Catfish, especially blues, channels, and flatheads, are important recreational and commercial species. Channel catfish also support a multi-million dollar fish-farming industry. To advance management of wild catfish stocks and improve production on fish farms, scientists have extensively studied factors affecting catfish growth. The faster a catfish grows, the sooner it reaches harvestable size and, subsequently, trophy proportions, so growth is important to catfish anglers.
Many variables can affect catfish growth, but throughout their range, temperature is a major factor. They begin active feeding and resume rapid growth at water temperatures above 70°F, and growth isn’t suppressed at high temperatures in natural waters if adequate food is available. It’s generally assumed that catfish grow faster in the South where the warmwater season is longer than in the North.
While this assumption isn’t necessarily false, it also isn’t true that catfish in northern waters always grow more slowly than southern cats. Witness the giant channel cats caught from Red River of the North in Minnesota, North Dakota, and Manitoba. How about the record-shattering flathead from Elk City Reservoir, Kansas? Add to the list the behemoth blues in the lower Missouri River and the Mississippi River from St. Louis downstream. Yes, the lower Mississippi flows through the Deep South, but the water originates from the north and flows through the hot summer climate of the southern states in 10 to 15 days, hardly enough time to gain a lot of subtropical degrees.
So what do we know about the relationships between climate—annual temperature conditions—and catfish growth? Biologists have assembled a lot of data on catfish growth in lakes and rivers throughout North America, but that tells us little about how climate affects growth without further analysis. The best way to test this is to look at catfish growth across the climatic gradient of North America. This is where a landmark study by Dr. Andrew Rypel comes in.
Channels, blues, and flatheads maintain strong populations in rivers and streams with flowing water, but also in the standing waters of lakes and reservoirs. Because rivers provide different food resources than lakes, and dealing with flowing water can require a catfish to expend a lot of energy, growth might differ substantially between standing and flowing water. Rypel compared the growth between standing and flowing waters in addition to his climate comparisons.
Show Me the Data
Growth rate assessments usually rely on what biologists call length-at-age data. For example, a growth assessment of channel catfish in Lake A reveals fish are 6 inches at the end of their first year of growth (age-1), 8.5 inches after their second year of growth (age-2), and 11 inches after their third year of growth (age-3). In Lake B, channel cats are 6 inches at age-1, 9 inches at age-2, and 12.5 inches at age-3. Length-at-age data show the fish in Lake B are growing faster than in Lake A during their second and third years.
To understand how climate and habitat affect catfish growth, Rypel analyzed length-at-age data from published papers, fishery agency reports, and unpublished field data from fishery biologists. He analyzed data for 46 blue catfish populations, 125 channel catfish populations, and 44 flathead populations.
Grow with the Flow
Blue catfish grew faster in flowing-water environments, Rypel found. Although blues are adaptable and can thrive in reservoirs, they’re more often recognized as denizens of the main channel of large rivers. Their success in reservoirs may be due to the widely varying conditions the species encounters in large rivers. For example, blues quickly move onto the slack-water floodplain of large rivers to feed when the waters rise. But the issue is growth: Blues tend to grow faster in rivers. Better growth may be related to the types of forage fish available to them in rivers.
Some trophy blue cat anglers insist there’s a link between big blues and fatty riverine forage species, like skipjack herring. The supreme effectiveness of cut skipjack as a bait for trophy-class blues supports anglers’ thinking, and they might be right. Skipjack herring is a highly migratory species, and their numbers have declined substantially where rivers are dammed to build reservoirs. However, blue catfish have relatively fast growth in reservoirs where skipjack persist, such as those on the Tennessee River where hydroelectric generation creates flowing-water conditions and locks and dams allow fish passage.
Channel catfish had faster growth in standing-water, mainly reservoirs. Like blue cats, channel cats can live in a variety of habitats. But, as evidenced by the millions of pounds reared in aquaculture ponds every year and the excellent fishing they provide in ponds and small impoundments, they often thrive in standing water. Channel cats in rivers gravitate to slack-water areas. They tend to occupy slower-flowing pools and, like blues, are quick to move onto floodplains to forage on a wide variety of plant and animal foods. They usually grow faster in standing water.
Flathead catfish growth didn’t differ between flowing and standing water. While studies have shown they have specific and consistent preferences for logjams in rivers and streams where they occur, many reservoirs support strong, fast-growing flathead populations. The lack of effect of flowing water on flathead growth also may indicate that other environmental factors are more important. Flatheads are voracious fish eaters and consume a wide variety of preyfish. Likely, flatheads grow fast anywhere they have abundant forage fish. The flatheads’ ability to exploit food supplies and the lack of competition for that food probably has a lot to do with their success wherever they’ve been introduced.
Temperature Is Important, Sometimes
Blue catfish growth was positively related to average temperature—they tend to grow faster in warmer climates. Growth of channel catfish and flathead, however, was not related to average temperature—neither species tended to grow faster in southern versus northern waters. So say the statistics. But we know catfish growth occurs when water temperature is above 70°F. A likely explanation for these seemingly inconsistent results is that other environmental factors have a strong enough effect on growth that they mask the effect of temperature.
We know food supply—the type and abundance of food—plays a large role in catfish growth. But for a rich food supply to benefit catfish growth, we also have to consider the abundance of the catfish sharing the food and, possibly, the abundance of other species competing with catfish for food. Data on these variables weren’t available for Rypel’s models.
Rypel’s analysis revealed information useful to biologists and anglers—expect faster-growing blues in rivers in warm climates, faster-growing channels in lakes and ponds north and south, and fast-growing flatheads anywhere they have a lot to eat. But further data analysis uncovered a surprise.
When the data were adjusted to account for differing lengths of the growing season, growth tended to increase with latitude. In other words, growth rate during an equal-length growing season was greater in more northern waters than southern waters. So, if you put northern and southern catfish in tanks side-by-side under equal conditions, the northern catfish should grow more. Rypel calls this a “latitudinal countergradient”—more growth is expected during a longer growing season, but growth is faster per unit of time where growing season is shorter.
Latitudinal countergradient helps account for some exceptionally fast-growing channel cats and flatheads in northern waters (such as channel cats in the Red River of the North), and also partly explains the lack of a relationship between growth and average daily temperature for these two species. Even though blue catfish added more size where growing season was longer, they also showed a latitudinal countergradient when growth was adjusted to account for the length of the growing season. The countergradient effect for blues was weaker than for channel or flathead catfish, perhaps because they are known to move such long distances in large rivers.
The latitudinal countergradient concept is important from a fishery management perspective. First, if northern fish have a tendency toward faster growth during a shorter growing season, they must feed more during the briefer window of opportunity for growth. That means more food must be available when the water is warm. This reasoning provides a management target: Make sure food is abundant during the short growing season when water temperature is above 70°F.
The need for more food during the summer growing season, however, also creates management challenges because that is when other warmwater fish also are consuming the most food. There may be one more hurdle to management to provide abundant food during the warm season: Other fish species also may have latitudinal countergradients. To date, latitudinal countergradients also have been found in striped bass, northern pike, and lake sturgeon.
There are genetic concerns, too. If the tendency to feed more during the briefer warm growth period is genetic, fishery managers (and pond owners) need to be certain to stock only locally-sourced fish. For example, stocking southern channel cats into northern waters may result in a population that doesn’t feed more and grow more during the brief growing season, resulting in a disappointingly slow-growing population. On the other hand, stocking northern channel cats into southern waters may result in faster growing fish.
We discourage anglers and pond owners from moving fish around because many factors can affect growth and survival, and the genetics of fish species must be conserved throughout their range. However, the latitudinal countergradient is an interesting concept for forward-thinking fishery managers to test.
Foresight is Better
While hindsight may be 20/20, improvement can only be achieved by looking forward. Learn from past experiences, but don’t be shackled by “conventional wisdom.” Many factors affect growth of catfish, and which factor limits growth certainly varies among populations. The latitudinal countergradient revealed by Rypel’s research shows that a northern zip code is not, by itself, an indicator of slow catfish growth and poor catfishing. Rather, northern catfishing can be very good, and in some cases better than many southern waterbodies.
Fishery managers will likely take note of the latitudinal countergradient of catfish as they invest more time and resources into developing quality fishing opportunities for trophy catfish anglers.
*Dr. Hal Schramm is a professor in the Department of Wildlife and Fisheries at Mississippi State University. He’s a frequent contributor to In-Fisherman publications on science topics. Dr. Andrew Rypel is a Research Fishery Ecologist with the Wisconsin Department of Natural Resources and studies effects of global climate change on fish populations and improved conservation management of fisheries resources.
Taken from Elk City Reservoir, Kansas, on May 14, 1998, the fish stretched the tape a whopping 61 inches and sported a pleasantly plump, 42¾-inch girth. Paulie was crappie fishing at the time, and hooked it on a jig-and-minnow. Like many world records, it was not without controversy. It was verified while alive by Kansas Department of Wildlife and Parks fishery biologist Sean Lynott. But details of the catch—such as the relatively light tackle Paulie was using, and his statement that it didn’t put up much of a fight—raised eyebrows in the cat community. Still, the record stands to this day as a testament to the immense proportions flatheads are capable of attaining.