Sustainable fisheries management

Because fishers usually harvest larger fish, fish populations adapt to the pressure of heavy fishing by evolving to mature earlier and at smaller sizes. This phenomenon is concerning both for the economy and the environment: economically because smaller fish means a smaller catch for fishers, and for the environment because the broader ecosystem impacts of these changes are unclear.

Through a growing body of research, the IIASA Evolution and Ecology Program is elucidating to what extent observed changes in fish populations are caused by non-reversible genetic changes, or evolution, and to what extent the changes are phenotypic, or influenced by the environment rather than genetics. The distinction is crucial for sustainable management, since fisheries-induced genetic changes tend to be slow or impossible to reverse.

In the study, the researchers addressed this question by exploring the relative impact of two factors: density dependence, meaning that when the cod population shrinks through fishing, more food is available, so that fish can grow faster and mature earlier; and life-history evolution, meaning that a trend towards genetic changes causing earlier maturation is induced by the selective harvesting of larger fish.

Using an eco-evolutionary model that could reproduce 74 years of historical data on age and length of maturation in Northeast Arctic cod, they found that a combination of these two factors was likely responsible for the observed changes in fish size. Their carefully calibrated quantitative model also revealed that the cod population might have collapsed around 1980 had it not undergone some life-history evolution that made it more resilient to the high fishing pressures it experienced after World War II.

The study thus shows that ecological and evolutionary dynamics do not work in isolation, but rather as part of a complex system. In order to prevent fisheries collapse, it is important to understand these dynamics and how they interact.