ASIAN CARP, SCHMASIAN CARP! – Meet the Meanmouth

I’ll never forget the first fish I ever caught on the St. Croix River. I was seven years old. My dad and I were trolling one warm summer evening. He kept our seven-and-a-half-horse Evinrude putt-putting along at its slowest possible speed. Still, my little Mirro-lure vibrated more than swam as it bucked the current.

I got a strike. It could have been any of seven or eight species of game fish commonly caught on lures in the St. Croix. Whatever it was, at that tender age I was sure it was a monster. After a minute or two of awkward pulling and reeling I lifted the splendid barely-one-pound specimen out of the water.

The fish’s glistening flanks were surprisingly dark. The sun, about to settle into the bluff treetops on the Minnesota side, brought out olive-gold highlights and muted vertical stripes. I thought it was the most beautiful, exotic creature I’d ever seen. I knew it was a bass, but my dad, grinning proudly, made sure I knew exactly what kind of bass. “That, son, is a keeper!—a beautiful St. Croix smallmouth black bass.”
     I still caught a few bass. But this time, some-
     thing was radically, shockingly, different.

UPS AND DOWNS

Smallmouth black bass (Micropterus dolomieu)

In 60-plus years since that magical moment, I've caught at least a thousand more smallmouth black bass in the St. Croix. It’s the species the river is best known for, perfectly suited to its clean, cool currents, sandy-to-gravely bottom, rocky crags, eddies and deep pools—one many credit as, pound for pound, one of the scrappiest game fish in the world.

Many of my favorite fishing spots lie along a secluded slough that diverts for a mile or so into the Wisconsin-side woods before rejoining the river. I'd been noticing that for some years, with the water level reaching its customary late-summer low earlier and earlier, it’s been getting harder to float into it after late June.

One early-July day a few years back, I found the slough already cut off from the river's flow. I had to drag my canoe over sandbars and through muddy shallows to get in. The backwater’s now-stagnant waters were murky, and weeds I'd never before seen on the river thrived.

I was concerned about whether the bass would still be there—usually only carp and a few intrepid pike tolerate these unappealing waters. Surprisingly, I still caught a few bass. But this time, something was radically, shockingly, different.

Largemouth black bass (Micropterus salmoides)

These were all largemouth bass. Or at least they seemed so to my unskilled eye. The extension of the jaw's upper, maxillary, bone to a point clearly behind the fish's eye, and a dark swath running lengthwise along each side (rather than the vertical-stripe pattern typical of smallies) both suggested it.

So where did these “bucketmouths” suddenly come from? Where did the smallies go? How could such a changeover happen so fast? Was this an irreversible trend—perhaps yet another close-to-home sign of global climate change? Had I seen the last of this handsome, valiant breed—the fish that had defined piscine beauty for me a half century ago?

     I caught seven little bass in the slough. 
     But this time I couldn’t tell what they were.

SPLIT THE DIFFERENCE
In the past few years, I’ve once again caught smallies in the slough, but also the occasional largemouth. The difference hasn’t seemed to depend strictly on water level and quality, so I’m still wondering what’s going on.

This summer of 2015 has been an extraordinary one for this part of Minnesota—relatively cool temperatures and plenty of rain. For the first time in memory, my favorite slough has been navigable all summer long—and well into the fall. I’ve been fishing in there seven or eight times, and have delighted in catching lots of smallies, many of them that amazing dark color that so struck me when I was a boy. And good-sized ones at that.

Yesterday, on my latest outing—likely last of the season—I caught seven little bass in the slough. But this time something was quite different. I couldn’t tell what they were. Every single one had some of the characteristics of a smallmouth and others of a largemouth. Some of those traits fell halfway in between.

Black bass caught and released in St. Croix River, October, 2015

Smallmouth and largemouth black bass

Instead of the maxillary extending back to the middle of the eye’s pupil (smallmouth) or well behind the eye (largemouth), it extended just to the back of the eye. Rather than a compound dorsal fin (smallmouth) or one with a distinct break between the forward, spiny part and the rear, softer part (largemouth), there was something halfway in between.

Instead of vertical stripes and bronze highlights (smallmouth) or a mossy green hue with a dark horizontal stripe (largemouth), these specimens had fairly uniform gray-green coloring with reddish dorsal and anal fins and tail.

   If the meanmouth is really here, what does that 
   say about the health of our beloved St. Croix?

MEET THE MEANMOUTH
So, have I been witness, in the course of just three or four years, to the evolution of a new, hybrid black bass species on the St. Croix? This cross has been noticed and documented elsewhere; it’s called the meanmouth*. More commonly considered the offspring of a smallmouth and a spotted bass (Micropterus punctulatus), meanmouths of the small-/largemouth version do exist, though they are thought to be “extremely rare due to the difference in habitat preferred by the respective parents.”**

But that would clearly support the case for meanmouths evolving in the St. Croix, because both habitats are present: clear, cool, flowing smallmouth waters during half the summer; still, murky, warm largemouth waters the rest of the summer. Wouldn’t that be the ideal habitat for a hybrid?

While we St. Croix River aficionados tremble at the thought of an impending invasion of Asian carp—bigheads, silvers or both—here, sneaking in under the radar, comes the meanmouth. Not invasive, nor destructive (that we know of), but of concern nonetheless.

If the meanmouth is really here, what does that say about the health of our beloved St. Croix? I don’t know if the Minnesota and Wisconsin DNRs are aware of these apparent changes in the river's black bass population, but perhaps more observations from other fishermen—preferable some with more ichthyological knowledge than I possess—would help them at least to quantify and track the extent of the changes.

*The origin of the name "meanmouth" is recounted in this quote from In-Fisherman:
“The term “meanmouth bass” was born when Childers observed a school of largemouth-smallmouths attacking a female swimmer. “The bass leaped from the water and struck her on the head and chest,” he wrote, “and drove her from the pond.” On another occasion, he watched meanmouths attack a dog that ventured into shallow water.”

** BassFishingGurus.com