Certain Individual Fish Contribute Far More to Ecosystem Health than Others, Research Shows

Understanding unique contribution of highly active individuals could support sustainable fisheries management.

It seems intuitive that every human being has unique behaviors and this affects how they live, eat, and move. And while we might be able to imagine this for large creatures like bears or mountain lions, it is often much harder for humans to extend this idea of individuality to small fish like snappers or groupers.

photo of abaco island estuary
Research in mangrove-lined estuary on Abaco Island shows the importance of highly active individual fish in maintaining ecosystem health. Image credit Craig Layman.

It took us until 2017 to understand that individual behavior can drive the collective functioning of schools of fish, and until 2018 to say conclusively that fish feel conscious pain, though it may not be the same as the pain we feel. Now, a study published in Scientific Advances shows us not only that fish contribute in major ways to keeping their habitat healthy and nutrient-dense, but that some individuals within species are far more important when it comes to this. These ‘super-urinator’ individuals move around more, and spend more time foraging, which means they contribute more nutrients to their ecosystem through their urine and spread it more widely.

The study’s authors examined the behavior of 33 individual gray snappers (Lutjanus griseus) and 25 cubera snappers (Lutjanuscyanopterus) at a mangrove-lined estuary on Abaco Island in the Northern Bahamas. The fish, both of which are exploited tropical species, were surgically implanted with transmitters so that researchers could study their movements. Nine receivers were placed throughout the study site in the shallow estuary.

The study shows that “individual-level behavioral traits are poorly captured at the population-level” and that nutrient supply from individuals was different from the population average over 80 percent of the time. If accounted for, this individual variation would nearly double the estimated nutrient supply to their ecosystem.

The problem is that those engaging in fishing activities like spearing and angling are more likely to catch these active individuals. “Fishing specifically selects for these extra-important individuals, which in turn has disproportionately negative effects on the ecosystem,” Jacob Allgeier, lead researcher and assistant professor at the University of Michigan, said.

Through this research, Allgeier concluded that a definition of biodiversity which only focuses on contributions and impacts to ecosystems at a species level, and not individual animals, might not be the best approach to conservation.

Allgeier has been working on understanding marine ecosystems in the Bahamas and Haiti for more than a decade. In 2012, he realized that fish are perhaps the most important contributors of nutrients to their ecosystem, which they excrete through their urine and which algae and seagrass require for growth. In fact, their contribution is one of the largest sources in marine ecosystems, so much so that the amount of urine can impact how much and how quickly algae and seagrass grew.

In marine food chains, fish were long considered predators, feasting on plants and animals alike. But it turns out they also recycle the nutrients they consume, and provide growth fodder for the bottom of the chain.

In 2015, more research involving 102 different species showed that different fish transfer these nutrients to the water at different rates. These rates depend on what species of fish live in the ecosystem, due to the fact that different species tend to eat different foods, and how large they are.

Now we know that within each species, too, individual contributions can differ a lot, and that loss of certain individuals can have a bigger impact on ecosystem health. For example, computer models used in the study to simulate fishing show that removing the most active fish can decrease nitrogen supply in the environment by up to 69 percent.

While Allgeier is not advocating fisheries management based on fish pee, he does think it is a useful proxy to understand alternative ways in which fishing is affecting ecosystems, particularly in nutrient poor systems. “Ecosystem based management needs to consider the potentially cryptic impacts that species and individuals have on ecosystems,” he said.

That’s why Allgeier now wants to track more fish. At least 500 more at the Abaco island, which could show how they individually contribute to providing nutrients for their surroundings, and how important particular individuals are within these different species.

“We are basically scaling this up to study more species and individuals and to quantify the specific implications for ecosystem function,” he said.

It might take more work to study fish at the individual level. But if we know which individual fish, as well as which species, are the most important to keeping marine ecosystems nutrient-rich, it might just lead to more targeted management and more impactful conservation interventions.

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