Bits & Pieces: Catfish Diets, Walleye Escapement, and Traveling Trout

This article originally appeared in the May 2022 issue of In-Fisherman.

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River Connections: Flathead Diets at Extreme Flows

Rivers are dynamic, going through periods of flood and drought extremes, creating unique challenges to river catfish anglers. As floods expand rivers into new habitats and droughts constrict the channel, reducing space, fish also must acclimate to new conditions, whether it’s shifting locations to areas with ideal current and depth, or to areas with abundant food resources, or both. Anglers adapt by adjusting fishing locations depending on where it’s possible to fish and where they predict catfish are located in the variable environment.

One aspect of catfish response is how flooding and drought extremes affect their diet. The years of 2011 and 2012 provided a unique opportunity to study flathead catfish diets in the Middle Missouri River. Record flooding in 2011 had the river overflowing into its floodplain, while drought in 2012 had confined the river into its channel all year. University of Nebraska researchers seized this opportunity to see how diets of flathead catfish differed between flood and drought during these two years.*

The researchers sampled flathead catfish using electrofishing during July, August, and September of both years, and foods were flushed from stomachs before all flatheads were released alive. A variety of diet items were found in stomachs including invertebrates such as mayflies, dragonflies, and other insects and crayfish; unidentified fish species along with catfish and freshwater drum, and on rare occurrences bass, sturgeon, paddlefish, and common carp; and a frog and snapping turtle.

Flatheads sampled in 2011, the flood year, had stomachs that were more full, and fish were in better body condition compared to flatheads sampled in 2012, the drought year. The percentage of flatheads with empty stomachs also was less during the flood year. A substantial change in the size at which flatheads switched from eating invertebrates to fish was also observed between years. In the flood year, flatheads switched to eating fish at 8 inches, while in the drought year the switch occurred at about 14 inches. In the flood year, there was a greater abundance of age-0 fishes in the river overall, the researchers report, which could have resulted in increased encounter rates between flatheads and prey fishes, possibly causing the switch to a fish diet at a smaller size. Movement of flatheads and prey fishes into the floodplain to seek refuge from high water also could have increased encounter rates.

Differences in diet structure between flood and drought years occurred in flatheads eating invertebrates but not in flatheads eating fish. In other words, the types of invertebrates eaten were different between years, but the types of fish species eaten were not. The drought year brought a decrease in the diversity of invertebrates in the diet, which according to the researchers may reflect general declines in macroinvertebrate diversity in rivers in low-water years. The researchers conclude that the study provided evidence as to how hydrological conditions can affect prey consumption and fish diet, and that in regulated rivers, organisms respond to floodplain connectivity.

—Dr. Rob Neumann

*Hogberg, N. P., and M. A. Pegg. 2016. Flathead catfish Polydictis olivaris diet composition during extreme flow events in a large river. J. Freshwat. Ecol. 31:431-441.

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Fish Management: Where Do Walleyes Go?

Native primarily to the Upper Midwest, walleyes are commonly introduced into reservoirs within and outside their native range to provide recreational fisheries. While walleyes can thrive in reservoirs, natural reproduction is often limited and regular stockings are required to maintain healthy fisheries.

Other than angler harvest, walleyes are lost from populations through natural mortality. Reservoirs, however, are unique because walleyes can also be permanently lost from the population due to escapement—the loss of fish over spillways or through water-control structures. While walleye escapement can be significant, how it contributes to fish loss relative to harvest and natural mortality is not as clear.

We measured walleye harvest, natural mortality, and escapement in two Iowa reservoirs—Big Creek and Brushy Creek—to better understand the significance of these factors in regulating populations. We also evaluated the effectiveness of a physical barrier on Big Creek spillway that was installed in 2012 to reduce walleye and muskie escapement. Walleyes were implanted with radio tags in both reservoirs beginning in October 2016 and tracked to monitor their locations through May 2019. Anglers were also asked to report tagged walleyes that they caught.

No walleyes escaped from Big Creek Reservoir, where the fish barrier was installed on the spillway. Walleye escapement at Brushy Creek (no fish barrier) was greater during higher lake levels during April; annual escapement ranged from 22 to 47 percent. Walleye harvest was higher from April through July compared to the rest of the year, with annual harvest ranging from 13 to 27 percent. Walleye natural mortality was positively related to water temperature and was estimated at 36 to 38 percent annually. Results of our study suggest large numbers of walleyes can be lost from reservoirs via escapement over spillways, which can be significant in maintaining reservoir walleye populations, and that physical barriers can be effective for reducing escapement.

—Robert Weber and Dr. Michael J. Weber, Iowa State University

*Weber, R, and M. J. Weber. 2021. Effects of harvest, movement, and escapement on reservoir walleye populations. N. Am. J. Fish. Mgmt. 41:775-790.

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Field Notes: No Boundaries for Bull Trout

Bull trout, one of the lesser known species of char, are native to northwestern North America from Alaska to northern California. They inhabit Arctic waters, Pacific coastal areas, and Missouri River drainages in mountain and coastal streams. This species (Salvelinus confluentus) exhibits four different life-history strategies: a non-migratory or resident form; a riverine form; populations that live in lakes but move to tributaries to spawn; and anadromous types that feed in marine waters and ascend rivers to spawn.

All populations are listed as Threatened under the Endangered Species Act, due to habitat degradation and fragmentation, dam construction, water quality problems, and introduction of non-native species. But they’re abundant in some waters where they provide excellent catch-and-release fishing. Adults eat a variety of fish species and readily strike flies, spinners, and spoons. They generally range from 18 to 24 inches, though the all-tackle record is 31 pounds 15 ounces and fish over 10 pounds are occasionally caught.

State and federal fishery agencies have monitored bull trout populations, and biologists with Idaho Power Corporation recently conducted a tagging study. One fish marked in the Hells Canyon stretch of the Snake River in Idaho has demonstrated the wandering nature of this species. After tagging in 2018 at 12 inches, it was recaptured nearby a year later, having grown to 16.5 inches.

In May, it turned up in northeast Oregon’s Imnaha River, a tributary of the Snake River. In July, it traveled more than 186 miles to spawn in a small mountain stream in the Salmon River watershed of central Idaho. The following winter, it was back home in Hells Canyon, but made its migratory journey to Oregon again the following spring. Considering that bull trout may live more than 12 years, it’s likely to continue its interstate travels.

—Steve Quinn

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