Bits & Pieces: Introduced Muskies, Optimum Temperatures, & Catfish Flavors

Management in Action: Effects of Introduced Muskies

Muskellunge are stocked into lakes by many state fishery agencies throughout North America to create trophy fishing opportunities. As muskie stocking expands and muskie fishing becomes more popular, more concerns arise among the public regarding the effects of muskie introductions on fish communities. These effects, however, are generally poorly understood.

Minnesota DNR fishery biologists recently published a study examining the effects of muskies on fish communities where they have been introduced.* They discuss that in Minnesota, the potential impact of stocked muskies on resident fish communities is the most common concern in comments from the public regarding muskie stocking in new waters. Other concerns by some in the public include fear of attacks on swimmers, introductions of invasive species by anglers, declining property values, overcrowding, and others. While social concerns were part of the impetus for implementing the study, the focus was on the biological effects of introduced muskies on fish communities, the authors note.

To assess potential changes in fish communities, the researchers compared relative abundance (CPUE, catch-per-unit-effort in test nets) of seven fish species and mean weight of two species before and after muskie introductions in 36 lakes, which were paired with reference lakes with similar habitat and management of the stocked lakes. Muskie lakes had established muskie populations, with stocking occuring for 20 to 56 years, providing adequate time for muskies to develop several year-classes and grow to trophy size. Because of the importance of ciscoes in muskie diets, the lake set also contained waters with and without ciscoes. This aspect of the study helped address the question of what muskies might consume should cisco populations continue to decline due to climate warming.

The study found no significant decline in walleye CPUE or mean weight related to muskie stocking, the authors report. In fact, walleye catch rates were higher in all lake groupings after muskie stocking, although the difference wasn’t statistically significant. Pike CPUE in stocked lakes remained stable following the introduction of muskies, while CPUE of pike increased in reference lakes, suggesting muskies might help moderate the abundance of small pike. Yellow perch CPUE increased in lakes after muskie stocking compared to reference lakes. While white sucker CPUE has generally declined in both sets of lakes over the past 30 years, reduction in CPUE was greater in muskie stocking lakes, the authors note.

While cisco CPUE has been generally declining over time in both lake groups, there was no observed change due to muskie stocking. The researchers found that in waters containing ciscoes, white sucker CPUE didn’t decline due to muskie stocking. The researchers suggest that given muskies’ preference for soft-rayed preyfish, it wasn’t surprising that white suckers would be eaten by muskies, especially in lakes without ciscoes.

The researchers concluded that the introduction of muskies has not adversely affected gamefish populations in Minnesota lakes, supporting the findings of previous studies. They suggest that although muskies can attain large sizes, muskie density (number of muskies per unit of lake area) isn’t sufficient to “shape” fish communities through predation.

–Rob Neumann

*Knapp, M. L., S. W. Mero, and D. F. Staples. 2021. Are muskellunge affecting fish communities in waters where they have been introduced? A re-examination of Minnesota’s stocked muskellunge waters. N. Am. J. Fish. Mgmt. 41:229-241.

The Nature of Fish­: Fish Optimum Temperatures

No fish species has a single tem­perature that it seeks, but each does have a range within which it can excel. This chart offers a sin­gle optimum temperature—a sort of “average” tempera­ture-based on our perusal of various science documents that list temperature ranges.

The larger indi­viduals of a given species often seek colder temperatures than the optimums suggested in the chart. Colder temperatures also play a role during fall, when fish are feeding more heavily to stock up for what may be at least a partial win­ter fast. Larger fish also often feed more effectively than smaller fish during fall—one reason it’s considered a peak period for trophy fish.

–In-Fisherman

Food Science: Catfish Flavors

The flavor of catfish is partially determined by their diet and the quality of water where they’re caught. Fish absorb chemical com­pounds in their fat layers from the food they eat. Veteran anglers remove the yel­low fat and red meat that, when cooked, gives the fillet an unpleasant flavor. In some polluted waters, anglers may endanger their health by not remov­ing this fat before cooking.

Even where the water appears unpolluted, cat­fish sometimes take on “off-flavors.” Scientists have isolated chemicals in cat­fish ponds that causes these off-flavors. According to the Southern Regional Aquaculture Center (SRAC), off-flavors are commonly described as “earthy,” “muddy,” “moldy,” and “musty,” among other disagreeable tastes.*

According to SRAC, metabolites produced by blue-green algae are a common cause of off-flavors in pond-raised catfish, the most common being the chemical 2-methylisoborneol (MIB). MIB causes a musty-medicinal off-flavor. Another common off-flavor tied to blue-green algae is caused by geosmin, which gives fish an earthy or muddy flavor that is reminiscent of the odor of a damp basement, according to the SRAC. A third off flavor is called “woody,” which the SRAC reports as not being intense and the chemical cause of this “wood chip” taste is unknown. This woody flavor purges from fish more slowly that those caused by MIB and geosmin.

The SRAC describes another set of off-flavors as decay/rotten, including “egg-sulfury,” “sewage,” “decaying vegetation,” and “rotten,” with all of these having an apparent common origin of decaying plant or animal matter. According to SRAC, one study found “rotten” off-flavors more common in winter in ponds with dead fish, perhaps as the catfish feed on decaying fish. It’s unknown whether the decaying-plant flavor is related to catfish eating decaying plants or from checmicals released as plants decay.

The SRAC also reports that petroleum off-flavors can develop when fish are exposed to gasoline, kerosene, motor oil, and other petroleum products, most often due to accidental spills in commercial operations. These off-flavors can last for weeks to months and purging them is especially slow.

One of the remedies suggested for purging off-flavors in farm-raised catfish is transfering them to ponds or tanks with fresh, odor-free water. For decades, catfish anglers have held their wild-caught fish in stock tanks filled with clean water to remove the muddy taste prior to cleaning. The SRAC reports that off-flavors are purged more quickly in warm water than cool water. They suggest that small, lean fish held in warm, odor-free water purge MIB off-flavors within 2 to 4 days. Large, fatty fish held in cold water may not purge the flavor for weeks or months.

–In-Fisherman

*Tucker, C. S., and M. van der Ploeg. 1999. Managing off-flavor problems in pond-raised catfish. Southern Regional Aquaculture Center Publication No. 192.

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