When most anglers think of species invading the Great Lakes, they think of gobies or lamprey. Few of us are worried about flatheads, but maybe we should be. Flathead catfish are native to the central United States, in between the Appalachians and the Rockies. They are a mainly river species that historically inhabited the Mississippi River and its tributaries. Now, however, flatheads are making their way into the Great Lakes, including Erie, Huron, and Michigan. Researchers from various states are digging in to figure out why.


One of the first invasion routes the researchers analyzed were authorized stocking events by federal and state agencies. In Ohio, only one stocking event happening in an Erie tributary; the DNR stocked the Huron River in 1968, but there was already a native population in the river. Michigan stocked flatheads more frequently, but only in six inland lakes where escapement was unlikely. Other bordering states have no such records. As such, it is more probable that the flathead migration was caused by other events.


Canals to the Great Lakes were a powerful catalyst for aquatic migration in the early and mid1900s. In fact, they’ve been the primary or secondary vector for over 140 introduced species. Upon closer inspection, though, the researchers concluded that the canals would only have been a route for relatively small numbers of flathead catfish. The canals were full of pollution, had great numbers of locks that were effective blocks to fish passage, and often closed due to lack of maintenance. The few flathead catfish that travelled through them would have been the exceptions to the rule.


The biggest culprit in the invasion seems to be unauthorized releases. Aside from one possible case of individuals escaping from an Ohio hatchery into the Maumee River, a tributary of Lake Erie, most of these transplants are made by anglers. Catfish are one of the most common fish to be removed from one body of water and stocked illegally in another body of water, usually where they aren’t native.


At least 20 sites in the Great Lakes watershed have been flagged as places where unauthorized movement of flathead catfish has occurred. In reality, this is just a fraction of the sites where illegal stocking is happening, but it’s impossible to get a concrete number because of the covert nature of the act.


Once the flatheads are in a waterway connected to the Great Lakes, natural dispersal takes care of the rest. Individual cats are highly variable in how often and how far they travel, but they all are capable of travelling hundreds of miles. If there are no impediments like low water levels or dams, one catfish can pass into a new watershed quickly and with no exceptional energy expenditure. Most of this migration occurs during the summer months, when relatively high water temps allow these cold-intolerant fish to move. Luckily, this cold-intolerance has kept flatheads out of Lake Superior so far.


Like all introduced species, flathead catfish upset the balance of the Great Lakes ecosystems. These apex predators are able to put a decent dent in native fish communities, especially in bullhead populations. While most anglers regard bullheads with disdain, the various bullhead species fill an important ecological niche as both predators and prey. The decimation or removal of bullhead populations could topple entire aquatic communities, especially because the Great Lakes fish communities are already struggling against other invasive species.


Unfortunately, the invasion is expected to continue. Once started, migrations like this are difficult to stop. Although colder temperatures will limit how far and how quickly flatheads can travel, climate change is likely to remove this barrier in the future, too. Scientists are hopeful that Asian carp mitigation structures might slow the movement of flatheads as well. For now, more research needs to be done on the extent of their effect on Great Lakes communities and how to reduce it.


Originally published in Outdoor News Minnesota.

P.L. Fuller, G.E. Whelan. The flathead catfish invasion of the Great Lakes. Journal of Great Lakes Research 44 (2018) 1081-1092.