Overfishing and the environmental degradation of the oceans are hitting fish populations with a "double whammy" from which many may not recover, a study has found.
Research has shown that measures to conserve fish stocks may actually be making things worse by creating vulnerable populations.
Policies such as taking only the biggest fish are altering the age structure of fish populations, which makes them more vulnerable to total collapse under environmental stress.
The study, which used 50 years of data, is unique in that it directly compares for the first time commercially important fish stocks with fish populations that are not targeted by ocean-going fishing vessels.
This has enabled scientists to tease apart the effects of environmental degradation and overfishing to see how much of the problem of global fish declines is due to each factor.
One of the main conclusions of the study - published in the journal Nature - is that commercially important fish stocks are suffering from the "double jeopardy" of overfishing which makes them more susceptible to environmental damage.
Professor George Sugihara, of Scripps Institution of Oceanography at the University of California San Diego, said that fishing amplifies the natural peaks and troughs of population size. "Fishing can potentially not only lead to declining stock levels, but we show it actually causes populations to fluctuate more through time, which could put them at greater risk of collapse than we previously thought," Professor Sugihara said. The research was different from many previous studies in that it estimated fish populations from the amount of freshly hatched fish larvae caught as samples in the ocean. Other estimates have been based on the number of fish landed in ports by fishing vessels, which does not take into account the size of fish populations that are not targeted by the fishing industry.
Professor Sugihara and his colleagues believe that by taking bigger fish, the industry is selectively removing those members of the population that are more able to act as buffers against random environmental variation and add year-to-year continuity to the population.
As fishing continues there is a tendency for the average age and size of a targeted population to go down - ending up with a stock composed largely of immature, juvenile fish that are less able to cope with environmental change caused by pollution, global warming or natural events such as El Nîno.
"This so-called age-truncation effect suggests that fisheries need to be managed not only to maintain a harvest of target or total biomass level, but also to maintain a certain age structure in the stock," Professor Sugihara said. "Instituting practical maximum size limits or encouraging the use of marine reserves to protect the larger individuals are possible solutions."
Professor John Beddington of Imperial College London, who took part in the research, said that heavily-fished populations are unlikely to contain many fish older than a few years.
This means they are almost entirely reliant on the successful growth of fish larvae and baby fish - or "recruits" - to maintain fish numbers year on year.
"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."