Evolutionary and Demographic Consequences of Over-Exploitation: Synopsis
Over-exploitation affects life history. Changes to age and size at maturity, longevity, and quite possibly fecundity and egg size are the product of phenotypic and genetic responses to fishing. Phenotypic and genetic changes to life history traits are of interest to conservation biologists and resource managers because of the inextricable links that exist between these fitness-related characters and population growth rate. The former, manifested ultimately by age-specific schedules of survival and reproductive investment, are the primary determinants of individual fitness, usually represented by intrinsic or finite rate of increase. The geometric mean of these individual fitness measures determines the growth rate of the population (r), a key parameter that is inextricably linked with sustainable rates of harvesting, probability of persistence, and rate of population recovery. The potential for fishing to cause evolutionary change is not appreciably different from other forms of predator-induced mortality, given its propensity for effecting differential mortality among genotypes. Many fisheries scientists and resource managers have been either reluctant to acknowledge the potential for fishing to elicit genetic changes or doubtful as to whether such changes are truly harmful. The latter point is a fundamentally important one because it raises questions as to whether human-induced evolution brought about by fishing is likely to significantly affect population attributes such as maximum sustainable yields, population resistance to natural environmental stochasticity, or probability of recovery following collapse. Among numerically depleted fishes, none has declined further than Newfoundland's northern Atlantic cod. Since 1962, this species have decreased 99%, representing a loss of 1-2 billion individuals; as of 2008, the population has shown little sign of recovery.