Study: Wild brook trout do suffer from stocking
Finding confirms PF&BC’s program
Harrisburg — Recent research conducted by the U.S. Geologic Survey in New York shows that stocking brown trout into streams harboring wild brook trout is harmful to the native trout populations.
The finding, which confirmed the beliefs of fisheries biologists and other officials in the Keystone State and beyond, mostly validates stocking practices employed in recent years by the Pennsylvania Fish & Boat Commission.
USGS researchers found that direct interactions between brown trout and brook trout – such as competition for food – over time diminish brook trout populations.
Repeated stocking of brown trout in brook trout habitats can drastically decrease brook trout numbers, noted James McKenna, USGS scientist and lead author of the study
“There is great potential for brown trout stocking to reduce native brook trout populations,” he said.
Improper brown trout management could threaten vulnerable brook trout populations, according to the study, which is published in the North American Journal of Fisheries Management and available online.
The Pennsylvania Fish & Boat Commission, which has been stocking trout for more than a century, in recent years has been trying to protect vulnerable wild brook trout populations from interference from stocked browns, pointed out Dave Miko, the agency’s chief of fisheries management.
“Our approach has evolved over time, and we now take into account the species mix of what we stock and the wild fish that are present in systems,” he said.
“Actually, beginning in 1983 with Operation Future, the commission began to take a close look at the effects of stocking trout on top of wild trout, taking wild trout populations into account when developing stocking allocations.”
Of the 1,200 stream sections the commission now stocks, only nine of them have brook trout-only wild populations, and in seven of those, the agency stocks brook trout only.
Only in Double Run in Sullivan County and Moose Creek in Clearfield County are fish stocked over brook trout-only wild populations.
Double Run, a small Class B water, gets just 300 rainbows preseason. Moose Creek, which is a Class C water, is stocked both preseason and inseason with both brooks and brown trout.
There are many stream sections that have both brooks and browns reproducing in them, Miko explained. All of the wild brown trout streams at one time would’ve been native brook trout streams.
“Wild trout populations in streams can differ significantly,” he said. “A stream can have just a handful of wild trout or it can have 3,000 wild fish.
“So we are definitely going to continue to stock streams that have just a handful of wild fish in them to continue to provide opportunities for fishermen.”
Pennsylvania differentiates between wild trout stream sections: Class A waters are “the best of the best” having the most wild fish and water chemistry and are never stocked; Class B stream sections have thriving wild trout populations, but some are stocked; Class C and Class D wild trout waters have some wild trout and are usually stocked.
“We stock the lower classes of wild trout streams because they do not offer enough opportunities to anglers,” Miko said.
However several Fish & Boat commissioners the last few years have advocated not stocking Class B streams to see whether their wild trout populations might improve and expand and become Class As.
“We do know that stocking trout over wild fish can suppress wild trout populations,” Miko said.
“What this new study does is add some additional support to the belief that stocking of brown trout in general can displace wild brook trout,” Miko said, adding that brown trout tend to outcompete brook trout whenever the stream habitat is beneficial to both species.
“In higher elevations and headwaters streams, brook trout tend to have strongholds there and brown trout don’t infiltrate,” he said.
“That could be because of the acidic water chemistry of the headwaters and water temperatures are also a big factor. Some headwaters, in general, are colder than brown trout prefer.”
Brown trout can withstand temperatures up to 78 degrees; 72 degrees is the top temperature for brook trout.
“To me the USGS report confirms that we’ve been doing the right thing because we only have those two stream sections that are wild brook trout only that we have been stocking,” Miko said.
“We are always trying to balance providing opportunities for anglers with a resource that we are charged to protect, and we are very careful.
“I think we do a pretty good job of that – we stock 3.2 million adult fish.”