Long-term changes in life history characteristics and reproductive potential of northern Gulf of St. Lawrence cod (Gadus morhua) and consequences for the stock productivity

Variations in life history traits determining reproductive potential of fish stocks are often large enough to invalidate the use of spawning stock biomass (SSB) as an unbiased index of reproductive potential. In northern Gulf of St. Lawrence cod (Gadus morhua), observed annual total egg production (TEP) differed from those estimated based on constant life history traits (i.e. direct proportionality between SSB and TEP). Variability observed in age structure, sex ratio, growth, maturity, and fecundity associated with environmental conditions, exploitation and SSB level clearly demonstrate the highly dynamic nature of TEP and complex interactions between environment, fishing and SSB. The sensitivity of reproductive rate to independent changes of each life history trait was estimated using demographic models. Natural mortality (M) had a larger influence on reproductive rate than variations in growth, maturation and fecundity. Correlations between age structure, growth, reproductive traits, and M demonstrated the importance of including M as a dynamic parameter in the estimation of TEP. TEP should be preferred to SSB as a measure of reproductive potential as, besides providing a means of estimating TEP, the monitoring of life history traits required to estimate TEP will also enable improved tracking of changes in stock productivity and resiliency.