Tracking the mysterious striking northern sunfish

Northern sunfish and longear sunfish look like aquarium fish.

But they’re natives, albeit ones in a state of flux.

Until a few days ago, I had not heard of northern sunfish. Then Matthew Ignoffo posted about catching some over the weekend, “I honestly didn’t even know they occurred around here. They have to be in the conversation for prettiest fish anywhere.”

I thought the same and reached out.

“I was in a headwater creek in the lower reaches of the Kankakee River watershed,” he replied. “I love creeks, so I often wade less traveled stretches photographing native fish and mussels.

He “was using an ultralight setup hoping to get some later season colored up hornyhead chubs and I ran into the northern sunfish to my surprise. They hit a tiny 1/64th-ounce jig with shreds of a Gulp! Minnow.”

That led me on a colorful path, beginning with making sure it was northern sunfish.

“How can you not like a fish that has a color pattern that rivals or beats that found in most tropical fish species found in a pet store?” emailed Chris Taylor, curator of fishes and crustaceans for the Illinois Natural History Survey.

“Yes, that looks to be a northern sunfish,” emailed Trent Thomas, streams biologist for the Illinois Department of Natural Resources. “That red border on the ear (opercular) flap is a pretty good indicator. The ear flap also tends to be at an upward angle when compared to the longear sunfish ear flap.”

Philip Willink, fish biologist with the INHS, tweeted another difference, “Notice how it is sort of greenish-purple-blueish overall. Longears tend to be more yellowish-orange. Not a great or consistent character, but that is what I usually see.”

Ignoffo later sent a photo of a longear he and his son found in southern Illinois illustrating some of those differences.

As of 2004 northern sunfish were recognized as a separate species, Taylor emailed Previously, northern sunfish (Lepomis peltastes) was recognized as a subspecies of longear sunfish (Lepomis megalotis).

Taylor emailed that northern sunfish are confined largely to northern Illinois, “most frequently encountered in the Kankakee, Fox, and Des Plaines river drainages.”

Thomas covers the east-central watersheds and gave this breakdown of distribution: “I believe my jurisdiction is at the southern limit of the northern sunfish distribution in Illinois. They are common in the Iroquois River basin, being present in our surveys at 18 of the 25 sites in 2015. Northern sunfish outnumbered longear sunfish 2 to 1 (190 vs. 94) in these surveys.

“Northern sunfish are even more dominant in the Vermilion River basin of the Illinois River drainage that flows through Oglesby, Streator, and Pontiac. In our 2019 surveys of that basin, we collected 738 northern sunfish vs. only 3 longear sunfish. The three longear sunfish came from the city pond in Pontiac.

“Further south, the Mackinaw River basin produced mostly longear sunfish (79%) vs. northern sunfish (21%). I have yet to identify a northern sunfish in the Sangamon River basin, Vermilion River basin (Wabash River drainage), or the Embarras River basin. There is a remnant population of northern sunfish in the Lake Fork of the Upper Kaskaskia River basin, where I consistently find them.

“Northern sunfish and longear sunfish almost definitely hybridize, with the offspring looking more like one parent species or the other and making identification of individual sunfish from these basins where both species co-occur difficult and uncertain.”

That has led to challenges in identifying them.

“Their strong similarities to the longear sunfish just took them longer for the advanced tools and techniques available to taxonomists to recognize them as a separate species,” Thomas emailed. “They are certainly a more northerly-occurring species, occupying about the northern third of the state and absent further south. Northern sunfish also tend to be a smaller species than longear sunfish, with a maximum size around 5 inches.”

As to northern sunfish history, Thomas noted, “They have always been around. Even Forbes and Richardson in “The Fishes of Illinois” (1920) recognized them, saying `Northward this species [long-eared sunfish] grades into a smaller dwarfish variety…’ These researchers found `no specimen exceeding three inches.’ So, the larger sizes we are seeing today are most likely the result of hybridization with longear sunfish.”

That led Thomas to speculate, “Noting this, the longear sunfish, being the southern counterpart, may have the potential to fully displace northern sunfish populations in the state at some point as water temperatures continue to become warmer.”