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American white pelicans who pause their migration at Strawberry Reservoir are filling their bellies with native species like Utah sucker for the most part, leaving cutthroat trout to the human anglers, according to new research from Phaedra Budy, Gary Thiede, Kevin Chapman and Frank Howe from the Quinney College of Natural Resources at Utah State Univerisity.
Strawberry Reservoir is one of the most popular sport fisheries in Utah and has considerable economic value for recreation purposes for the state. It has also become a refuge for migratory birds who traverse the deserts of the Great Basin along their path. Since man-made reservoirs are relatively new to the ecological landscape, researchers have to decipher how these introduced systems function on an ecological level, including understanding how birds interact with fish populations.
It’s expensive to stock and maintain cutthroat trout populations, and managers spend a lot of money and effort to keep stocked sportfish hatchlings alive and thriving. But over the last two decades, the abundance of cutthroat trout in the reservoir has varied — from a high of 464,000 adult fish in 2007 to lows of approximately 220,000 in 2012 and 2014. Beyond hatching and egg-to-fry survival, the primary culprits for cutthroat demise is predation by other fish, death by pelican, impacts from anglers (harvest and catch-and-release injury), disease and age. The group’s research sought to understand the impacts of the predator-prey relationships between fish-loving pelicans and cutthroat trout by examining what the pelicans ate.
Pelicans tend to eat local, for the most part. Over a two-year period, researchers found that pelican diets at the reservoir consisted of 85% Utah sucker, 6% Utah chub, 3% cutthroat trout, and 6% other prey. Utah sucker and Utah chub are abundant native fish whose expanding populations are a concern to managers, so the fact that birds use these fish as a staple food is good news. Diet samples collected from birds during the cutthroat spawning run contained more Utah chub (24%) and cutthroat trout (10%), but Utah sucker still comprised the majority of the birds’ diet even then. The number of adult cutthroat trout consumed by pelicans represented approximately 1% of the adult population of cutthroat trout in the reservoir, the research found.
“Cutthroat trout are fast swimmers and can outswim native chubs and suckers, and they stay too deep for pelicans when they are out in the open water,” said Budy, lead author on the research. “Pelicans eat what they can easily catch, and chubs and suckers are relatively slow swimmers and like shallow habitat where they are easy for pelicans to catch.”
The researchers also observed (anecdotally) that cutthroat trout tended to flee quickly when they sensed shadows of boats, while Utah chub and sucker loitered, apparently less concerned about what was going on above the waterline, she said.
Managers at the reservoir were also curious about the possibility of pelicans inhibiting trout from spawning. Pelicans sometimes form feeding “fences” — barriers at the edge of the reservoir blocking spawning tributaries, where they can easily catch fish in the shallows. The researchers found that this didn’t seem to be an issue at Strawberry Reservoir most of the time. Trout made it into spawning streams whether pelicans were present or not, according to data from electronically tagged fish. The researchers did find that on the days with the very highest densities of pelicans, trout travel could be delayed, and they identified a threshold for managers to intervene in order to avoid impacting trout populations long-term.
“Because pelicans are highly visible and congregate in large numbers at Strawberry Reservoir, anglers assume that they are eating tons of trout,” Howe said. “But the study shows that pelicans are not interested in the same fish species that are prized by human anglers. Knowing that the impact from pelicans to cutthroat trout is minor and short-lived will let managers focus on more important factors impacting trout populations at the reservoir.”
The pelicans actually seem to be doing managers a favor by removing competing native fish in far greater numbers than they could do themselves, and for free, Budy said. Meanwhile American white pelicans, a protected species, are getting a good meal out of it.