Rock snot is in Pine Creek

Waterville, Pa. — The recent discovery of rock snot in Pine Creek – the famous northcentral stream that cut the Pennsylvania Grand Canyon – brings with it the realization that  the invasive algae now spans the state.

Last year it was found in the upper Delaware River, that great wild-trout fishery in the northeast, and in the Youghiogheny River in the southwest. (It was also subsequently found in Dyberry Creek in the southwest.)

What that means for the state is not clear, but fisheries biologists fear the organism, properly called Didymosphenia geminata – or didymo for short – because unlike most other algae it thrives in the clean, cold water of trout streams.

Its disgusting  nickname comes from its slimy, greenish-tan appearance. When squeezed nearly dry, the algae has the feel of moist cotton or wool. It can literally blanket the bottom of a stream.

Biologists with the state Department of Environmental Protection discovered didymo in a sample collected in late June from Pine Creek, upstream of Waterville near the Hamilton Bottom Canoe Access Area. The sample was collected as part of routine stream monitoring.

A spokesman for the department noted that the biologists did not find “any evidence of a full bloom of didymo in the creek or nearby waterways.”

Although wonderfully scenic Big Pine, which is very popular with canoeists and kayakers, is heavily stocked with trout by the Fish & Boat Commission each spring, it is not truly a trout stream. It gets so warm by July 4 most years the surviving salmonids either die of flee its tepid currents up cooler tributaries.

So fisheries scientists aren’t sure didymo could take hold there the way it has in the upper Delaware, a cooler flow coming from bottom-release reservoirs, where it thoroughly covers the bottom in places.

“It is a pretty heavy mat of algae on the bottom to the point where you can’t drift a wet fly or nymph and not get fouled,” John Arway, executive director of the Fish & Boat Commission, said of the upper Delaware.

“But it is not like the pictures you see from New Zealand streams that are clogged with didymo. Still it is heavy enough in the upper Delaware that it interferes with fishing.”

Just how big a threat didymo is in Pennsylvania is not clear, even to Arway.

“When you put it in perspective – comparing it to other threats to the state’s fisheries – for example, it pales by comparison to acid mine drainage. AMD is a huge, major threat because it can eliminate all aquatic life in a watershed if a coal mine goes bad.”

Didymo, is not considered a  human health risk, but it can impact stream habitats and sources of food for fish, and make recreational activities unpleasant. According to Arway it can cause ecological damage by smothering other organisms that also live on the streambed and support the food web for the resident fish community.

Invasive species pose a threat to either recreation, ecology or economics, he noted, and in this case we know that rock snot can pose a threat because of the infestations we’ve seen in waters from New Zealand to the upper Delaware River.

“Our biggest fear is that didymo will spread to our mountain brook trout streams,” Arway said. “We are not really sure about where it’s going to do well or where it won't do well.”

The good news for heavily forested wild-trout streams in the mountains is that they are mostly dimly lit.

“Because didymo depends on sunlight, it won’t do well in a heavily shaded mountain brook trout stream,”he said. “It won’t thrive even though the temperatures are optimum. So it is a question of light, water temperature and nutrients.”

But didymo has a fairly wide range of temperatures and can survive it in water from about 39-80 degrees F., Arway pointed out. It does best at about 48 degrees F – way colder than Pine Creek, for instance, usually is.

So didymo “does not seem to be a super threat in Pennsylvania,” Arway believes.

“Even where it occurs it really hasn’t been shown to decimate trout populations, it just makes it difficult to fish for trout,” he said.

“For example, in the upper Delaware you can still catch fish and there’s still enough food in the river for the trout to live, but it is a real nuisance to fish or recreate in – whether it’s kayaking, river rafting or fishing.”

It can have some ecological impacts to other animals that require light penetration to the stream bed to live, so didymo can cause the whole ecological character of a stream to shift.

“But we haven't really seen any evidence that it will negatively affect a fish population.”

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