Of all the threats facing the ocean today, overfishing is among the most serious. Depleting the adult fish population impedes breeding, resulting in severe impacts on global food security.
When it comes to overfishing, the tendency is to focus on individual populations. But a new study in Science Advances suggests that this may not be the most efficient way of tackling the problem. Rather, it proposes that the wider aspect of ecosystem overfishing also be addressed.
‘There’s a lot at risk in global fisheries,’ said study author Jason Link, the senior scientist for ecosystem management at NOAA Fisheries who wrote the paper with colleague Reg Watson of the University of Tasmania’s Institute for Marine and Antarctic Studies.
‘Fish stock after fish stock is overfished, major features in an ecosystem are being ignored and there are real constraints to actual fisheries production that are often forgotten about. We need a system to address overfishing and our study provides a way of doing that. The key is to understand that we should be removing stuff at a rate less than or equal to which is produced.’
Link and Watson’s study introduces three large-scale ecosystem indices that link fisheries landings to primary production and energy transfer up the marine food chain. Thresholds for each index then determine whether ecosystem overfishing is occurring (these are based on the known limits of productivity in a particular part of the ocean).
The first index, the Ryther index, is the total catch in a given area. The second, the Fogarty index, is the ratio of total catches to total primary productivity in an ecosystem, while the third, the Friedland index, is the ratio of total catches to chlorophyll. The indices were named after late marine biologist John Ryther and NOAA Fisheries scientists Michael Fogarty and Kevin Friedland.
The study concludes that ecosystem overfishing occurs when the total catch of all fish is flat or declining, the effort necessary to catch the same amount of fish keeps increasing and the total landings relative to production in a particular ecosystem exceed suitable limits.
‘We take the total catch in a given ecosystem and link it to a particular area of that ecosystem,’ Jason Link said.
‘Using catch and satellite data, we then link the total catch to primary production in the ecosystem concerned and take the total catch and link it to chlorophyll, a measure of the small plants at the bottom of the food pyramid. Those three things give a clear indication of whether we are removing, by the catch, more or less than what we theoretically and practically should. Based on modelling efforts and empirical observations, we know that we can obtain this information on any given patch of the ocean.’
The researchers found that out of the world’s approximately 64 large marine ecosystems, about half, or 30 to 35, were experiencing ecosystem overfishing. Extreme thresholds were exceeded in nearly 40 to 50% of tropical and temperate ecosystems, and two specific instances were also found – increased pressure on tropical fish and a shift in some fish populations towards the poles due to climate change.
Looking at fish size (particular populations and fish as a whole across different species), they also found that areas with significant levels of overfishing tended to have smaller fish, with a much smaller mean size of catch. Regions where the greatest amount of ecosystem overfishing occurs also have the greatest impacts.
‘The north and south temperate regions have various challenges,’ he explained.
‘In the tropics, there is a large concentration of people and therefore more fishermen and fishing fleets. It’s easier for a vessel to go across longitudes than up and down in latitude so there may be a longer history of fishing in tropical and temperate latitudes. That, in turn, is probably linked to population densities and why we are fishing harder in those regions compared to elsewhere.’