Science Advisory Report 2007/002

Accounting for Changes in Natural Mortality in Gulf of St. Lawrence Cod Stocks

Summary

• Trends in lengths at age (growth), condition, and age at maturity were compared between the two Gulf of St. Lawrence cod stocks as these may be related to changes in natural mortality. The main difference in these parameters is that after a period of decline, growth in northern Gulf cod appears to have recovered to 1970s levels while it has not in southern Gulf cod.
• Total mortality (Z) estimated from survey data during the period of the moratorium in the mid to late 1990s was 0.56 and 0.58 for northern and southern Gulf cod respectively. Given that fishing mortality (F) was minimal, this indicates an M of about 0.4 on both stocks during the moratorium.
• It is possible to estimate M in age-structured population models based on survey and catch data (e.g. ADAPT). When identical formulations of ADAPT were applied to each stock, estimates of M obtained were consistent with values assumed in the 2006 assessments. These analyses indicated that over the last decade M has declined to about 0.25 in northern Gulf cod but remains high (about 0.4 - 0.5) in southern Gulf cod.
• The 2007 assessments for northern and southern Gulf cod should be generally consistent with respect to methods for estimating M in ADAPT. However, details will differ between the two stocks. For both stocks M should be fixed for a period prior to 1985 at 0.2 to be consistent with earlier studies that show M to be at this value or lower. For both stocks M should be fixed at 0.4 for the years comprising the moratorium - northern Gulf cod 1994-1996, southern Gulf cod 1994-1997. Periods of fixed M are necessary to anchor the ADAPT estimation.
• For the period from 1980 to 1993 M can be estimated in the southern Gulf. For northern Gulf M should be fixed at 0.2 up to 1985 and at 0.4 from 1986 to 1996. For the period from 1997/8 to 2006 M should be estimated in two blocks for both stocks unless there is statistical justification for three blocks.
• Scientific advice on the management of both northern and southern Gulf cod stocks is based on risk analysis. Uncertainty in the estimate of M must be formally included in the risk analysis.
• It is recommended that further analysis be carried out with respect to the estimation of M for these and other cod stocks taking into account variability in survey catchability, uncertainty in reported catches and sensitivity to assessment model assumptions and constraints.
• Reasons for the recent difference in natural mortality between the two stocks remain unclear.

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