Important rainbow trout strain still detects predators despite hatchery life

By on June 12, 2015
The researchers studied the effects of chemical cues on fingerling Hofer strain rainbow trout. (Courtesy: Christopher Kopack)

A disease-resistant German rainbow trout strain was a lynch-pin for the species’ recovery in Colorado after whirling disease devastated rainbow populations in the state’s streams, according to Colorado Parks and Wildlife.

The German rainbow, or Hofer strain, grows quickly and baffles the whirling disease parasite, making it a prefered strain for the state’s hatcheries. But Hofers also have a hard time staying alive once they’re out of the stocking truck.

Christopher Kopack, a research associate in Colorado State University’s Department of Biology, said that studies of predator gut contents in systems where Hofers were stocked found the strain was a prefered food source.

“Some unpublished data has actually shown that this particular strain can disappear from a water system within a month,” Kopack said.

But the state has had success with Hofer crosses, which mix disease resistance with the sturdiness of other strains and have recently been found reproducing naturally. Anglers are also reporting good rainbow catches in many of the state’s big rivers, including the Arkansas and Colorado.

Some biologists are still left wondering why the original Hofers fare so poorly in the wild. Kopack said he and his colleagues were curious whether the strain had lost its natural abilities to detect and avoid predators after so many generations living the easy life in the hatchery setting.

They tested that in the lab using a relatively new technique of exposing fish to chemical cues and tracking how fish respond. They acquired some Hofer rainbows from a state hatchery and exposed them individually to one of two chemical cues, or a combination of both. One chemical was an alarm cue extracted from other rainbow trout that they’re thought to release when a threat is nearby. The other was a chemical released by brown trout, an important predator of stocked rainbows in Colorado stream, that prey species can use to detect when the browns are close.

The results, published in the journal Fisheries Research, showed that even the highly domesticated Hofers still have an innate ability to detect predator cues and change their behavior accordingly. They spent less time swimming and exploring and more time frozen.

The trout were observed in tanks in the lab. (Courtesy: Christopher Kopack)

The trout were observed in tanks in the lab. (Courtesy: Christopher Kopack)

The researchers thought that the trout might respond more strongly to a mix of both the predator and alarm cues, but that turned out not to be the case. One possible explanation for that is that the trout may have been responding as strongly as they could to the individual cues, so the fish exposed to both couldn’t show a stronger reaction.

So if Hofers can still detect predator cues, why can’t they stay alive in streams? According to E. Dale Broder, doctoral candidate with Colorado State’s Department of Biology, responding to a chemical cue in the lab is different than actually avoiding a hungry brown trout in the wild.

“It’s thought that a lot of the actual predation could occur in the first or second encounter with a predator,” said Border, co-author of the study. “So there is just not opportunity for them to learn.”

But there is some thought that some learning could happen in the hatchery. If the young Hofers are exposed to chemical cues while they’re still being reared, that might leave them a little better prepared the first time they encounter a predator in the wild.

“A hatchery fish could learn to recognize, for example, that this predator pheromone is dangerous,” Broder said. “So post stocking, they would initiative these antipredator behaviors very quickly, as soon as they recognized the predator pheromone.”

The researchers are still studying how well that kind of predator training could actually work. They’re also looking into how introducing environmental variables into the hatchery setting — like structure and different flows and temperatures — could make fish more fit for the wild.

These studies could benefit other species as well. If additional experiments with rainbow trout go well, the work could be applied to help species of conservation concern like the Arkansas darter, Kopack said.

Whirling disease, caused by a spore that deforms fish spines and causes them to swim in circles, came to Colorado in 1986 when a private hatchery accidentally imported infected fish that were stocked across Colorado. The disease was widespread by the 1990s and natural reproduction of rainbow trout had more or less ceased. But the resistance of the Hofers and 20 years of work from Colorado Parks and Wildlife has the species looking stronger than it has in years in the state.

“It’s been a long road, but bringing back populations of fish that were essentially extirpated from Colorado can only be called a huge success,” said George Schisler, the department aquatic research team leader, in a release.

Top image: The researchers studied the effects of chemical cues on fingerling Hofer strain rainbow trout. (Courtesy: Christopher Kopack)

About Jeff Gillies

One Comment

  1. Pingback: Press about our recent paper | Dale Broder

Leave a Reply

Your email address will not be published. Required fields are marked *