Reduced catches of small oceanic 'forage' fish like sardines and anchovies may be required in some ocean areas in order to protect the larger predators that rely on these species for food.
This is a finding of the first major study of the ecosystem effects of fishing forage species: 'Impacts of fishing low trophic level species on marine ecosystems', reported today in the journal Science.
Dr Tony Smith of CSIRO's Wealth from Oceans Flagship led the international team of 12 authors from Australia, the United States, the United Kingdom, South Africa, France and Peru.
"Forage species such as anchovy, sardine, herring, mackerel and krill often are the main food source for larger predatory fish, marine mammals and seabirds," Dr Smith said.
"They account for more than 30 per cent of global fisheries production for use directly as human food and indirectly in livestock feeds, and demand is rising."
Previous studies have raised concerns about the flow-on effects on seabirds of forage fishing off Peru and South Africa and in the North Sea, and of rising krill catches on whales in the Southern Ocean.
"In this study we used three different types of models to examine the broader ecosystem effects when forage fish are harvested at levels that maximise sustainable yields," Dr Smith said.
"We found forage fishing had large impacts in the five areas studied (the northern Humboldt, southern Benguela and California currents, North Sea and south-east Australia).
"These impacts were both positive and negative, and varied across forage species, ecological groups and ecosystems."
The greatest impacts were seen for forage species that dominate their local food supply, such as Peruvian anchovy in the northern Humboldt ecosystem, and for forage species that are highly connected to many other species across the food web.
Some ecological groups declined by more than 60 per cent as a result of forage fishing at conventional levels. Marine mammals and seabirds were often affected.
"The modelling showed that halving fishing rates for the high-impact species would greatly reduce the impact on ecosystems, while still achieving 80 percent of the maximum sustainable yield," Dr Smith said.
"This reduced level of fishing could improve economic outcomes for forage fisheries while also improving yields for some other commercial species."
He said these results could be combined with other management measures, (such as closing areas near marine mammal and seabird breeding colonies to fishing), to achieve ecological objectives while ensuring forage fish continue to contribute to global food security.
The Marine Stewardship Council (MSC), which partly funded the study, is revising its guidance on assessing forage fisheries in line with these outcomes. More than 10 per cent of global fishery production is assessed for sustainability within the MSC program.