Resilience of the world's fish stocks

April 29, 2013

Philipp Neubauer, who is due to start work at Dragonfly in August, has published a paper modelling the recovery of the world’s fish stocks from overfishing in the journal Science this month. The full paper may be accessed via the publications page of the RAM legacy stock-assessment database.

“We found that marine fish populations were surprisingly resilient to overfishing and could generally rebuild to sustainable levels within a decade or so – if fishing was reduced substantially at the first signs of overexploitation,” he says.

With his collaborators, Philipp looked at populations that had experienced at least one period when their abundance dropped below 50% of a common management target. They identified and then modelled the variables responsible for recovery times and their relative impact. To do this, they had to accommodate stocks that hadn’t yet recovered. By treating depleted stocks as 'ailing patients' and applying statistical methods from clinical trials, they were able to determine the factors that affect a stock’s recovery.

Philipp is currently completing a post-doc at Rutgers University in the US, and did his PhD in marine ecology at Victoria University of Wellington. He and his collaborators were keen to find out if the slow recovery of overexploited populations was due to insufficient reductions in harvest rates or an erosion in the resilience of the population.

“We found little to support a loss of resilience. Recovery is all about the harvest rates. If you can detect depleted fish populations early and respond quickly with appropriate reductions in catch, there is no reason not to expect a quick recovery.”

The study also yielded some surprises. “We found that fish stocks that had been moderately overfished for long periods (of 50 years or more) tended to recover faster than those that had been depleted more rapidly (over 30 years or less). It seems that over the longer time period, natural selection favoured fish that can reproduce when they are smaller – before they are able to be caught.”

But when fish stocks are driven to severely low abundances, recovery can take decades and recovery projections become uncertain.

“The bad news is that, globally, we don't have a good track record of making the required fishing cuts when depletion is first recognised. Of 62 currently depleted stocks, less than a quarter are fished at rates needed for rebuilding or below. While we see many encouraging signs in places like the US and Australia where rebuilding efforts were mandated some years ago, other regions, like Europe, are only now taking action despite years of publicised overfishing.”

The research has attracted interest from media around the world, and Philipp and Olaf Jensen, also from Rutgers, spoke to the BBC World Service’s Science in Action programme on 18 April.