Commercial fishing causes dangerous fluctuations in fish populations

The research, which is published in today’s Nature, involved compiling the largest ever survey of both exploited fish and non-exploited fish off the California coast. The research team looked at how the abundance of both types of fish varied over a 50 year period, and found the first evidence that exploited species’ population levels vary far more than non exploited species’ in the same ecosystem.

Researchers concluded that this increased variability in exploited fish stocks was most likely to be caused by the effect fishing has on the age structure of a population. Heavily-fished populations are unlikely to contain any fish older than a few years old, and as such are wholly reliant on the successful growth of larvae into baby fish (recruits) to maintain population numbers year on year.

Professor John Beddington from Imperial College London’s Division of Biology, who worked on the study, explained: “Intensive fishing makes populations vulnerable because if they rely on recruits to replenish their numbers, there is always the danger that some kind of environmental factor will devastate the recruits in one season. This would leave the population close to collapse, with very few young fish coming into the group to replace those being caught.”

Professor Beddington adds that the increased variability has serious implications for the way in which fish stocks are managed: “Typically fish populations are managed by governments setting total allowable catch limits (TACs), but a fixed TAC which doesn’t take into account the variability of abundance over time, may mean that in some years it is completely incompatible with the population size. This means that fishing vessels could unwittingly overexploit the population, even though they are abiding by set limits.”

Researchers hope that their findings will mean future decisions about fishery management take into account the variations caused by fishing, to safeguard the future of key fish populations.