According to new research by Phaedra Budy, Gary Thiede, Kevin Chapman and Frank Howe of Utah State University’s Quinney College of Natural Resources.
Strawberry Reservoir is one of Utah’s most popular sport fisheries and has considerable recreational economic value to the state. It has also become a haven for migrating birds that cross the Great Basin deserts on their way. Since artificial reservoirs are relatively new to the ecological landscape, researchers need to decipher how these introduced systems work at an ecological level, including understanding how birds interact with fish populations.
It is expensive to stock and maintain populations of cutthroat trout, and managers spend a great deal of money and effort to keep stocked sportfish hatchlings alive and thriving. But over the past 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 around 220,000 in 2012 and 2014. Beyond the hatching and survival from eggs to fry, the main culprits for the disappearance of the cutthroat are predation by other fish, death by pelican, angler impacts (injuries during harvesting and catch and release), illness and age. The group’s research aimed to understand the impacts of predator-prey relationships between fish-loving pelicans and cutthroat trout by examining what the pelicans ate.
Pelicans tend to eat locally, for the most part. Over a two-year period, researchers found that the diet of pelicans at the reservoir consisted of 85% Utah sucker, 6% Utah chub, 3% cutthroat trout and 6 % other prey. The Utah sucker and Utah chub are abundant native fish whose expanding populations are of concern to managers. The fact that birds use these fish as a staple food is therefore good news. Diet samples taken from birds during spawning ground spawning contained more Utah chub (24%) and cutthroat trout (10%), but Utah sucker still made up the majority of the diet of birds. birds at that time. The number of adult cutthroat trout eaten by pelicans was about 1% of the adult cutthroat trout population in the reservoir, according to the research.
“Cutthroat trout are fast swimmers and can swim faster than native chubs and suckers, and remain too deep for pelicans when in open water,” said research lead author Budy. “Pelicans eat what they can easily catch, and chubs and suckers are relatively slow swimmers and like shallow habitats where they are easy for pelicans to catch.”
Researchers also observed (anecdotally) that cutthroat trout tended to flee quickly when they sensed the shadow of boats, while Utah chub and sucker loitered, apparently less concerned about what was going on. above the waterline, she said.
Reservoir managers were also curious about the possibility of pelicans preventing trout from breeding. Pelicans sometimes form feeding “fences” – reservoir edge barriers blocking spawning tributaries, where they can easily catch fish in shallow water. Researchers found that this didn’t seem to be a problem at Strawberry Reservoir most of the time. Trout arrived in spawning streams whether or not pelicans were present, according to data from electronically tagged fish. The researchers found that on days when pelican densities were highest, trout travel could be delayed, and they identified a threshold for managers to intervene to avoid having a long-term impact on fish. trout populations.
“Because the pelicans are so visible and congregate in large numbers at Strawberry Reservoir, anglers assume they are eating tons of trout,” Howe said. “But the study shows that pelicans are not interested in the same species of fish that are prized by human anglers. Knowing that the impact of pelicans on cutthroat trout is minor and short-lived, managers will focus on larger factors impacting trout populations at the reservoir.”
Pelicans actually seem to be doing managers a favor by taking out competing native fish in far greater numbers than they could on their own, and for free, Budy said. Meanwhile, American white pelicans, a protected species, get a good meal.