Research Document - 2011/138
Recovery Potential Assessment for the Laurentian South Designatable Unit of Atlantic Cod (Gadus morhua): The Eastern Scotian Shelf Cod Stock (NAFO Div. 4VsW)
By R.K. Mohn and S. Rowe
Abstract
In its 2010 assessment of Atlantic Cod, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) designated the Laurentian South Designatable Unit (DU) as Endangered due to a 90% decline in abundance of mature individuals over the previous three generations. The Laurentian South DU consists of three populations or stocks: southern Gulf of St. Lawrence cod (management unit 4T and 4Vn (Nov-Apr)), 4Vn resident cod (4Vn (May-Oct)), and eastern Scotian Shelf cod (4VsW).
Recovery potential assessments (RPAs) were introduced by DFO Science to provide the information and scientific advice necessary to meet various requirements of the Species at Risk Act (SARA), including decisions regarding the listing of a species under the Act and the development of a recovery strategy. Specifically, as part of the assessment process, scientific information is needed to support the development of social and economic cost assessment scenarios for recovery, to better inform public consultations, and to support other entities involved in the decision of whether to add the species to Schedule 1 of the Species at Risk Act. The recovery team also requires this information to develop a recovery strategy, and if necessary, one or more action plans.
The objective of this research document is to address the terms of reference for the Atlantic Cod RPA as it relates to the eastern Scotian Shelf cod stock, a component of the Laurentian South DU. Historic stock trajectories are described and the stock is projected into the future to assess the probability of achieving recovery targets assuming that current productivity conditions were to persist in the future. This research document also addresses the major threats to the survival and recovery of eastern Scotian Shelf cod and the limiting factors. Measures that can help its recovery are listed.
The spawning stock biomass (SSB) of 4VsW cod reached the lowest level observed in the 53-year record in 2003 at about 7,500 t. Recently, it has rapidly grown to 64,000 t and is approaching the long term mean (75,000 t). Natural mortality of 4VsW cod aged 5 years and older (5+) was estimated to be unusually high in the 1990s and early 2000s (peaking at approximately 1.1) but has recently declined to 0.36. For 4VsW cod, most other components of productivity were at their lowest values in the period 1990-2000 and then have shown some recovery since then. Weight-at-age, condition, and area occupied all show such a pattern. There have also been improvements in recruitment rate in some recent years. Since the closure of the directed fishery in 1993 fishing mortality (ages 5-15) is estimated to have been 0.035, a small fraction of natural mortality. More recently it has dropped further to about 0.01, a negligible level. A limit reference point (LRP) has been established for 4VsW cod, based on 40% of the spawning stock biomass at maximum sustainable yield (MSY) during the productive period before 1990. The LRP is estimated to be 50,000 t. Estimated SSB has been below the LRP since 1992 with the exception of the 2009 estimate of 64,000 t, 25% above the LRP. If 1994-2009 productivity conditions persist in the future, projections indicate that SSB of the 4VsW population would be expected to decline below the LRP and then stabilize in the long term at a low level, even with no fishing. Productivity conditions have improved in the past few years compared to the average of those used in the projections. Removals of 4VsW cod at the level of the bycatch fishery since closure of the cod-directed fishery in 1993 have no detectable effect on the probability of survival or recovery. The only additional action that can be taken to improve the chances for recovery of 4VsW cod would appear to be action to reduce the rate of natural mortality on adult (5+) cod. Predation by grey seals is considered to be a significant component of natural mortality but its relative contribution is of unknown magnitude. Even without establishing the degree of causality, it is noted that the Sable Island grey seal population was under 50,000 animals when 4VsW cod was productive; the current Sable herd size is around 300,000, six times larger.
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