The channel catfish (Ictalurus punctatus Rafinesque 1818), an important component of the Upper Mississippi River System (UMRS) fish community, is abundant and makes up a significant portion of the total fish biomass of our large rivers. This species is avidly sought by both sport anglers and commercial harvesters. One of the goals of the Long Term Resource Monitoring Program (LTRMP) is to monitor and evaluate long-term trends in the populations of the channel catfish.
From the inception of the program in 1989 to the present, the field stations of
the LTRMP have been monitoring fish populations in several types of aquatic
habitat in six reaches of the UMRS (the UMRS includes the Illinois River and
the upper Mississippi River).
The channel catfish, Ictalurus
punctatus.
These habitats include tailwater just downriver of a navigation dam, the impounded area just upriver of a dam, flowing side channels, the main channel, and backwater contiguous areas that are at least seasonally connected to the river main channel. The Illinois Natural History Survey operates the two LTRMP stations in Illinois: one on the La Grange Reach of the Illinois River and the Great Rivers Field Station on Reach 26 of the Mississippi River (Figure 1). By combining data from the two Illinois stations with data from the other four stations, we can assess the status of channel catfish populations throughout much of the UMRS.
One problem in making population assessments is that each type of sampling gear (different types of nets and electrofishing boats) is selective for certain sizes or species of fish. Moreover, the vulnerability of fish to the gear varies with environmental conditions, such as water depth, current, and turbidity, and biological factors, such as the presence of submersed vegetation. Hence, it is important to sort out what changes in fish catch are attributable to real changes in the fish populations and what changes are due to other factors. To obtain a variety of sizes of fish under a variety of environmental conditions, the LTRMP utilizes several different gears for population assessment, including day and night electrofishing, small and large hoop netting, fyke netting, seining, and trawling. The multiple gears are employed using standardized methods. The gears that have captured the most channel catfish are hoop nets (small and large), accounting for 68.1% of the total catch. Another 25% have been caught using fyke netting, day electrofishing, and trawling.
More than 42,000 channel catfish have been captured, measured, and returned to the rivers by LTRMP fish biologists. There is a striking variability in catch (and presumably abundance) of channel catfish from one reach to another and usually among habitat types within a reach. The La Grange Reach has yielded 15,912 fish (38.1%), the largest share of the combined catch, while Reach 4, along the Minnesota portion of the upper Mississippi, has yielded the least, only 1,707 (4.1%). We have noticed a strong south to north gradient in day electrofishing catch rates. During nearly all years of sampling, the catch rates have been significantly higher at Reach 26, Open River Reach, and La Grange Reach than at Reaches 4, 8, and 13. Catches by electrofishing have ranged from over 10 channel catfish per hour at La Grange Reach in 1997 to less than 1 per hour at Reach 4 in 1994. Catfish prefer habitats with woody debris, bank cavities to hide in, and moderate currents, so it is not surprising that more than two-thirds of the fish collected by all of our gears have been in side channel and main channel borders (hence the common name "channel" catfish) while only 10% have been found in backwater and impounded habitats.
Map indicating the six reaches of the Upper Mississippi River System,
including two operated by INHS.
In addition to differences in abundance of channel catfish among reaches and habitats, there are marked differences among years in production of young fish. In 1991, for example, there were large numbers of young channel catfish in nearly all of the study reaches. Sampling the same reaches year after year allows us to "watch" the fish grow. Channel catfish from the "class of '91" were first measured when they were less than 4 inches long; by 1997 many were over 20 inches long--an excellent growth rate for the Midwest.
Ongoing research involves assessment of effects of environmental conditions on sampling efficiency so that we can compensate for these effects and better assess the effects of environmental variations on the fish populations themselves. For example, the low catfish yields in the Minnesota reach of the Mississippi may be attributable to more abundant submersed vegetation there. The vegetation not only hides and protects young catfish from predators but also from fish biologists and their sampling gear! Once we can adjust catch rates for differences in sampling efficiency, we can assess the true abundance of catfish and other fishes, and associate differences in abundance with differences in habitat and water quality, seasonal water regime, and other factors, including natural and harvest mortality. Such information is vital in setting harvest regulations and managing rivers to sustain catfish populations.
Todd M. Koel, Ruth M. Sparks, and Richard E. Sparks, Center for Aquatic Ecology
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