Claire Corlett

Fish Food, Fish Tanks, and More
Salmon in the Great Lakes: Part 1, Early Life History

Salmon in the Great Lakes: Part 1, Early Life History

Each spring students from a hundred and
eighty schools around Michigan release salmon raised in their classrooms. These are Chinook salmon, a species is native to the Pacific
Northwest. Millions of Chinook salmon are stocked in
the Great Lakes Basin in each year along with trout and salmon of other species. Some are stocked in the net pens like
these, which are maintained by volunteers who
feed the young fish as they imprint on river water. Others are raised exclusively in
state-run hatcheries or by teachers and students in a classroom setting. The
Michigan Department of Natural Resources’ Salmon in the Classroom program provides
teachers with salmon eggs and the training necessary to care for and stock
young fish. Caring for salmon as they grow from egg
to smolt is a powerful experience that teachers can incorporate into lessons on
Biology, Earth Science, Chemistry and Social Studies. Salmon in captivity have the advantage
of living in an environment with no predators, optimal temperature, and plenty
of food, but natural reproduction also occurs in many streams and rivers that
feed into the Great Lakes. In fact, over half of all Chinook salmon in
Lake Michigan are now naturally spawned in clear cool streams like the Pere
Marquette River in northern Michigan. In Lake Huron, the vast majority of Chinook salmon are now wild descendants of stocked fish. Scenes like this are evidence of improved
water quality and stream habitat in Michigan rivers but natural reproduction of
non-native fish also has a downside. Many people see it as a mixed blessing
at best. To understand why, this video series will
explore the history of the Great Lakes region and the ecological changes that
have occurred over the past two centuries. First we will examine the early life history of Chinook salmon in more detail. In the wild, eggs are
fertilized and deposited over depressions in gravel or cobble. These
nests are called redds and they’re typically located in stream reaches with
fast-flowing and well-aerated water. Most eggs are spawned in
September, October, or November. Eggs incubate in the redds through the winter and hatch
into sac fly or aelvins in late winter or early spring. The yolk sac provides a nutrient rich
source of energy for growing fry. In captivity, warmer temperatures speed up
the life cycle. These sac fly were hatched indoors in December. Like their wild
counterparts they’ll need to swim to the surface and fill their swim bladders
with air before beginning to feed on their own. After emerging from the gravel young
salmon are called parr. This life stage can be distinguished by purple
thumbprint-like markings known as parr marks. Naturally spawned parr remain in
the river, often moving to eddies near the edge of fast, gravely areas where
they were born. These wild parr in the White River are
feeding on small insects and other food trapped in the surface film. The current
is relatively slow here but the tiny parr still work hard to maintain their
position, snatch prey, and avoid predators
including kingfishers and herons. Looking up isn’t enough to avoid
predators either. Brown trout like this one are voracious fish eaters that feed
at any time day or night. Trout are most common in the cold, gravely streams where salmon are spawned. As young salmon move downstream they encounter a wider
variety of predators in the murky water of large rivers. Coolwater fish like
this walleye, and warmwater fish like this bass lurk among the rocks and logs
of the big river. The transition between the river-dwelling parr stage and the
open lake phase of life is known as smoltification. During this transition, salmon lose their
parr marks and develop a bright silver coloration. The larger a salmon is during
smoltification, the better chance it has of evading predators. One look at this classroom tank is
enough to know that salmon here have no problem getting enough to eat Abundant food and optimal temperature
will give them a size advantage over their wild cousins. Although being born wild offers the
advantages of natural selection. Regardless of where salmon were spawned and
raised, all of them eventually find a home in the Great Lakes, where they will
feed and grow for one to four years. Chinook salmon are eating machines in the big
lake, where they can gain five to ten pounds per year on a diet of invasive
alewife and other open water baitfish. Later videos in this series will
explore how alewife and other unwanted invaders have changed the ecology of the
Great Lakes and take a closer look at the end of the salmon’s life cycle when
mature salmon turn dark in color and return to their natal streams or
stocking sites. This video was produced by Michigan Sea
Grant in cooperation with the Michigan DNR.

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