Destructive invasive algae threatens Gunpowder trout

The diatomaceous algae, Didymosphenia germinate, called Didymo or “rock snot” by anglers, is easily transferred by user groups (trout anglers, kayakers, waders, canoeists) to other streams. A single cell of Didymo can infect new waters.

The biggest user group threat is to trout anglers. Didymo and decisions by the DNR will change fishing habits, how to fish, the flies used and possibly even result in regulation bans of some fishing equipment.

Didymo has been around a long time, with records indicating infection in the United States as early as the late 1800s. The invasive has devastated western trout waters and New Zealand.

It was originally associated with cold, nutrient-poor waters but may have morphed into something different. It is now found in warmer waters and associated with tailrace fisheries, such as the Gunpowder, fed by water released from Prettyboy Reservoir.

Didymo is not like whirling disease, which directly affects trout to cause fry and fingerling to swim in circles. Whirling disease has disappeared or been suspended, since all infected hatchery trout were destroyed last year. Current studies on stream-caged sentinel rainbow fry will reveal the success of this in June, said Don Cosden, DNR chief of inland fisheries.

Didymo forms a thick yellow-, white- or cream-colored mat on the stream bottom. In extreme cases, it prevents trout from getting to bottom-dwelling insects — their groceries — such as caddis flies, mayflies and stone flies. It is not harmful to humans.

Charlie Gougeon, DNR Central Maryland head honcho for trout, noted that the DNR found algae mats some two weeks ago when checking streams for trout fry populations and spawning success. Samples collected by Susan Rivers, DNR macro-invertebrate and fish-health specialist, proved it to be Didymo.

In DNR strategy meetings earlier this week, plans were to provide disinfecting stations at parking lots and road crossing all along the Gunpowder from Prettyboy dam to Loch Raven. Educational posters are already up.

Proposals are to have cleaning stations filled with a 5 percent salt solution for anglers to wash their boots. Felt-soled boots are particularly bad since they hold cells that can be transferred to other streams.

Disinfecting stations are likely to be large versions of self-filling dog water bowls by which a large inverted container refills a basin when the water level becomes low. A one-minute salt-solution soak kills all Didymo cells.

“We think that the vector was an angler dancing around in infected streams and then fishing here,” said Gougeon.

Infected streams in New York, Vermont, West Virginia and southwestern Virginia are the closest to area streams.

Rock snot was not present in the Gunpowder during last fall’s electro-shocking studies and is expected to have arrived here sometime in January. The heaviest concentration is around the Falls Road and York Road areas, but it can be found down to Glencoe.

Rivers explained that currently there is no known way to rid streams of it once it arrives.

An additional concern is the possibility that trout fishermen can infect other area streams as they move around the state to fish. “We have to look at other areas,” said Rivers. She noted that since Didymo was found in the catch-and-release area, those areas on the North Branch of the Potomac, Savage, Youghiogheny, Casselman, Patuxent and Hunting Creek are being monitored.

Rock snot could affect stream insects available to trout and might entail a shift in fishing methods from typical nymphs, emergers and dries of mayflies, caddis flies and stone flies to smaller offerings of midges, Baetis and Tricos. These are able to get through the rock snot mat.

In addition, legal types at DNR are looking at the possibility of banning the use of felt-soled waders in the Gunpowder and other trout streams as a way of preventing Didymo spread.

Anglers can help by using a one-quart (one-pound) box of salt per five gallons of water to soak felt-soled boots at home to kill this damaging invasive. A suggestion for canoeists and kayakers is to carry salt and spread it liberally in the bilge water to kill Didymo cells, rinsing later to remove the corrosive effects of salt from any internal metal parts.