Maine’s Thissell Pond “Reclaimed” from Invasive Species
January 24, 2018
The following article was provided by the Maine Department of Inland Fish & Wildlife and written by Regional Fisheries Biologist Tim Obrey
In 2012 and 2013, the Maine Department of Inland Fish & Wildlife performed the first chemical reclamations in the Moosehead Lake Region since 1984, when Sawyer Pond was reclaimed after white perch were illegally introduced. A reclamation is usually the last option—the nuclear option—on the list of fisheries management techniques. We devote our careers to protecting and enhancing fisheries and aquatic habitat, so taking things to this level is often the last thing we want to do. But there are a few occasional cases when hitting the “reset button” is the best alternative for long-term success.
We follow strict state/federal guidelines and utilize a piscicide called rotenone that only affects gill-breathing organisms. Some fish species are more susceptible to rotenone, so by using precisely calculated concentration rates, we can target certain invasive fish species to restore a healthy ecosystem.
In 2013, we did just that at Thissell Pond. This pond was first reclaimed in 1962. At that time, there were few game fish in the pond, but suckers and other minnows were abundant. After the initial reclamation, the pond was stocked with brook trout and a very nice fishery developed. In fact, by 1980, the hatchery trout had made use of the prolific springs in the pond for spawning sites and became a self-sustaining population. We no longer needed to stock the pond. These events do not happen often. In most cases, annual stocking is required to maintain fishing. Unfortunately, smelt were illegally introduced into the pond in the 1980’s, probably by a well-meaning angler who thought it would help the fishing. It did not.
Instead, the adult smelt utilized these same springs in the pond for their spawning in April and May, just as the trout fry were hatching. I don’t know if you’ve ever looked closely at a smelt and their eating apparatus, but they look like a mini-barracuda and they love to eat small fish. Newly hatched trout fry probably taste pretty good after a tough day at the spawning site. Needless to say, the brook trout population went into serious decline.
We stocked large numbers of splake for a few years in an attempt to eliminate the smelt. It almost worked. The splake initially decimated the smelt population and there were some champion splake in the pond. But a few smelt survived, and they quickly re-established and there were just a few wild brook trout remaining. Ultimately, it was determined that there was no way to restore the wild brook trout with the presence of smelt in this pond.
On a warm day in August, we deployed the rotenone and reclaimed the pond. We saw a few dead trout, fewer splake, and more smelt than we could count. Windrows of smelt washed ashore over the next few days. The chemical breaks down quickly in warm weather and we kept a close eye on the pond over the next few weeks.
We saw plenty of insect activity and the crayfish survived. This was very positive because crayfish were an important part of the diet for brook trout in Thissell Pond. By September the pond was clean and clear and eerily quiet.
The pond silently restored itself over the coming months and the total lack of fish allowed the native invertebrates to reestablish and become abundant. We stocked some very small fingerling brook trout the following October, over a year removed from the reclamation.
These first-generation wild strain fish, which originated from nearby Sourdnahunk, weren’t small; they were miniscule by comparison to normal hatchery-strain fish of the same age and we were worried about their ability to survive. But apparently size didn’t matter. When they hit the water, they were ready to belly up to the buffet table that was Thissell Pond. We had reports the next summer that some of these fish exceeded 12”. They are now older and still growing rapidly. We have stocked the pond each fall with fish from the same source.
MDIFW staff and the camp owners had previously constructed artificial trout spawning boxes in the pond to improve the amount of brook trout natural reproduction. This summer, we went to the pond to assess the condition of those boxes, which would be important for the establishment of a self-sustaining trout population. Much to our surprise, we found young wild brook trout that had obviously been spawned in the pond that spring. They were just a few inches long and were hanging on the edges of one of the spring-fed spawning boxes.
The angler reports from this summer were very encouraging, and the presence of natural reproduction is a great sign. We plan to continue to stock the pond with Sourdnahunk strain fingerlings until we can document sufficient recruitment to maintain the fishery, but clearly this pond is well on its way to a full recovery.