Science Advisory Report  2011/034

Recovery Potential Assessment (RPA) for the Southern Designatable Unit (NAFO Divs. 4X5Yb and 5Zjm) of Atlantic Cod (Gadus morhua)

Summary

• Cod in the Southern designatable unit (DU) are assessed as two separate management units:  Southern Scotian Shelf and the Bay of Fundy (NAFO Div. 4X and the Canadian portion of 5Yb), and eastern Georges Bank (NAFO Div. 5Zjm).
• 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 5Zjm stabilizing at low levels over the past decade, while continuing to decline in 4X5Yb.
• The demersal juvenile stage (4 – 35cm long) is the most habitat-dependant period in the life-cycle of Atlantic Cod. Physical disturbance of structural components of habitat can reduce its value and increase mortality of juvenile cod. Existing data lack the spatial resolution required to evaluate the amount of suitable habitat available to demersal juveniles and whether it has changed in the past three generations, especially in the offshore. However, there is no indication that the amount of suitable habitat is currently limiting recovery of cod in this DU.
• Within the Southern DU, the area of occupancy for cod declined slightly in the 1990s but appears to be stable since 2001. Despite the slight decline in calculated area of occupancy, the range and overall distribution of cod within the Southern DU has not changed since the 1970s.
• The structure of cod populations in the Southern DU is complex, involving seasonal migration patterns, mixing grounds and some exchange between putative stocks. However, there is no evidence of a decrease in the number of populations.
• Natural mortality of Div. 4X5Yb cod aged 4 years and older (4+) is estimated to be unusually high (0.76 for 1996 to 2008), whilst natural mortality of Div. 5Zjm cod aged 6 years and older (6+) is also elevated (0.5 for 1994 to 2009).
• Average recruitment in the Southern DU stocks has decreased to less than half of the pre-1992 level. Recruitment has generally been higher for 4X5Yb cod when ages 3+ biomass has exceeded 25,000t and for 5Zjm cod when ages 3+ biomass has exceeded 30,000t. In recent years, biomass has remained below these values and recruitment has been poor.
• Conservation limit reference points (LRPs) have been calculated for 4X5Yb and 5Zjm cod, based on Beverton-Holt stock recruitment models. The Precautionary Approach (PA) reference point, B_lim, was calculated as 24,000t for cod in Div. 4X5Yb and 21,000t for Div. 5Zjm. Estimated 4X5Yb spawning stock biomass (SSB) has been below the LRP since 2002 and was estimated to be 10,600t at the beginning of 2009. Estimated 5Zjm cod SSB has been below the LRP since 1994 and is currently estimated to be 9,260t.
• Projections were undertaken for both stocks and at the DU level. Future productivity conditions are very uncertain. Thus, these projections should not be interpreted as forecasts of future stock status because they depend on assumptions about future productivity and fishing mortality. The probability of current conditions continuing for a long period of time is unknown. These projections are explorations of the consequences of particular productivity assumptions.
• If current productivity conditions were to persist in the future, projections of the abundance and biomass of cod in 4X5Yb and 5Zjm indicate the population is expected to increase, even with moderate fishing (i.e. to the level of the established fishing mortality threshold, F_ref).
• For 4X5Yb cod, with no fishing there is a greater than 95% probability of SSB being at or above the LRP of 24,000t by 2020. Fishing at half the current fishing mortality reference level there is a greater than 95% probability of SSB being at or above the LRP by 2033 and the probability of SSB being at or above the LRP within 36 years when fishing at the reference level of 0.2 is 72%.
• For 5Zjm cod, with no fishing there is a greater than 95% probability of SSB being at or above the LRP by 2028. Fishing at half the current reference level there is a greater than 95% probability of SSB being at or above the LRP by 2033 and the probability of SSB being at or above the LRP within 36 years when fishing is at the reference level of 0.18 is 82%.
• Projections of the combined mature abundance of the 4X5Yb and 5Zjm stock components indicate that the population is expected to increase under current productivity conditions over the next 36 years.
• The major potential sources of mortality identified for Southern DU cod were natural mortality (including seal predation), along with fishing above F_ref, discards and bycatch.
• Reductions in fishery removals, at least to the level of F_ref, and bycatch mortality are the only identified mitigation measures for which the increases in survivorship can be calculated.

This Science Advisory Report is from the Fisheries and Oceans Canada, Canadian Science Advisory Secretariat, Zonal Advisory Process, February 21-25, 2011 on Recovery Potential Assessment (RPA) for Atlantic Cod (Newfoundland and Labrador, Laurentian North, Laurentian South, Southern Designatable Units). Additional publications from this process will be posted as they become available on the DFO Science Advisory Schedule.

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