Once Bitten, Twice Shy

If you’ve ever wondered how fish react to fishing pressure, a study on brown trout offers tantalizing clues.

To gauge different responses to angling pressure between heavily fished and relatively untouched populations, biologists measured the catch rates and various behaviors of brown trout in two backcountry New Zealand rivers.

The Ugly River is a wilderness stream accessible only by hiking 10 hours through untracked and difficult terrain. The second site was the Owen River, which is easily accessed by road and has high fishing pressure for a backcountry river. Both are 50 to 100 feet wide, provide the same habitat, and have similar densities and size structures of brown trout.

Let’s Go Fishing

Teams of guides and experienced fly anglers methodically fished similar stretches of each river on four three-day trips. In 12 days, 157 browns were caught on the Ugly River, 51 on the Owen. The fish represented 43 percent of the Ugly River’s estimated population of browns and 11 percent of the Owen’s population. Fish in both rivers averaged about 3.8 pounds.

Both the number of trout seen and hooked declined on successive days of each three-day trip on the Ugly, presumably because fishing pressure pushed the browns tight into cover. Researchers allowed two to four weeks to elapse between each three-day fishing trip, and noted that trout sightings and catches returned to high levels on the first day of each trip. In other words, fish responded to angling immediately, but the effects on trout behavior disappeared after two weeks.

On the more pressured Owen River, the numbers of fish seen and hooked were much lower on the first day and did not change during successive days of each trip.

Also, the trout in the Ugly River were less likely to be spooked, more likely to be caught on the first cast, and overall required fewer presentations. Fish in the Owen that did not spook were more likely to remain at their feeding station while anglers cast to them.

What It Means

Relatively unfished brown trout are less angler-shy and more vulnerable to capture. But the “virgin waters” effect is short-lived. As was apparent in the Ugly River, a single capture drastically changes a fish’s behavior.

Whether this response is learned or has a physiological basis—such as elevated levels of stress hormones or the depletion of energy reserves after capture—is not easily determined, but the effect lasts several days.

There was also a “chronic” effect of fishing in the Owen’s browns. The trout were less visible and less catchable. And, while fish occupying visible feeding stations were less likely to spook, they were also less likely to take a fly.

Do these findings apply only to brown trout in New Zealand streams? I doubt it. Fish in small, clear streams are highly vulnerable, but dozens of anglers fishing even a large lake or reservoir day after day can “educate” a lot of fish.

I commonly hear fisheries management colleagues who try to maintain large populations of quality fish lament the effect of “increasingly skilled anglers” and how technology has raised angler efficiency. There is little doubt that angler skill and efficiency is increasing, but we may be wrong in assuming that the fish don’t change, too.

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