Pelicans prefer native fish to popular sport fishing reservoir

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Pelicans are picky eaters, say researchers who examined food choices made by fish-eating birds in a Utah reservoir.

According to new research from scientists at Utah State University, American white pelicans feast on native fish such as Utah sucker, but overlook game species such as trout cutthroat.

Strawberry Reservoir is one of the most popular fishing spots for anglers in northern Utah. But when migratory birds take flight from the deserts of the Great Basin, they linger on the man-made body of water – to go fishing.

Recognizing the economic importance of sport fishing to the state and the cost of stocking the reservoir with game fish, researchers had to determine how such a new feature in the landscape works ecologically. They also studied how pelicans interact with different fish populations.

In 2014, researchers placed solar-powered transmitters on pelicans to record their migratory routes, which saw them fly from Central America and Mexico to the western United States during the warmer months and then back. south for the winter. (Utah Division of Wildlife Resources)

“Cutthroat trout are fast swimmers and can outrun native chubs and suckers, and stay too deep for pelicans when in open water,” said Phaedra Budy, lead author of an article published in the Canadian Journal of Fisheries and Aquatic Sciences. “Pelicans eat what they can easily catch, and chubs and suckers are relatively slow swimmers and like [a] shallow habitat where they are easy for pelicans to catch.

Over the past 20 years, the count of cutthroat trout in the reservoir has varied from highs in 2007 of 464,000 adult fish to lows of around 220,000 in 2012 and 2014. Fish stocking and maintenance game are expensive. Meanwhile, cutthroat trout deaths have been blamed on predation by other fish, predation by pelicans, harvest and release by anglers, and age and disease.

To find out if pelicans are to blame, the team looked at exactly what these birds like to eat.

For the most part, pelicans eat local fish species. Over the two years of the study, scientists found that the pelicans’ diet consisted of 85% Utah suckers, 6% Utah chub, 3% cutthroat trout and 6% other prey. Because chubs and suckers are prolific native fish, with an expanding population of concern, it was good news for wildlife managers that pelicans preferred them.

Kevin Chapman releases an American white pelican. Analysis of pelican diets at Strawberry Reservoir in Utah showed that the vast majority of their diet included abundant native fish rather than sport fish such as cutthroat trout. (Frank P. Howe)

This underlines the research conducted by the university in 2016.

During the cutthroat spawning season in early May through mid-June, diet samples were collected from pelicans. The researchers found that the birds ate more Utah chub (24%) and cutthroat trout (10%), even though Utah suckers still made up the bulk of the birds’ diet. They also found that the pelicans only ate one percent of the adult population of cutthroat trout in the reservoir.

Cutthroat trout are a wary species, researchers say, because they move away quickly when they sense the shadow of boats. Utah chubs and suckers are not as wary or concerned about anglers.

Pelicans form feeding barriers or “fences” when they line up at the edge of a reservoir to block spawning tributaries as young cutthroat trout migrate to deeper water. Wildlife managers wanted to know if there were ways to prevent pelicans from picking up young trout when swimming in shallower water.

Migratory pelicans prepare to spend the night in a water reservoir before heading south on October 28, 2009, in Emek Hefer, central Israel. An estimated 500 million birds fly over the region twice a year on their annual migrations from Europe and Asia to Africa and back. Researchers at Utah State University have noted that pelicans form “fences” on the shoreline of reservoirs, blocking spawning tributaries. (David Silverman/Getty Images)

“Because pelicans are highly visible and congregate in large numbers at Strawberry Reservoir, anglers assume they are eating tons of trout,” said study co-author Frank Howe. But, he says, the study shows that pelicans and anglers aren’t interested in the same fish.

“Knowing that the impact of pelicans on cutthroat trout is minor and short-lived will allow managers to focus on larger factors impacting trout populations in the reservoir,” he said.

Howe and his colleagues found that trout from Strawberry Reservoir swam in spawning streams even when pelicans were around. But the days with the highest pelican densities on record also saw delays in the trout journey. Therefore, scientists have developed a threshold number of pelicans that wildlife managers will use as a general rule for dispersal to save trout in the long term.

By eating native chub and suckers, pelicans can eliminate competing native fish in far greater numbers than wildlife managers can do on their own and at no cost, Budy said.

Game fish at Strawberry Reservoir include sterilized rainbow trout, cutthroat trout, kokanee salmon and crayfish. In 1990, the reservoir was poisoned to eliminate all aquatic life in an effort to eliminate so-called “trash fish” such as the Utah chub. However, soon after, Utah chubs reappeared. Cutthroat trout was introduced to precede chub. Currently, there are strict catch and release regulations for game fish.

Another challenge has been the unhealthy level of phosphorus introduced by the Strawberry River due to cattle grazing along its banks. A grass and tree planting program was completed in 2019 to improve wildlife habitat and reduce phosphorus, although grazing was still permitted.

Edited by Siân Speakman and Kristen Butler

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