An Undetected Fish Invasion

By Wes Ashe, Fisheries Resource Biologist, Belgrade Lakes Region

Fish invasions are often overwhelming to native ecosystems – altering the species assemblage, changing the insect community and forage base, and devastating native fish populations. Most of the time, fish invasions are obvious. Growing to gargantuan sizes, northern pike have forever altered the Belgrade Lakes. Voracious smallmouth bass have taken over northern portions of the Kennebec River and impacted wild brook trout and landlocked salmon populations. White catfish have saturated the lower Kennebec and Penobscot Rivers and likely far outnumber native catfish in some of those reaches. But sometimes fish invasions are subtle and go unnoticed for years, even decades.

In 2017, MDIFW Fisheries Biologists were assessing illegally introduced black crappie in Savade Pond (Windsor), a 62-acre water at the headwaters of the West Branch of the Sheepscot River. Using a 16-foot electrofishing boat along the weedy shoreline, quick pulses of electric current momentarily stunned Savade fishes in the shallow water. Not too long into the collection, a beefier looking sunfish splashed to the surface and was quickly scooped up. The fish wasn’t a crappie. It wasn’t a native pumpkinseed or redbreast sunfish either. After a minute or two of inspection and disbelief, biologists confirmed the fish species as a bluegill. Bluegill don’t belong in Maine waters, and despite their small size and unintimidating features, they are highly invasive.

By the time the electrofishing was finished, 2 pumpkinseed sunfish, 8 black crappie, and 30 bluegill were boated. Both pumpkinseeds, the only native fish netted that day, were returned to the water unharmed. The crappie and bluegill were brought back to the lab for analysis.

The next summer, biologists returned to the Sheepscot River to get a better handle on the distribution of bluegill in the watershed. What we discovered was alarming. Bluegill were collected and abundant in Turner Mills Pond (Somerville), Long Pond (Windsor), and Sheepscot Pond (Palermo). In Sheepscot Pond, a deep, oligotrophic water home to wild lake trout, salmon, and stocked brook trout, biologists were shocked at the findings. In just a few hundred yards of habitat, dozens of bluegill of all ages and sizes were amassed. They were everywhere. Pumpkinseed sunfish were not.

Bluegill and pumpkinseed are both members of the same family and genus. The two sunfish species look an awful lot alike, and most Mainers would just call them ‘a sunfish’. There are, however, a few features that distinguish the two. Pumpkinseed have a little red mark on the tip of their gill cover. Bluegill do not. Pumpkinseed have wavy emerald or blue streaks on their head. Bluegill do not. Pumpkinseed also have long, pointed pectoral fins, while bluegill pectorals are more rounded.

We have no idea when or where the Sheepscot bluegill populations originated. We know it was done intentionally or through negligence, illegally, and many years ago. No matter their source or objective, the larger and more competitive bluegill now greatly outnumber their closely related family member. The two are even hybridizing and their offspring (appropriately name “pumpkingills”) grow even larger and are more aggressive than both parents.

Until a few years ago, this bluegill invasion went largely unnoticed. There were no angler reports or Facebook posts. Biologists had no idea. Unlike a highly publicized and/or tangible pike or crappie infiltration, invasive bluegill have quietly impacted indigenous fishes in the Sheepscot River watershed for years. While this invasion has been more subtle than most, its outcome will be just as detrimental to the resident pumpkinseed population and the rest of the native ecosystem.

Do your part to help protect Maine’s fisheries. Never introduce any fish into Maine waters, it can alter the ecosystem forever and is illegal. This includes aquarium fish and plants. And remember, it is illegal to import any freshwater fish into the state of Maine without a permit from the Maine Department of Inland Fisheries and Wildlife.

Editor’s Note:

We received some questions about the likelihood of birds being responsible for this introduction.  We asked Wes for his thoughts:

“I did a little research on this and so have others. It seems bluegill eggs are slightly adhesive. That means they could potentially attach to birds and other critters that frequent waterbodies. But, there is no empirical evidence (i.e. scientific evidence) that water birds disperse fish eggs. Plus, the nearest bluegill watershed to the Sheepscot is somewhere in Region A – a long ways for a heron to fly with slightly adhesive eggs to remain and survive on its body. It is also plausible that the eggs could be dispersed unintentionally by humans via watercraft, minnow traps, etc., but again it’s unlikely they would survive the trip from a source water so far away. It is far more likely the original introduction occurred from an illegal and unpermitted stocking (whether that person knew how detrimental their actions were or not). Unfortunately, invasive bluegill (and other fishes) do not need to be transported by someone from a southern Maine watershed or another state. They can be ordered on the internet and delivered right to your door. That’s why it is so important for people to understand that just because you can buy something on the internet, it doesn’t mean it’s legal.”