Influence of growth and survival costs of reproduction on Atlantic cod, Gadus morhua, population growth rate

A stochastic, age-structured life history model was used to examine how age at maturity (θ), pre- (Z(imm)) and postreproductive (Z(mat)) mortality, and postreproductive growth rate can affect maximum reproductive rates of fish at low population size. Simulations suggest that annual (r) and per-generation (R₀) metrics of population growth for Newfoundland's northern Grand Bank Atlantic cod, Gadus morhua, are primarily influenced by changes to mortality prior to and following reproduction. At observed weights at age and Z(mat) = 0.2, r ranged between 0.135 and 0.164 for cod maturing at between 4 and 7 years. Incremental increases in either Z(imm) or Z(mat) of 0.1 were associated with 0.03-0.05 reductions in r. To effect similar reductions, individual growth rate would have to decline by approximately one half. At observed weights at age, increases in Z(mat) from 0.20 to 0.45 increased the probability of negative per-generation growth from 3 to 26% for cod maturing at 4 years and from 6 to 46% for cod maturing at 7 years. Thus, even in the absence of fishing mortality, little or no population growth by Atlantic cod may not be unexpected in the presence of environmental stochasticity, particularly when accompanied by increases in mortality and declining individual growth.