According to new research by Phaedra Budy, Gary Thiede, Kevin Chapman and Frank Howe of the Quinney College of Natural Resources at Utah State University.
Strawberry Reservoir is one of Utah’s most popular sport fisheries and has significant recreational economic value to the state. It also became a haven for migratory birds as they traversed the Great Basin deserts. Because 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.
Stocking and maintaining ruthless trout populations is expensive, and managers spend a great deal of money and effort keeping stocked sportfish alive and thriving. But in the last two decades, the abundance of wild trout in the reservoir has changed – dropping from 464,000 adult fish in 2007 to 220,000 in 2012 and 2014. The main culprits beyond hatching and survival from egg to fry are predation by other fish for loss of throat, death by pelican, angler effects (harvest and injuries during catch and release), disease and age. The group’s research aimed to understand the implications of the predator-prey relationship between fish-loving pelicans and throated trout by examining what pelicans eat.
Pelicans tend to eat mostly locally. Over a two-year period, the researchers found that the diet of pelicans in the reservoir consisted of 85% Utah sucker, 6% Utah chub, 3% throat trout, and 6% other prey. The Utah sucker and Utah mullet are both abundant native fish whose expanding populations are worrying managers. The birds’ use of these fish as a staple food is therefore good news. Dietary samples from birds during spawning included more Utah sweetbread (24%) and throated trout (10%), although Utah sucker still made up the majority of the birds’ diet at that time. According to the study, the number of adult cutthroat trout eaten by the pelicans was about 1% of the adult cutthroat trout population in the reservoir.
“Tree trout are fast swimmers and can swim faster than native mullets and suckers, and stay too deep for pelicans in open water,” said Budy, lead author of the study. “Pelicans eat what they can catch easily, and mullets and suckers are relatively slow swimmers and like shallow habitats where pelicans can catch easily.”
The researchers also observed (anecdotally) that ferocious trout tended to flee quickly when they sensed the shadow of boats, while the Utah freshwater snapper and sucker saunter ostensibly less concerned about what was going on.
Reservoir managers were also curious about the possibility that pelicans could prevent the trout from breeding. Pelicans sometimes form feeding “fences” – barriers at the edge of the reservoir that block their spawning arms, from which they can easily catch fish in shallow water. The researchers found that this doesn’t seem like a problem in the Strawberry Reservoir most of the time. According to data from electronically tagged fish, trout reached spawning streams with or without pelicans. The researchers determined that trout travel may be delayed on days when pelican density is highest, and set a threshold for managers to intervene to avoid a long-term impact on fish and trout populations.
“Because the pelicans are highly visible and congregate in large numbers in the Strawberry Reservoir, fishermen assume they eat tons of trout,” Howe said. “But the research shows that pelicans are not interested in the same species of fish that human anglers value. Knowing that the impact of pelicans on cutthroat trout is small and short-lived, managers will focus on the larger factors affecting trout populations in the reservoir.”
Pelicans actually seem to be doing managers a favor by removing competitive native fish for free, in greater numbers than they could do on their own, Budy said. Meanwhile, American white pelicans, a protected species, eat well.