Penn State researcher looking at impact of non-native trout on streams

University Park, Pa. -- Using a $35,000 grant from the state's Wild Resources Conservation Fund -- in a study believed to be the first of its kind in North America -- a researcher in Penn State's College of Agricultural Sciences is trying to determine if the millions of non-native trout stocked over decades for fishermen have affected the state's native aquatic organisms.

It's a question anglers and scientists have been asking for years -- what impact are all those rainbow and brown trout raised and planted by the Pennsylvania Fish & Boat Commission having on the state's stream life? In recent years, with exotic, invasive species of plants, animals, insects, fungi and viruses wreaking havoc on native species across the country, the issue has taken on a new urgency.

In Smoky Mountains National Park streams in Tennessee and North Carolina, a program is under way to eliminate brown and rainbow trout so native brook trout again will thrive unhindered by competition from exotic trout. In streams in America's West, brook trout and rainbows are being removed so native cutthroat and federally endangered bull trout again can become dominant. In light of these efforts, Caleb Tzilkowski decided to look at the situation in Pennsylvania.

"In a lot of our headwater streams, brown trout have displaced brook trout," says the Petersburg, Huntingdon County, doctoral degree candidate in wildlife and fisheries science. "A lot of us have wondered for a long time what effect non-native trout were having on our streams. I am trying to find out. This kind of research has been done in New Zealand and Japan, but we don't believe it ever has been done here before."

In his study, Tzilkowski, a long-time trout fisherman, is focusing on brown trout because rainbow trout rarely establish breeding populations. Brown trout, on the other hand, spawn in Pennsylvania streams and wild populations are common. It is not unusual for wild browns and brook trout to coexist in the same headwater stream. The wild brook trout likely have been there for thousands of years; the wild browns are descendants of stocked fish that were able to survive and reproduce.

"Brown trout get bigger and live longer than brookies," says Tzilkowski, who has a master's degree in ecology from Penn State. "When browns get bigger, say 12 inches or so, they switch their diet to fish. They can eat a lot of brook trout."

Tzilkowski's research, overseen by J.R. Stauffer, professor of ichthyology, utilizes 30 cages placed in Centre County headwater streams. Wild brook trout were placed in some cages, wild brown trout in others, and no trout in the rest. In the bottom rubble covered by each cage, trout food -- including invertebrate species such as aquatic insects and crayfish, salamanders, sculpins and minnows -- was inventoried. After three weeks, those organisms were inventoried again to determine what the trout had eaten.

Halfway through his two-year research project, Tzilkowski sees some trends. "After one field season, there is no detectable difference between what and how much similar sized wild browns and wild brookies eat," he says. "I don't expect to have conclusive answers because it will be hard to extrapolate this from the cages to the open streams. But it looks to me like the competitive advantage wild browns enjoy stems from them getting bigger to where they can eat a greater variety and larger foods."

Another aspect of his research has reassured him that stocked trout aren't immediately likely to threaten other non-game fish populations. A Penn State colleague is trying to replenish endangered shiner populations in the Brodhead Creek watershed in Monroe County. Trout are stocked each spring into the only stream (Marshalls Creek) where the endangered shiners still occur in the state, and because shiners are known to be excellent trout bait, Tzilkowski wondered how many of the endangered shiners stocked trout would eat.

Tzilkowski attempted to gauge what impact the stocked fish had on the shiner populations. What he found surprised him. "In the stomachs of the stocked trout we've examined thus far, we didn't find any shiners," he says. "In their stomachs we've found vegetation, bits of leaves and other stream debris. It seems as though stocked fish that are used to being fed eat most anything they can get in their mouths. Of course, some of the stocked fish may get acclimated to the streams and become efficient predators of shiners and other organisms later. In fact, it appears that large, naturalized brown trout in Marshall Creek do eat some of the endangered shiners."

Tzilkowski is concerned that Pennsylvania trout fishermen will misinterpret his research goals and be uncomfortable thinking of brown trout as an invasive species. "I sometimes worry about other anglers' reaction to our work," he says. "Many fishermen love brown trout, and I am not necessarily advocating their elimination here the way they are being removed from streams in the Smokies. But, when we are investing so much time and money trying to prevent the spread of invasive species, it makes sense to me that we should understand the effects of voluntarily spreading potentially invasive species around. Right now, the only effects we know that exotic trout have are the obvious ones, such as the displacement of brook trout. We have lots of wild brook trout and there are rare fishes in a lot of Pennsylvania streams. I'd like to keep it that way. It may be that we should stop stocking brown trout in certain watersheds.

"Hopefully my research will shed some light on that issue."