Missouri officials use diverse trout species to control parasites

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

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

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.


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