From ecoRI News (ecori.org)
The tiniest plants and creatures in the ocean fuel entire food webs, including the fish that much of the world’s population depends on for food and work.
In a paper recently published in Science Advances, National Oceanic and Atmospheric Administration (NOAA) Fisheries researcher Jason Link and colleague Reg Watson from the University of Tasmania’s Institute for Marine and Antarctic Studies suggest that scientists and resource managers need to focus on whole ecosystems rather than solely on individual populations.
Population-by-population fishery management is more common, but a new worldwide approach could help avoid overfishing and the insecurity that it brings to fishing economies, according to the paper.
“In simple terms, to successfully manage fisheries in an ecosystem, the rate of removal for all fishes combined must be equal to or less than the rate of renewal for all those fish,” said Link, the senior scientist for ecosystem management at NOAA Fisheries and a former fisheries scientist at the Northeast Fisheries Science Center, in Woods Hole, Mass.
The authors suggest using large-scale ecosystem indices as a way to determine when ecosystem overfishing is occurring. They proposed three indices, each based on widely available catch and satellite data, to link fisheries landings to primary production and energy transfer up the marine food chain.
Specific thresholds developed for each index make it possible, they said, to determine if ecosystem overfishing is occurring. By their definition, ecosystem overfishing occurs when the total catch of all fish is declining, the total catch rate or fishing effort required to get that catch is also declining, and the total landings relative to the production in that ecosystem exceed suitable limits.
“Detecting overfishing at an ecosystem level will help to avoid many of the impacts we have seen when managing fished species on a population-by-population basis, and holds promise for detecting major shifts in ecosystem and fisheries productivity much more quickly,” Link said.
In the North Sea, for example, declines in these indices suggested that total declines in fish catch indicative of ecosystem overfishing was occurring about 5-10 years earlier than what was pieced together by looking at sequential collapses in individual populations of cod, herring, and other species. Undue loss of value and shifting the catches in that ecosystem to one dominated by smaller fishes and invertebrates could have been avoided, according to the authors.
The first index used in the study is the total catch in an area, or how much fish a given patch of ocean can produce. The second is the ratio of total catches to total primary productivity, or how much fish can come from the plants at the base of the food web. The third index is the ratio of total catch to chlorophyll, another measure for marine plant life, in an ecosystem.
Proposed thresholds for each index are based on the known limits of the productivity of any given part of the ocean. Using these limits, the authors said local or regional context should be considered when deciding what management actions to take to address ecosystem overfishing. Having international standards would make those decisions much easier and emphasize sustainable fisheries.
“We know that climate change is shifting many fish populations toward the poles, yet the fishing fleets and associated industries are not shifting with them,” Link said. “That already has had serious economic and cultural impacts.”
The authors note that they are able to follow these shifts over time and see how they can exacerbate or even contribute to ecosystem overfishing.
Fisheries are an important part of the global economy. In addition to trade and jobs, fish provide the primary source of protein to more than 35 percent of the world’s population, and 50 percent of the people in the least developed countries, according to the authors. Regions where the greatest amount of ecosystem overfishing occurs are also where impacts can be the greatest.
The researchers looked at 64 large marine ecosystems around the world and found those in the tropics, especially in Southeast Asia, have the highest proportion of ecosystem overfishing. Temperate regions also have a high level of ecosystem overfishing, with limited capability to absorb shifting fishing pressure from the tropics as species move toward the poles.