The Catch-and-Release Trap: How Sportfishing Is Silently Endangering Giant Trevally

Researchers studied the impact of sport fishing on giant trevally, revealing that catch-and-release practices, while preserving fish in the short term, can alter fish behavior and movement over time.

Their findings suggest that sustainable fishing policies are needed to manage the effects of fishing pressure and maintain a healthy giant trevally population.

Impact of Catch-and-Release on Giant Trevally

To protect vulnerable fish species, some sport fishing practices use catch-and-release, where fish are returned to the water after being caught. However, frequent fishing can still impact fish behavior and threaten the sustainability of fisheries. An international team of biologists, led by the University of Massachusetts Amherst and the University of South Florida, partnered with conservationists and industry experts in Seychelles to investigate this issue for the first time.

Their study, published on December 12 in the Canadian Journal of Fisheries and Aquatic Sciences, found that the giant trevally, a prized sport-fishing target, is sensitive to intense fishing pressure. This insight emerged from a unique “natural experiment” triggered by the COVID-19 fishery closure, offering important guidance for managing the species across its range in the Indian and Pacific Oceans.

Sustainable Sportfishing Initiatives in Seychelles

“A few years ago, both the Island Conservation Society, which promotes conservation in the Seychelles Islands, and the Alphonse Fishing Company, (now known as Blue Safari Seychelles Fly Fishing) a flyfishing guide service located in the Seychelles, approached my group concerned about the sustainability of sportfishing in the Seychelles,” says Andy Danylchuk, professor of fish conservation at UMass Amherst and the paper’s senior author. “It was the perfect convergence of a research question that my team cares deeply about — how recreational fishing can be a sustainable part of the ‘blue economy’— brought to us by stakeholders looking to implement policies and management guided by science.”

As opposed to harvest-orientated recreational fishing whose effects can be more easily gauged in terms of population numbers, catch-and-release should theoretically result in less mortality, meaning there should be more fish in the water for the anglers who are eager to catch them.

Researchers partnered with conservationists and industry partners in order to better manage the sought-after game fish. Credit: Alphonse Fishing Company (now Blue Safari Seychelles Fly Fishing)

Challenges of Catch-and-Release Practices

However, there are a host of unintended consequences that may be associated with catch-and-release, which can be difficult to quantify. These include increased mortality from hooking a fish in critical locations, such as the gills, post-release predation, altered movements, and “hook shyness,” which occurs when some fish in a population are so used to being targeted that they no longer bite at the lures or flies anglers cast to them.

Hook shyness can be especially concerning with a species like the giant trevally, a “bucket list” prized sport fish for many anglers who are willing to spend thousands of dollars on a once-in-a-lifetime trip to catch them. If catch rates decline because of hook shyness, anglers can become disappointed, which can ultimately affect the local tourist industry. In some locations, to protect against such a loss of income, local guides may resort to baiting the fish with chum in order to lure them closer to their paying customers — something that can exacerbate the problem of altering the behavior and movement patterns of giant trevally.

Assessing Giant Trevally Fishing Practices

“The Island Conservation Society and the Alphonse Fishing Company wanted to know whether or not the current fishing practices for giant trevally in the Alphonse Group of islands in the Seychelles was sustainable,” says Lucas Griffin, the paper’s lead author who completed this research as part of a post-doctoral fellowship at UMass Amherst and is now an assistant professor of marine biology at the University of South Florida. “And so, we’ve been working with them on a series of studies to assess just that.”

The first, published in Ocean and Coastal Management in 2021, charted out what the giant trevally fishery looked like and how the Island Conservation Society and Alphonse Fishing Company have successfully managed it. The second, published in Fisheries Research in 2022, showed that catch-and-release has a minimal impact on the short-term activity and survival rate of giant trevally. “Our most recent published paper pulls more of the pieces together to shed light on whether angler presence alters the behavior of giant trevally,” says Griffin.

Insights from a Natural Experiment

To conduct their research, they relied on a combination of acoustic telemetry (where a transmitter is surgically implanted in the giant trevally and receivers are placed throughout the coastal environment to rack individual fish as they move), and a mark-and-recapture program (where the team worked with anglers to implant microchips similar to what you’d put in your pet cat or dog that could be used to record how many times, and in which locations, individual fish were caught).

They had initially planned to work with the Alphonse Fishing Company and Island Conservation Society to use short-term closures of different fishing spots to see what happened to the giant trevally under “natural” conditions — and then COVID essentially shut recreational fishing down altogether for nearly two years. “It’s strange to say, but the pandemic created a once-in-a-lifetime natural experiment,” says Griffin. “The entire fishery shut down, so we could really see how giant trevally live in the total absence of fishing pressure.”

Behavioral Adaptations and Ecosystem Balance

The team found that, while giant trevally can adapt their behaviors and movement patterns to fishing pressures, they don’t react to short-term closures — even those as total as the one during COVID. Furthermore, their data shows that over time, giant trevally may indeed become hook shy, and they point to evidence that some groups of giant trevally have shifted their home ranges away from prime fishing spots. Conversely, the mark-and-recapture program is showing a considerable population size of giant trevally, and that new giant trevally may be entering the fishery which may offset any issues of hook shyness.

Taken together, all of this evidence points to the conclusion that although giant trevally may be resilient to the acute effects of catch-and-release, they may indeed be sensitive to the overall extent of fishing pressure and that putting more anglers on the water may exacerbate the problems of hook shyness. A healthy ecosystem and high abundance of giant trevally may make fish more resilient to the effects of angling.

Towards Sustainable Angling Practices

“We are part of the ecology of these fish,” says Griffin. “If we want to be catching more of them, we may need to consider limiting the number of anglers that target them in any given location.”

Reference: “Beyond the hook: do angler-fish interactions in a catch-and-release recreational fishery modify fish space use and catchability?” by Lucas P. Griffin, Gail Fordham, George Curd, Pierre André Adam, Christopher Narty, Keith Rose-Innes, Devan Vd Merwe, Eleanor Brighton, Sascha Clark Danylchuk, Caitlin McGarigal, Marin A.W. Allen, Steven J. Cooke and Andy J. Danylchuk, 12 December 2024, Canadian Journal of Fisheries and Aquatic Sciences.
DOI: 10.1139/cjfas-2023-0329

Funding for this research was provided by the Seychelles Conservation and Climate Adaptation Trust, the Alphonse Foundation and Bonefish & Tarpon Trust, while in-kind support was provided by Blue Safari Seychelles, Blue Safari Seychelles Fly Fishing (formerly Alphonse Fishing Company) the Islands Development Company, the Island Conservation Society, the Seychelles Fishing Authority, Patagonia Inc., Thomas & Thomas Fly Rods, Cortland Lines and Nautilus Reels.

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