Hatchery fish can be both easier or harder to catch

Angling success is higher when fishing for hatchery raised trout than it is for trout grown in a more natural setting, even when that more natural environment is created in a hatchery.

A recent study in Finland found that brown trout grown in a normal hatchery environment are more vulnerable to anglers because they have a greater tendency to explore and to grab at any food supply they see.

In the study, scientists created two rearing environments: a normal hatchery environment with concrete pens and a more natural or enriched hatchery environment that provided fish with cover, a rocky bottom and real insects during their early lives. Both types were fed pellets prior to release, but natural insects were also available in the enriched environment.

The hatchery fish reared in the normal setting grew faster and larger than the hatchery fish reared in the more natural setting, but they also lost weight and soon became hungry when released into the wild, making the trout more susceptible to angling. Trout grown in the enriched environment tended to be more cautious.

The researchers/anglers tested their hypotheses by releasing the fish into simulated natural environments in two batches and then fished for the trout using fly fishing gear and natural-looking artificial flies.

The ability to create hatchery environments aimed at angling success creates an ethical dilemma for fisheries managers, according to Anssi Vainikka, assistant professor in aquatic ecology at University of Eastern Finland, and leader of research on contemporary evolution of animal personalities and life-histories in harvested freshwater systems.

“We hope that the anglers and managers throughout the world would understand that the nature is the best teacher for the fish, and if we need to compensate for our detrimental actions in the wild by releasing hatchery fish, we should raise the fish in as natural environment as possible and use more actively their ability to learn before the release,” Vainikka said.

The researchers discovered that the trout from the enriched environment learned how to feed on natural prey (insects) and that was “of great importance in decreasing vulnerability to angling among domesticated trout,” the study says.

“It is well known that hatchery fish are vulnerable to predation due to lack of experience with predators,” Vainikka said. “Our results show that they are also vulnerable to angling because they do not know what to eat and, therefore, actively seek to try almost anything.

“For anglers, our results are a bit disappointing because they show that one doesn’t really need much skill to catch a hatchery fish,” he concluded.

The researchers believe that hatcheries could adopt more natural-like components, creating what they call enriched environments within a hatchery, in order to produce more angler-resistant fish. Such hatchery environments could train the fish to avoid anglers and predators before they are released to a stream or lake.

“We have also observed in multiple cases that the younger the released fish are, the more likely they learn the habits required in the wild,” Vainikka said. “Releasing large fish for put and take fisheries is good for the anglers, but the fish themselves have serious troubles in learning to feed and avoid predators and anglers, and could therefore be considered even unethical.”

The study’s primary finding was that brown trout that showed high activity or high exploration behavior right after release into a more wild environment were the most vulnerable to fly fishing.

In addition, the study surprisingly found that density of the fish is conducive to catching fish, at least when the angler is using flies that mimic natural nymphs. In other words, the fewer the fish in a given area, the more difficult to bring the fish to a hook.

“We studied the vulnerability of fish to fly fishing in different fish densities, and we assumed that the fish would learn socially to avoid [the] hook,” Vainikka said. “If this was the case, we should have caught the least fish from the tanks with most fish, but we observed the opposite. This is good news for anglers, because the trout appear to be the more difficult to catch the fewer they are.”

Image above: Photo Courtesy ODFW. Leaburg Hatchery was constructed in 1953 by the U.S. Army Corps of Engineers (COE) to mitigate for lost trout habitat caused by construction of Blue River and Cougar dams and other Willamette Valley projects. The hatchery is used for egg incubation and rearing of summer steelhead and cutthroat trout, and for rearing trout.

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