June 7, 2021 — Small pelagic fish that school in open water—think sardines or anchovies, are eaten by all kinds of predators. Seabirds, marine mammals, and bigger fish feed on these small pelagics giving them the moniker “forage fish.”

Forage fish support several fisheries, particularly “reduction fisheries,” where fish are caught and reduced into fishmeal and fish oil for livestock and aquaculture. The anchoveta fishery off the coast of South America is the largest in the world, and nearly all catch is reduced. From a food production perspective, reduction fisheries turn fish that humans don’t like to eat into other kinds of meat that humans do. That isn’t to say forage fish aren’t fished for human consumption—they are and have one of the lowest carbon footprints of any food, but the majority of catch is reduced. Eat more anchovies and sardines, people!

However, forage fish also play a foundational role in many ocean ecosystems. They buoy the diets of marine birds and mammals like whales, puffins, albatross, and other vulnerable species while also indirectly supporting valuable fisheries, e.g., salmon and tuna feed on forage fish. Their role in the food chain has led to some calls to limit forage fish fisheries to boost the populations of their higher-value predators. This makes intuitive sense, but new research out this week by Free et al. shows it’s more complicated than simply “more prey, more predators.”

In 2012, a prominent forage fish paper was published that advised a highly precautionary approach to commercial fishing of forage fish. They suggested that to be as conservative as possible, even fisheries currently considered well-managed should be reduced by 50% to enhance and maintain predator populations. It kicked off a decade of forage fish population modeling and scientific discussion. The major criticism of the 2012 paper was that the ecosystem model used in the paper assumed that commercial fishing had an outsized impact on forage fish populations and did not account for ocean conditions. However, forage fish populations are highly sensitive to environmental conditions. For example, long before humans were fishing them, the Pacific Sardine went through periods of significant population boom and bust. This environmental sensitivity complicates the understanding of fishing impact, especially because the predators eat far more forage fish than are taken via fishing. Surly overfishing is bad, but would further reducing fishing below sustainable levels benefit the broader ecosystem?

Scientists did more research. In 2017, a paper by Hilborn et al. showed little correlation between forage fish populations and their predators. The authors argued that if forage fish have natural boom and bust cycles, their predators should have the resilience to find other kinds of prey in times of bust (and indeed, most marine predators that forage on small pelagic fish have a broad diet and are highly mobile). Hilborn et al. challenged the 2012 paper’s recommendations for a highly precautionary approach to forage fish fisheries. However, it was still a relatively simple analysis—the authors used population data to show correlations (or the lack thereof) between the abundance of forage fish and changes in their predator populations. They found that just 5 of the 50 predators examined in that study showed a positive correlation to forage fish population.

The 2017 paper showed correlation but not causality—the paper published this week gets closer to causality by controlling for possible confounding factors, namely by using a predator dynamics model that accounted for forage fish boom and bust cycles. This hadn’t been in previous models. Further, the 2017 paper only looked at U.S. ecosystems; this paper included ecosystems in Europe, South Africa, and the Humboldt Current off South America, giving a more global view of forage fish ecosystem dynamics.

Read the full story at Sustainable Fisheries UW