The science of sustainable seafood, explained

Impacts of fishing forage fish on the fish that feed on forage fish

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 of it 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 advocacy groups wanting to limit forage fish fishing 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.”

photo of a whaleshark eating from a fishing net full of forage fish
Forage fish and a predator | Shutterstock

A brief history of forage fish population modeling

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 cut in half 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 commercial fishing had an outsized impact on forage fish populations and did not account for ocean conditions. 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 suggest 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 they do, 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, but it was 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 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.

The updated model, results, and management suggestions

The Free et al. paper used a model of intermediate complexity, a step up from single-species correlational models, but not quite on the level of a highly complex ecosystem model. There’s good reason for that—the highly complex ecosystem models are too broad to look at specific predator/prey dynamics and seldom include enough taxonomic resolution. The intermediate complexity was about as advanced as they could go to look at particular predator/prey interactions.

The researchers state in the paper that the model “had high power to detect influence of forage fish on predators.”

They ran the model to examine 45 different predators that relied on forage fish for at least 20% of their diet and had similar findings to the 2017 paper—few significant relationships between forage fish abundance and predator abundance.

Our results indicate that forage fish abundance rarely impacts predator productivity, which suggests that the extra-precautionary management of forage fish would rarely achieve the intended benefits for marine predator populations.

The authors gave several real-life case studies of resilient marine predators that support their results. For example, great skuas in the North Sea have switched prey in response to the overfishing of sand eel and have not seen population declines. Little penguins in southeast Australia also adapt well. They change forage locations based on previous years’ catch rates and communicate to other penguins about it. However, compared to marine mammals and predatory fish, seabirds were less resilient overall.

Though the analysis showed few cases of forage fish abundance affecting predator abundance, there are some important exceptions to note: Local populations can matter, especially around breeding grounds. Overfishing in those areas was shown to have the most harmful effects on predator abundance.

There was one other finding worthy of pause: in some cases, when forage fish populations went up, predatory fish populations went down. A strange result for sure. It’s possible that forage fish feed on the planktonic juveniles of the predatory fish, reducing the amount that make it to adulthood, but we can’t know for certain.

Marine predators need protection, but reducing forage fish fishing isn’t the answer

Fishing can undoubtedly impact high-trophic level animals, but fishing less low-trophic level fish doesn’t seem to have the intended conservation effect. Instead, the authors offer three better suggestions to protect marine predators:

  1. Reduce bycatch and incidental mortality, a serious threat to both seabirds and marine mammals, through modifications to fishing gear or dynamic ocean management.
  2. Protect breeding sites by restoring habitat, removing invasives, and reducing human disturbance.
  3. Restrict fishing close to breeding sites.
Picture of Max Mossler

Max Mossler

Max is the managing editor at Sustainable Fisheries UW.

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