Research Document - 2015/069
Recovery Potential Assessment (RPA) for the Southern Designatable Unit (NAFO Divisions 4X5Yb and 5Zjm) of Atlantic Cod (Gadus morhua)
By K.J. Clark, D.S. Clark, I.V. Andrushchenko and D.P. Swain
Abstract
In its 2010 assessment of Atlantic Cod (Gadus morhua), the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) designated the Southern Designatable Unit (DU) as Endangered. Cod in the Southern DU are currently assessed as two separate management units: Southern Scotian Shelf and the Bay of Fundy (NAFO Division 4X and the Canadian portion of 5Yb) and Eastern Georges Bank (NAFO Division 5Zjm). A recovery potential assessment (RPA) was conducted in February 2011 by Fisheries and Oceans Canada (DFO) Science to provide the information and scientific advice required to meet various requirements of the Species at Risk Act (SARA), including decisions regarding the listing of Southern DU cod under the Act and developing a recovery strategy. This document addresses the terms of reference for the Atlantic Cod RPA in relation to the Southern DU.
The abundance of cod in the Southern DU has declined in number and biomass since the early-1990s. Trends differ between the two components in the Southern DU, with cod numbers in NAFO Division 5Zjm stabilizing at low levels over the past decade, while continuing to decline in Division 4X5Yb. The structure of cod populations in the Southern DU is complex, but there is no evidence of a decrease in the number of populations. The range and overall distribution of cod within the Southern DU has not changed since the 1970s and there is no indication that the amount of suitable habitat is currently limiting the recovery of cod in this DU.
Conservation Limit Reference Points (LRPs) were calculated for 4X5Yb and 5Zjm cod, based on Beverton-Holt stock recruitment models. The Precautionary Approach (PA) reference point, spawning stock biomass limit (Blim), was calculated as 24,000t for cod in Division 4X5Yb and 21,000t for Division 5Zjm. Estimated 4X5Yb SSB has been below the LRP since 2002, and was estimated to be 10,600t at the beginning of 2009. Estimated 5Zjm cod spawning stock biomass has been below the LRP since 1994, and is currently estimated to be 9,260t. Average recruitment in the Southern DU stocks has decreased to less than half of the pre-1992 level. Thirty six year (five generation) projections were undertaken for both stocks using recruitment data from the entire time series. Biomass was projected to increase above the LRPs for both stocks within this time period if fishing mortality was maintained at or below reference levels.
Sources of potential mortality identified for Southern DU cod included natural mortality, fishing, discards and bycatch in other fisheries. Natural mortality of Division 4X5Yb cod aged 4 years and older was estimated to be unusually high, whilst natural mortality of Division 5Zjm cod aged 6 years and older was also elevated. The only mitigation measures for which increases in survivorship could be calculated were reductions in fishery removals.
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