The dawn of Darwinian fishery management

Let us compare a livestock farmer and a fisher. The farmer selects and breeds individuals that exhibit the most desirable characteristics. This is good practice, because it increases the prevalence of these characteristics in the next generation of the stock. In contrast, the fisher catches large, fast-growing fish, so their desirable characteristics are less likely to be passed on to the next generation of the stock (Figure 5.1). Fish that grow quickly tend to be caught sooner and therefore may produce fewer offspring. Fish that delay maturation tend to be caught before they have the chance to reproduce, so the fish that are left to breed are those that mature at a younger age. Fish that limit their current investment in reproduction in order to increase future reproductive success will often be harvested before such savings have a chance to pay dividends. The mortality imposed by fishing can therefore act as a selective force that favours slower growth, earlier maturation and higher reproductive investment. Clearly, the selections made by the farmer and the fisher work in opposite directions. The farmer selects desired characteristics that improve his or her stock, whereas the fisher selects characteristics that may inadvertently reduce a stock’s productivity and resilience. Therefore, fishery scientists need to incorporate both ecological processes and evolutionary processes in their research programmes in order to ensure the best scientific basis for fishery management. The notion that fishing can affect the genetic composition of exploited populations has been recognised for more than a century. Perhaps the earliest account can be credited to Cloudsley Rutter (1903), a US salmon biologist, who warned more than a hundred years ago: ‘[A] stock-raiser would never think of selling his fine cattle and keeping only the runts to