Archive for June, 2021

Jun 6 2021

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 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.”


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 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.

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 will 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. Though animals generally choose breeding grounds because of their resilience—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—extra protection of forage fish could reduce predatory fish populations. It is thought that forage fish feed on the planktonic juveniles of the predatory fish, reducing the amount that make it to adulthood.

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.

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Jun 1 2021

Ray Hilborn: MPAs aren’t the answer to ocean biodiversity, sustainability efforts

A global movement to create additional marine protected areas (MPAs) has been steadily gaining traction in recent years, with the initiative picking up milestone victories in the past few months.

In January, newly inaugurated U.S. President Joe Biden signed an executive order committing to a “30 by 30” goal, whereby the United States would designated 30 percent of its land and territorial waters to conservation by the year 2030. The move heightened the potential that MPAs will be used as a tool to tackle climate change.

A recent study supports the hypothesis that MPAs could be beneficial for climate change, maintaining biodiversity, and boosting the yield of fisheries. According to the study, strongly protecting at least 30 percent of the ocean – primarily in the 200-mile exclusive economic zones of coastal nations – would result in substantial environmental and commercial benefits.

But University of Washington Professor of Aquatic and Fishery Sciences Ray Hilborn told SeafoodSource that the study – and the concept of MPAs – are both flawed. The study, he said, made some assumptions and contains inconsistencies that effectively invalidate the conclusions it reached.

“It’s a classic example of where the peer-review process totally failed to identify inconsistencies, bizarre assumptions, and improper conclusions,” Hilborn said.

The study, he said, made different assumptions on different types of fishing effort.

“It happens that each of the assumptions they made about fishing effort is the one that makes MPAs look better,” he said.

A key example, Hilborn said, is how the study approaches trawling. The study made biodiversity calculations based on fishing effort shifting in geography as MPAs are put in place – which itself poses problems, he said. However, the study assumed that an MPA ban on trawling wouldn’t result in increased fishing effort in other areas.

“When it comes to the impact of trawling and the impacts on biodiversity, they assume when you close an area, the effort disappears,” Hilborn said.

The study found a ban on trawling in designated MPAs would have a carbon benefit – but that is true only if that trawling effort doesn’t move holds, Hilborn said.

“If you move the effort, the carbon benefit disappears,” Hilborn said.

Hilborn said the study also assumes an “instantaneous connection” between different species around the world – when in reality, species in separate oceans aren’t going to interact. And the analysis wasn’t actually global, as South Asia and Southeast Asia were not accounted for in the study.

“This isn’t a global analysis, because they don’t have trawl effort in Southeast Asia,” Hilborn said.

Protecting biodiversity is a key issue that needs to be tackled, and the core motivation behind MPAs and Biden’s 30 by 30 plan are sound, Hilborn said.

“[The] 30 by 30 [movement] is not ambitious enough,” Hilborn said. “We need to protect the biodiversity of 100 percent of our [exclusive economic zone].”

Protecting biodiversity in the oceans is not best accomplished via MPAs, especially in light of climate change, Hilborn said. In fact, while advocates have touted MPAs as a means to fight climate change, in reality, they do little to help, he said.

“They want to see 30 percent of the oceans permanently closed,” Hilborn said. “That’s absolutely the wrong thing to do. With climate change, things are shifting.”

Hilborn used the interactions between fisheries and the critically endangered North Atlantic right whale as an example of how a proposed MPA might not work as intended. In recent years, the species has been the center of an ongoing push for increased protections, and recently NOAA outlined new regulations to protect the species.

Climate change has forced the 400 or so remaining North Atlantic right whales to chase food sources that are now located in parts of the ocean with more fishing effort, primarily in the Gulf of Saint Lawrence. That movement highlights how MPAs would struggle to protect species in the ocean, Hilborn said.

“If you had closed areas to protect northern right whales 20 years ago, they’d be in all the wrong areas,” he said.

Protected areas on land, he added, make sense because of the nature of human interaction with the land.

“The reason you want parks on land is that human use is transformative. If you put a city on it, or you farm it, it’s gone,” Hilborn said. “In the ocean, fishing doesn’t really change the structure of the ecosystem. We don’t kill the plants which is what farming does, we don’t harvest the second trophic level, we just harvest the top of the food chain.”

Plus, many of the actual threats to the ocean aren’t coming from the ocean itself, or from fishing.

“If you look at what the threats to the oceans are, they’re ocean acidification, climate change, invasive species, various kinds of pollution, land runoff, and none of those are impacted by MPAs,” Hilborn said.

A great example is the large dead zone that forms in the Gulf of Mexico every year.  The dead zone is created by excess nutrient pollution from agricultural areas – mainly related to fertilizers washed into the gulf through the Mississippi River and other inland waterways. NOAA makes annual predictions for how large the dead zone will be, based on things like rainfall. An MPA in the area to protect that environment, Hilborn pointed out, would have no effect on the biodiversity of the ocean in the region.

“You could make it an MPA and ban everything, you could ban shipping, you could ban mining, you could ban fishing, and you’d have no effect on the dead zone,” he said.

Protecting biodiversity is possible, but MPAs are the wrong tool for the job, Hilborn said.

“You don’t need no-take in order to protect the biodiversity. Again, high profile things, marine birds, marine mammals, turtles, sharks, those are things where there’s very specific – gear specific – things that impact them,” he said. “Closed areas aren’t going to help, because they’re all so mobile.”

The solution for those species, he said, is simple.

“Take sharks or turtles – all you have to do is stop killing them,” he said.

Current fisheries management agencies already serve as a tool for protecting biodiversity, and Hilborn said additional effort can be made using those existing agencies.

“What I would like to see is very explicit targets in what are we trying to achieve in biodiversity, and for each one of those targets, what’s the best tool to achieve it,” Hilborn said. “In almost every case, you’re going to be modifying fishing gear, and how fishing takes place, rather than closing areas to all fishing gears.”

MPAs, he said, are essentially just regulating a few activities in an area, without addressing wider issues.

“Fundamentally, all MPAs are doing is regulating fishing, and maybe oil exploration and mining,” he said. “It’s just the wrong tool. The illusion that you’re protecting the ocean by putting in MPAs, it’s a big lie.”

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