Catfish growth, part 1

Where they grow the fastest

Mississippi offers diverse and outstanding catfishing opportunities. Fisheries scientists have compiled abundant information about factors affecting catfish growth to support management of wild catfish stocks and to improve production of catfish on fish farms.

Catfish growth is important to catfish anglers, too — the faster a catfish grows, the sooner it reaches harvestable size and, in time, trophy proportions.

Many factors affect catfish growth, but food supply and water temperature top the list.

Channel catfish eat a wide variety of foods, ranging from grain and nuts to fish, with invertebrates like insect larvae and crayfish usually forming the bulk of the diet throughout most of the year. Fish become increasingly important to channel catfish as they grow.

The diet of blue catfish is similar to channel catfish, but they also seem to have a taste for carrion — dead, decomposing fish and other nasty delicacies. Blue catfish have gained a little notoriety for feeding on mussels, particularly the invasive zebra mussel that has caused huge economic problems in the Great Lakes and colonized the Mississippi River.

As you might expect for a fish that grows to over 100 pounds, high-energy forage fish are an important menu item for big blue cats.

Flathead catfish are much more specific in their diets. Although young flatheads eat invertebrates, they rapidly transition to fish. Adult flatheads eat fish. A lot of fish.

Abundant food — of the right size and in the right places — sets the stage for fast growth.

Water temperature affects growth because it determines when catfish feed actively. All three catfish feed actively and grow fastest when the water temperatures are above 70 degrees Fahrenheit.

Biologists have always assumed blue, channel and flathead catfish grow faster in the South, where the growing season — the time when water temperature exceeds 70 degree — lasts 6 months or longer compared to northern waters where sufficient temperature for high growth only lasts four or five months.

But do catfish really grow faster in the South than the North? Answering that questions is not as simple as comparing a northern lake to a southern reservoir or a northern river to a southern river because many factors interact to effect growth.

The best way to determine where catfish have the fastest growth is to evaluate growth data from numerous lakes and rivers across a wide climatic gradient from north to south.

Fisheries scientist Andrew Rypel, formerly at the University of Mississippi and now with the Wisconsin Department of Natural Resources, did just that.

Rypel assembled all the catfish growth data he could find in the published fishery journals and unpublished fishery agency reports. In total, he analyzed data for 46 blue catfish populations, 125 channel catfish populations and 44 flathead catfish populations.

Statistical analysis of the robust, nationwide data set only partly supported conventional wisdom that blue catfish grew faster in the South than in the North. Growth rates of channel and flathead cats, however, did not differ between northern and southern waters.

Statistics don’t lie, but it’s hard to believe them when water temperature is fundamental to growth, and there are obvious differences in the length of the growing season between northern and southern climates.

A likely explanation for these seemingly inconsistent results is that other environmental factors have strong-enough effects on growth that they mask or overshadow the effect of temperature.

Food supply is a likely candidate for the “overshadowing factor.” Food supply includes the type and abundance of food. But for a rich food supply to benefit catfish growth, biologists also have to consider the abundance of the catfish sharing the food and, possibly, the abundance of other species competing with catfish for the food.

Unfortunately, data on these variables were not available and not included in Dr. Rypel’s analysis.

Further data analysis revealed another explanation for the lack of a North-South difference in channel and flathead catfish growth: When the catfish growth rate was adjusted for length of the growing season, northern fish actually grew faster during each day of the growing season.

In other words if you put northern catfish and southern catfish in two tanks with identical temperature conditions, the northern catfish would outgrow the southern catfish.

This novel finding has some interesting management implications and warrants further research, but it also explains the abundance of huge channel and flathead catfish from northern waters.

Blue, channel and flathead catfish flourish in lakes and rivers. Have you ever wondered whether they grow faster in flowing or standing water? The second part of this discussion, to be included in the November issue will answer that question, as well as provide some average growth rates for Mississippi catfish.