June 02, 2014
Successful catfish anglers know habitat is important. But too often anglers think about habitat only in terms of where adult cats live — a logjam for flatheads, a deep scour hole for blues, or a slowly flowing pool or chute for channels. Large catfish, and plenty of them, don't just happen. Biologists who manage catfish habitats have to think about habitats for all life stages. And sometimes this takes some really broad thinking.
Catfish are cavity spawners. Hollow logs, a hole in a bank, a washout under a log or large boulder, a cavity in a riprap bank, even some of man's discards provide spawning spots. Spawning cavities share two characteristics: Shelter from current and protection for the developing embryos from marauding predators.
Expect better spawning habitat in unaltered streams and rivers with heavily wooded riparian zones (the strips of land along rivers). The stable banks and exposed rootwads provide spawning cavities. Trees that die and fall into the river provide logs and logjams that also can serve as spawning sites. If spawning habitat is lacking, adding it is relatively easy. Structures such as ceramic or concrete drainpipes are effective. Keeping the structures from washing away is the difficult part of habitat enhancement.
If spawning is adequate, survival of the young catfish is the next critical step in producing strong catfish populations. The habitat requirements for juvenile blue, channel, and flathead catfish are similar and rather simple — the young of all three species occupy channel and channel-border areas. In large rivers, I've found concentrations of young-of-the-year blue and flathead catfish on sandbars and over large sand shelves. Young channel cats in smaller rivers are known to concentrate in riffles.
Although occasionally found around brushpiles, abundant cover doesn't appear to be as essential for catfish as it is for the young of other sportfishes. Forsaking cover, the solution to high survival of young cats is rapid growth. And that means good food resources.
Young catfish — blues, channels, and flatheads — all start on a diet of invertebrates. Initially, the fry feed on zooplankton. As they grow, they switch to progressively larger invertebrates like midge larvae and aquatic worms. Desirable forage for 4- to 10-inch cats includes larger aquatic insect larvae like mayflies and large midge larvae. With further growth, terrestrial invertebrates, crayfish, snails, and thin-shelled freshwater mussels become important. By 12 inches, fish become an important component of a catfish's diet. For flatheads, the diet switches to almost exclusively fish. Blues and channels remain omnivorous, feeding on a mixture of terrestrial and aquatic insects, worms, crayfish, mussels, snails, and fish.
Since survival means fast growth, and fast growth means abundant food of the right quality and size, good nursery habitat — and thus good recruitment of young catfish — means good habitat for their preferred foods.
Life changes dramatically when catfish reach about 12 to 15 inches. Now they are the eater and rarely the eaten. They can boldly take up residence where they choose, or travel to feeding locations at will.
While all three catfish may be found in or near cover, the flathead is clearly the most secretive. Flatheads are commonly found in logjams in water generally less than 10 feet deep. In largerivers and riverine reservoirs, large rock (riprap) installed to armor banks and forestall erosion is a favored haunt of adult flatheads.
Blue cats are regarded as creatures of the main channel. That may be true in smaller rivers, but I don't think the use of main channels has been fully explored for large rivers. Sampling the main channel of large, free-flowing rivers like the lower Missouri and Mississippi is difficult and dangerous, and biologists are just beginning to get a handle on the fish life in these deep, dark, swiftly flowing, and ostensibly barren habitats.
In the lower Mississippi River, small adults are abundant along ledges where sandbars drop into the main channel, but we collect a lot of big blues at the "toe" (where the bank slope meets the flatter bottom of the main channel) of steep, natural and riprapped banks. In most rivers where blues are abundant, the apparently preferred habitat — the location where we are most likely to collect a dozen or more big blues — is a relatively small but deep scour hole created by an eddy current.
Channel cats should have been named "pool cats." Common to abundant in large and small rivers, adult channels seem to prefer pool habitat — areas of deeper water and slower current. Although some biologists note associations with cover such as large rock, logjams, or debris piles, channel cats are often abundant in areas without a lot of cover. Of all the sportfishes I have worked with, channel catfish best typify the term, "habitat generalist."
Blues, flatheads and, especially, channel cats have simple habitat requirements that are easily met in most rivers. All three species appear to tolerate moderate habitat alteration, and it appears that good populations of at least one of the "big three" catfish prevail in any river throughout the cats' range, provided water quality is adequate. For example, many Midwestern rivers provide good fishing for channel catfish despite the scarcity of other sportfish.
Habitat management is commonly the fishery manager's strategy of choice to improve fish populations. While apparently simple habitat requirements and enduring catfish populations are good news for anglers and managers, there's a downside. Because time and money are always limiting, fishery managers usually call for research only when they face a problem or recognize an imminent need.
Cat-fishery problems have been few, and as a result we know relatively little about how to manage habitat to enhance their populations. As the popularity of catfishing grows, more anglers put more fishing pressure on catfish populations, and more political pressure on fishery managers for good catfish resources.
I foresee a time when effective habitat management strategies will become important. But will managers be ready to act? Compared to other sportfishes, we have a lot to learn before we can develop habitat management plans for blues, channels, and flatheads. But maybe management should not focus on single catfish species. Most managers would agree that strategies that restore natural river function and benefit the general fish assemblage are good for catfish, too.
The Current Puzzle
All three catfishes are native to rivers, but they also thrive in reservoirs. And millions of pounds of channel catfish are raised in fish farms every year. It's hard to conclude that current is all that important. Some research my students and I did on the lower Mississippi River a few years ago offers some insight into water-current preferences of catfishes.
We compared the abundance of catfishes in secondary channels and main channels throughout the length of the lower Mississippi. Secondary channels are former channels parallel to the main channel, separated from it by a sandbar or an island. Current velocity usually is less in the secondary channel than in the main channel. Catfish were collected by electro-fishing at sandbar and steep bank habitats during falling-river stages (summer) and low-river stages (fall).
Blue catfish, both juveniles and adults, were more abundant in the main channel at all river stages. Abundance of channel cats was similar in the main channel and secondary channel, but abundance in both channel types was higher during low river stages. Possibly the channel catfish occupied deep-water habitats during falling-water stages and moved to nearshore habitats during low water.
Flathead catfish were more abundant in the main channel during falling-river stages, but the adults were more abundant in the secondary channel during low-river stages. Possibly the adults seek slower flow at this time.
Blue, channel, and flathead catfish are highly piscivorous, meaning they eat a lot of fish. Often, the "preferred habitat" of predators is not necessarily the habitat they prefer but the habitat preferred by the preyfish. I doubt, for example, that reservoir tailraces are the preferred habitat of big blue cats, but they pack in to dine on the abundant and often disoriented or injured forage fish that concentrate in the reservoir outflow. Gizzard shad is the dominant forage fish in the lower Mississippi River, and some of the distribution of cats we observed is likely related to shad behavior. This could explain the flatheads' moving into the secondary channels during low flow, a habitat with abundant shad.
We noted a clear exception to the predacious cats following the prey. Some of the secondary channels had rock dikes at the upstream end that blocked water flow during low river stages. The absence of flow in these secondary channels was clearly marked by a thick algae bloom that developed in the slack, fertile waters. Gizzard shad densely colonized these lake-like conditions. We collected no catfish from secondary channels when flow was absent, despite the abundant forage. In large rivers, no flow means no cats.
Winter, The Critical Period
Although flow preference may be a mystery during spring through fall, flow requirements during winter, at least for channel cats in temperate rivers, are clear — no current. Wisconsin River channel cats move downstream to deep scour holes in and near the mouth of the Mississippi River, where they find good water quality but reduced current. Some of these fish annually migrate almost 100 miles to these wintering sites. Similar behaviors have been noted for channel cats from Missouri River tributaries.
I am not aware of scientific studies of wintering behavior of blues. However, commercial fishermen harvest thousands of pounds of blue catfish in the winter fishing gill-nets in the deep scour holes below wing dikes of the lower Mississippi River. Even though the Mississippi is relatively low in the winter, current in the channel is still 3 to 4 mph. The dike pools provide refuge from the relentless lower Mississippi current.
Flatheads have long been regarded as homebodies, but a recent tracking study by University of Missouri scientists documented movement from tributary streams to wintering habitat in the Missouri and Mississippi rivers, a pattern reminiscent of channel catfish in the Wisconsin River. While the smaller tributary rivers provided deep pools that were used by some of the flatheads in one stream, the big rivers may provide more stable habitat conditions. Even a large, deep pool in a tributary stream is more prone to fluctuations in water current than a deep scour hole in a large river. Also, more reduced-current habitat would be available in big rivers during winter.
It may be more than coincidence that evidence for wintering areas comes from the northern half of the catfishes' range. Blue catfish in the Tennessee River appear to remain active throughout the winter. In contrast to colder northern waters, water temperatures in the middle and lower Tennessee River impoundments only briefly drop into the low to mid-40°F range. Temperatures are similar in Lake Texoma, where arm-busting blue cats are active all winter long.
Fishery managers need to learn if the presence of slackwater during winter is essential to high survival of adult cats in northern waters. If so, dams can prevent tributary catfish from reaching wintering areas. And slackwater habitat is of little use if water quality is poor. In the upper Mississippi River, slackwater habitat abounds in winter, but when covered with ice many of these areas are oxygen-deficient without flows of fresh water. Managers are evaluating innovative ways to provide areas of reduced flow and good water quality.
Floodplains - Food Factories for Catfish
The floodplain is the land lateral to the river that historically was inundated during periods of high water. For large floodplain rivers like the Missouri or the lower Mississippi, the floodplain may have been up to 100 miles wide. In developed countries like the U.S. and most European nations, the floodplains are still there — you can recognize them as vast flat fields of cotton, corn, soybeans, or rice.
Levees now line the rivers, floodproofing these fertile lands that once belonged to the river and its aquatic inhabitants. Studies of tropical, relatively undeveloped rivers like the Amazon in South America have linked fish production to the food-rich floodplain. This relationship is codified as the "flood-pulse concept," and it's the prevailing paradigm of river ecologists.
The inundated floodplain provides a rich supply of plant and animal foods used by fishes. As the river rises onto the floodplain, a variety of fishes — including blue and channel catfish — follow the water's edge, grazing on the new, untapped food resource. Many species of fish spawn on the floodplain, further enriching the food supply for fish-eating predators. Fishes, like the flathead catfish, that do not move onto the floodplain may also benefit when the receding waters carry the fish produced on the floodplain back to the river.
It stands to reason that the larger the floodplain and the longer it's inundated, the greater the fish growth and production. While true for tropical rivers, this relationship has received scant support in temperate rivers of the U.S., so we looked at the relationships between growth of blue catfish and floodplain inundation in the expansive lower Mississippi River floodplain. Our first analyses failed to show any relationships. Does the flood-pulse concept not apply to the Mississippi River?
Although it's warm in Mississippi in April and May when the Mississippi floods, the river is cold — the flood waters are strongly influenced by melting snow and ice in the north. Catfish are warmwater fish that do not resume high feeding rates necessary for fast growth until water temperatures rise above 55°F to 60°F. We found blue catfish growth was positively related to the area and duration of floodplain inundation after the water warmed to 60°F and above.
So the flood-pulse concept appears to pertain to blue catfish growth in the Mississippi River if temperature is considered. Unfortunately, channelization and levees constructed to allow economic development of the floodplain have resulted in a smaller floodplain, faster downstream passage of water, and a briefer flood pulse. The faster passage of water reduces warming, and in most years the floodwaters have receded from the floodplain before the water reaches 60°F.
If our theory is correct, catfish production may be much less than it was 100 years ago, before the Mississippi and its floodplain were drastically altered. Reliable reports indicate 100'‘plus-pound blue catfish were common at Mississippi River fish markets in the late 1800s. Did the expansive and prolonged flooding of the Mississippi River floodplain play a role in the abundance of giant blue cats?
Channelization is a nasty word to fishery biologists. It involves straightening a river, removing shallow bars, steepening and smoothing banks, and removing snags to allow waterways to more quickly move floodwaters. Channelization, while accomplishing social and economic benefits to people in flood-prone areas, removes the habitat diversity essential for productive fisheries.
Numerous fishery and aquatic ecology studies provide ample evidence of the dark side of channelization. But several recent studies have documented greater abundance of adult channel cats in channelized segments of rivers. In the stream we studied in Mississippi, flathead catfish were moderately abundant in an upstream, unchannelized reach which was rich with gravel bars and logjams. Blue catfish were abundant in a downstream impoundment, and channel catfish were abundant in the channelized reach in between.
A speculative explanation might go something like this. Channel catfish are highly nomadic, and therefore can obtain their life requirements by roaming throughout 10, 20, or even 100 miles of river (or, in this case, a tributary river-reservoir system). The channelized reach of the river may represent a habitat with adequate forage, but an absence of other potential competitors like blue and flathead catfish.