MDC fish hatchery uses diverse trout species to control parasites

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ST. JAMES, Mo. – The Missouri Department of Conservation's (MDC) Maramec Spring Fish Hatchery is protecting its trout with more trout. The hatchery, which raises rainbow trout, is trying a new way of preventing parasitic infestation using brook trout.

Rainbow trout, along with several other west-coast fish species, are susceptible to a parasitic copepod called Salmincola californiensis. The tiny, shrimplike parasites, which are about the size of a pencil eraser, attach to fishes’ gills, where they leave eggs and complete their life cycle.

While the copepods are not a problem in the wild, they can become prolific in hatcheries that raise fish in high-density conditions. The copepods can attach in such high numbers that they weaken the fish, making them more prone to disease, and even causing the fish to suffocate.

According to Maramec Spring Fish Hatchery Manager Wes Swee, the U.S. Food and Drug Administration currently does not recognize any chemical treatments to control parasitic copepods.

A study at a California hatchery concluded that placing brook trout upstream from rainbow trout somehow filters copepod larvae from the water supply, reducing infestation. The reason for this is undetermined. Swee said one theory is that copepod larvae are specific to certain west-coast fishes and cannot complete their life cycle on other species, such as east-coast-native brook trout. According to Swee, the copepods attach to brook trout but do not produce eggs as they do on rainbow trout.

To test the theory on Missouri trout, the MDC is raising a small number of brook trout at Maramec Spring Fish Hatchery. In June the trout will be put in one pool above a pool of copepod-free rainbow trout from another hatchery. If successful, brook trout could be an effective biological control method for managing parasitic copepods in fish hatcheries.

Biological control methods are not a new concept. In fact, they are preferable when a food product like fish is involved. A comparable biological control method is the use of ladybugs to reduce aphid infestation in gardens.

“Using a biological pest control is better than using chemicals that may affect the safety of a food item such as fish,” Swee said.

Although the copepods themselves do not affect a fish’s food quality, protecting hatchery-raised rainbow trout from parasites is an effort to ensure the quality of Missouri’s trout fishing.

“Controlling the copepods will allow us to continue to stock healthy fish for the public,” Swee said. “The long-term goal is to provide Missouri anglers with healthy, more vigorous and more attractive rainbow trout that will put up a good fight for fishermen.”

Rainbow trout is the species most commonly stocked by MDC to provide trout fishing opportunities in designated Missouri waters. A limited number of brown trout, produced in MDC hatcheries, are also stocked in selected waters each year.

“There are no plans to release the brook trout into Missouri waters,” Swee said.

For more information on Maramec Spring Trout Park visit and search “Maramec Spring Trout Park.” For information on Missouri’s other fish hatcheries, search “Fish Hatcheries and Trout Parks.”