Fishing causes dangerous fluctuations in fish populations

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Commercial fishing can cause serious fluctuations in fish populations that it can leave them in danger of collapsing completely, says research published today.

Scientists compiled the largest ever survey of exploited and non-exploited fish species found off the California coast and plotted their abundance over a 50 year period. The research, which is published in the journal Nature today, provides the first evidence that the population levels of exploited species vary much more than non-exploited species living in the same ecosystem.

The authors believe that their results indicate that the increased variability in exploited fish stocks is likely to be caused by the effects of fishing on the age structure of fish populations.

In areas where fish are heavily exploited, fish rarely reach an age of more than a few years old before they are caught, often before they have had the opportunity to reproduce, something that is particularly problematic in some of the deep water fishes with low population doubling times.

Professor John Beddington from the Division of Biology at Imperial College London, who worked on the study, said:

"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.

"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."