that can be taken to improve the chances for recovery of southern Gulf cod would appear to be action to reduce the rate of natural mortality on adult (5+) cod.
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