September 10, 2021
By Dr. Hal Schramm
Catfish are the subject of a lot of lore. Maybe it's because they're big or because they're invisible in their turbid river homes or the dark depths of reservoirs. I've heard about blue cats as big as a Volkswagen observed by divers inspecting dams from at least six different people. The observer was always a friend of a relative like an uncle or cousin. Maybe you've heard those stories, too. The last time the story was told to me, I listened with enthusiasm and replied, "If you know exactly where it was spotted, let's go try to catch it."
The scary thing isn't a leviathan catfish able to swallow a dog or a human, but that a few of the people who shared the story actually seemed to believe it.
The fishing world is full of dock talk and other "facts" that make good conversation. Some are hard to swallow and shouldn't be. Others might be credible, and there's no catfish angler that doesn't want to be on the front edge of new ways to catch catfish or catfish news. I recently visited with In-Fisherman Managing Editor Rob Neumann and we talked about a handful of catfish myths we've heard over the years. We don't always definitively agree or disagree, but we offer you biology.
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Catfish Always Rely on Smell to Feed Neumann: There's no doubt catfish are super smellers and tasters, but like other fish, they have additional senses they can use in feeding, such as vibration detection via their lateral line. Most evidence I've found also points to them as having good vision, probably darn good vision in clear-water situations. I wouldn't put it past them for having at least limited vision in muddy water with low light, too, like walleyes. In fact, based on the characteristics of their eyes, their visual acuity might be on par with walleyes, as their eyes share similar characteristics. I'll probably get some heat on that one, but I did say "might."
Like walleye eyes, channel catfish eyes have a tapetum lucidum, a structure in their retina that can allow them to see in low-light or dark conditions with little ambient light. Also, specialized cells called cone cells in the channel catfish retina have a single visual pigment that has peak sensitivity to red wavelengths. Another group of cells, rod cells, used for seeing in low-light conditions, contains a visual pigment peaking in the green part of the spectrum. Possessing two visual pigments suggests that channel catfish might possess some level of color vision. This is like the walleye's dual visual pigments, peaking in the red-orange and green parts of the spectrum.
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Schramm: Another sense some catfish species use that might surprise you is electroreception, the ability to perceive natural electrical stimuli. The brown bullhead has been repeatedly shown to be able to detect prey items by electroreception. All living organisms create very low-current electric fields, so the ability to detect these electric fields would be advantageous, especially for fish that feed on prey that move little, such as bottom-dwelling invertebrates.
Electroreception occurs in various fish, like sharks and rays and some bony fishes. What is odd, though, is that the brown bullhead is the only North American catfish for which electroreception is known. I don't know of any studies that have shown blue, channel, and flathead catfish don't have electroreception, but researchers usually don't publish negative results. Considering the popularity of blue, channel, and flathead catfishes and the ready availability of channel catfish for lab studies, I think it's safe to conclude that electroreception is, for whatever reason, restricted to brown bullhead among North American catfish.
Neumann: Recent research also shows that certain species of catfish also use changes in pH as a means of detecting prey. Louisiana State University researchers and their Japanese colleagues report Japanese sea catfish are equipped with sensors in their barbels that can locate prey by detecting slight changes in the water's pH level. They found that the catfish use this ability to detect sea worms, a primary prey. When the sea worms "breathe" they release tiny amounts of carbon dioxide that changes the pH of the surrounding water, giving away their location to the sea catfish.
: Another thing to consider is that the carbon-caused change in pH forms a gradient, just like the oils and odors oozing from a cut skipjack or a gob of your favorite stinkbait, that leads catfish to their prey. It's unknown whether the catfish we target in North America have pH detection ability, but regardless, myth busted.
Catfish Don't Bite During the Spawn Neumann: Â It's well known that catfish are cavity spawners and stay in their hole until the eggs hatch and fry disperse. So it's logical to assume that they don't bite while they're spawning. But we know that not all catfish spawn at the same time, so on any day during the spawning season there are always prespawn and postspawn fish that are feeding.
Schramm: I agree. And even if all the catfish did spawn at or close to the same time, the male runs the female off and tends the eggs and fry. There would be "spawned out" females trying to rebuild their energy supply. So although the bite might be difficult at times, there's good reason to fish during the spawn. Myth busted.
Catfish Bury in the Bottom in Winter Schramm: Biologists know from movement studies of blue, channel, and flathead catfish that river cats move to winter homes, which are typically deep holes, often with big cover like large rocks, and very slow or no current. Telemetry studies that can determine a fish's precise location at frequent time intervals have found that flatheads in northern waters don't move for days or even longer when they're in their winter homes. Covered with mud, unlikely. But I agree that they might be coated with a thin layer of silt adhering to their mucus.
Neumann: Right, Hal. This lore seems to have its origins with flatheads, which can go into a state of torpor, especially in northern waters, but not quite hibernation. Torpor is driven by temperature, which in turn drives metabolism and activity. You see underwater videos like those taken by Brian Klawitter on the Upper Mississippi River of large groups of flatheads lying motionless on bottom in wintering areas, long enough to accumulate silt on their backs. On those same videos you also can see channel cats actively swimming above the flatheads. We also catch channel cats in winter, even through the ice. And winter is also known to produce big blues in rivers and reservoirs. Myth busted!
Catfish Feed Heavily in Fall to Prepare for Winter Schramm: I've heard the same suggested for several species of fish, and documenting this would be difficult for fish in the wild because there are too many variables changing at the same time. For example, water is cooling, day length is getting shorter and, most importantly, the food supply is changing.
Unlike some warm-blooded mammals that pack on energy-rich fat before they hibernate or to get them through a long winter of sparse food, fish are cold-blooded and their body temperature fluctuates with water temperature. As their body temperature goes down, so does their metabolism. Fish feed when levels of fats or sugars in the blood decline below a threshold that signals hunger—a need to consume more energy. With low metabolism, the circulating levels of fats or sugar are depleted slowly. In turn, the fish feed infrequently and food is generally available. Thus, basic fish physiology suggests that fish don't need to "feed up" for the winter.
Optimum feeding rate and preferred temperature of channel catfish is near 86°F and decreases above and below this temperature. In fisheries where the water temperature climbs above 86°F, like in ponds or shallow, slowly flowing streams in the Midwest or South, food intake would be expected to increase in the fall when water temperature drops back to the optimum temperature. But an increase in feeding at cooler temperatures wouldn't be expected. This increase in food consumption to about 86°F and then decreasing at warmer temperatures is borne out by feeding rates used in catfish farming.
The preferred temperature or temperature of optimum feeding rate for blue catfish hasn't been established. Flatheads, however, have a higher preferred and optimum-feeding temperature of 88°F to 91°F. If temperature dictates feeding, I would not expect flatheads to increase food intake in the fall.
Possibly the availability of forage trumps temperature and hunger. Shad and some minnows are known to bunch up or school in the fall. Aggregations of right-size forage could trigger additional feeding by catfish, which eat fish when available. Increased feeding in early fall when water first starts to cool is possible for blue and channel catfish, but not because they are "feeding up" for the winter. I don't expect increased feeding by flatheads in the fall.
Neumann: Great explanation, Hal. I'd say you busted this one.
Catfish are Champion Burpers Neumann: When you've held channel catfish to unhook them or snap a photo, you may have heard them make a burping or croaking sound. You also can feel vibrations when they create these sounds. It's not burping as we know it, however. Stridulation sounds in channel catfish are produced during forward fin sweeps (called pectoral spine abduction), when ridges on the spine's base rub against a bone called the cleithrum.
Next time you hold a catfish and hear these sounds, watch as one of the pectoral fins sweeps forward each time the sound is created. "Croaks" can vary in duration, frequency, pattern, and loudness. This ability has been observed in other catfish species. Scientists think the sounds are disturbance calls, or a defense mechanism warning predators of the presence of sharp spines and thus deterring predation.
Schramm: You might also hear burping or "whooshing" sounds, which are clearly coming from the throat of blue cats. But this isn't a burp as how we burp, which is expelling excess gas from our stomachs. In blue cats, rather, the gas is coming from their gas bladder, either due to pressure on the gas bladder while you're holding the fish, or because the gas is expanding and being forced from the bladder after being caught from deep water.
If you've reeled up blue cats from deep water you might have seen bubbles of air that were expelled as it got closer to the surface. Blue cats are physostomous, meaning there's a physical connection for gas to move between the swim bladder and the gullet.
Channels and flatheads are physoclistous -- the gas bladder is not connected to the gut. Channels and flatheads rely on special tissues to regulate the amount of gas in the swim-bladder, a much slower process, so they can't "burp," like blue cats. Indigestion? No Pepto-Bismol needed. Burp myth busted.
Catfish are picky eaters Schramm: Catfish have senses of smell and taste that are among the most sensitive of all fishes, so they can detect very small variations in compounds. That's probably why stinkbait recipes are so closely guarded. Flatheads are primarily piscivorous, feeding on a variety of live baitfish. Blues and channel cats are opportunistic feeders, feeding on what is available from corn to crayfish and soybeans to shad. Channels and blue cats take "opportunistic" to a new level by feeding on dead animal matter, too.
Neumann: And being opportunistic means we can't always assume what they're eating. And in fact diets are quite flexible. Blue cats in Lake Norman in North Carolina are a great example. Most folks would think they'd eat shad year-round, but a study by Virginia Tech researchers showed at certain times of year, Asiatic clams dominated their diet. There are seasonal cycles to catfish feeding, and they often follow prey availability. Not that shad wouldn't be a great bait year-round, but it's not always what's in their bellies. And on the coastal James River in Virginia, one might figure the same, with shad and herring being the dominant prey.
But another study by Virginia Tech scientists show blue cats are eating mostly blue crabs. But they eat fish, too, and quite a variety of them.
Schramm: I love stuffed flounder. Catfish stuffed with crab sounds delicious, too! What you say reminded me about a food habits study of blue catfish we did on the Mississippi River. We found blues ate a wide range of foods, but primarily shad. There were, however, brief periods when many of them were full of zebra mussels that they probably picked off rock wing dikes. And about the crab-eating blue cats in the James: these non-native catfish have colonized Chesapeake Bay and are causing quite a stir because they're threatening the lucrative blue crab fishery.
Don't give away the family bait secret, but never hesitate to try different baits. Myth not quite busted.
Catfish have poisonous spines Neumann: Getting spined by a catfish results in a painful sting, often associated with redness, inflammation, and swelling. The leading edges of the single dorsal and twin pectoral spines of many species of catfish contain venom glands. When the spine punctures the skin and underlying tissue, a spine's membranous outer layer ruptures, releasing venom into the wound.
A study by University of Michigan researcher Jeremy Wright finds that at least 1,250 and possibly more than 1,600 species of catfish may be venomous—far more than previously believed, but not all catfish are venomous. Catfish venoms poison nerves and break down red blood cells, producing severe pain, reduced blood flow, muscle spasms, and even respiratory distress in some cases. Because none of the catfish Wright examined produces more than three distinct toxins in its venom, each catfish species probably displays only a subset of possible effects, he notes.
Schramm: That stings! Readers might be wondering about catfish species in North America. Here, venomous species include some catfishes, bullheads, and madtoms. Channel cats are believed to have venom in their spines. In that study Rob discussed, the author reports that black bullhead and flathead catfish lack any structures that could be identified as venom glands. The toxins in North American catfishes generally are harmless, but a sting can lead to bacterial and fungal infections at the puncture wound, or pieces of spine that might break off in the wound. Myth confirmed, for some catfish.
And about that Volkswagen-size catfish ... I ran some numbers. Because a VW Beetle is very similar to half an ellipsoid (actually kind of a straight-sided ellipsoid), and fish weight per unit of volume is usually about the same as water (62.4 pounds per cubic foot), that dam catfish would weigh in the neighborhood of 11,000 pounds! Like I said, if anyone sees one that big, lets go catch it!
Dr. Hal Schramm is an avid angler, fishery biologist, and freelance writer. He regularly contributes to Catfish In-Sider Guide.
