May 2021 - 41

Fish reproduction
When fish in Wyoming color up, school up and
stop eating they are spawning. Fisheries biologists pay
close attention to this natural process, which assures an
ongoing supply of fish in the wild and for us to catch.
The spawning process varies from species to species.
Take lake trout. Imagine your mate giving you a
head-butt in your ribs when he's in the mood. While
that might not sound romantic to human women,
during the fall female lake trout get the hint, releasing
a few hundred eggs at a time. The nudge comes at
night, and could come from more than one male,
which fertilize the eggs with their milt, which is fluid
containing the sperm of a male fish. The eggs fall
into the gravel at the bottom of the lake, which helps
protect them from hungry predators until they hatch
in the spring.
Arctic grayling do the deed a little differently.
While they are broadcast spawners, meaning they
release eggs into the water rather than building nests,
they reproduce in the spring. Male grayling establish territories in gravelly, shallow tributaries. When
females arrive, the fish school up. Females release
their eggs as the males release their milt. The fertilized
eggs fall into the gravel, where they remain until they
hatch in the fall, get flushed downstream or get eaten
by other fish.
Cutthroat and rainbow trout, which also are spring
spawners, have another way of reproducing. Females
dig redds the gravel then lay their eggs. Males fertilize
the eggs in the redds where the eggs remain until they
hatch three weeks later if the water temperature is
ideal in the 50s. It takes up to four months longer if
the water is colder. Brown and brook trout also make
redds when they spawn, though they do it in the fall.

Spring versus
fall spawners
How and when fish spawn varies from species to
species, giving that species situation-specific advantages or disadvantages.
" One of the reasons brook trout displace cutthroat
trout in high-elevation streams is the advantage brook
trout have as fall spawners over spring-spawning cutthroats, " said Matt Hahn, Wyoming Game and Fish
Department fisheries biologist. " Brook trout hatch in
the spring, so they are larger than the cutthroats when
the cutthroats emerge. The cutthroats are vulnerable
to direct predation by brook trout. "
Fall spawners don't always dominate. It's site-specific. In streams where flow is limited in late summer
spring spawners usually do better, Hahn said. During
the spring, there's lots of available spawning habitat.
Gravel beds have recently flushed clean. There might
not be as many options in the fall when water flow

The biggest difference between male and female Arctic grayling is the dorsal fin on the male, left, sweeps as far
back as the adipose fin, and females exhibit a shorter, more rounded fin. Female grayling release their eggs into
the water instead of building a redd for the eggs. (Photo by Patrick Clayton)

Brown trout like this one spawn in the fall and make nests called redds where females lay eggs and the males fertilize
them. (Photo by Jack Ballard)

Brook trout spawn in the fall and eggs hatch in the spring. Fish that spawn in the spring, such as cutthroat trout, are
vulnerable to predation by brook trout. (Photo by Lisa Ballard)

		

Wyoming Wildlife | 41



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