Friday, December 9, 2011

The Pitter Patter of.....Little Fish!

Within the cold, dark Heath trays of Dworshak’s incubation room an exciting transformation is taking place. Eggs that were spawned from returning Chinook salmon last August are beginning to hatch. Alevin, or yolk bearing fish larvae, are breaking free from their shells and beginning their first stage of life as fish. Although free from the confines of the corion, or egg shell, alevin are quite limited in mobility and still depend on their yolk sac for nutrition. In the wild alevin remain in the shelter of the gravel redd for several weeks after hatching. Once most of the yolk sack is absorbed the tiny fish will emerge from the gravel as fry, about 1 inch long.
Stacks of Heath style incubators stand like soliders in Dworshak's incubation room.

At the hatchery, alevin remain in the safety of their Heath trays making a slow but steady transformation from larvae to fry. Newly hatched Chinook have many developmental milestones to reach before they can survive life outdoors.  Just after hatching these tiny alevin don’t appear very fish like. Externally their fins will continue developing over the next few weeks and internally their digestive system has yet to form completely. 

Newly hatched Chinook alevin hardly resemble fish...

...but in a few months they will look like this!
Temperature plays a huge role in the rate at which salmon develop. The ideal rearing temperature for Chinook salmon from egg to fry is about 8 °C (46.4° F), but rearing temperatures can range from between 4-12° C (39-54° F). A rule of thumb is the colder the water the slower development will be. In fact it is common practice in aquaculture to use temperature units (TU) to estimate the critical stages of egg and fry development.  One TU is equal to one degree Fahrenheit above 32° F for every day of incubation. So under an ideal rearing temperature of 46° F Chinook salmon gain 14 TUs a day. In general, Chinook salmon eggs reach the eye up stage at 450 TUs, will hatch at 750 TUs, and will emerge around 1600 TUs.
Eyed Chinook eggs with a few newly hatched alevin

The 2011 brood year of Chinook salmon are at about mid-hatch and will continue their exciting, yet inevitable transformation into tiny fish. The majority will grow up strong and healthy at the hatchery and enter the Clearwater River in the spring of 2013. It is amazing to know that many of these fish will survive and live in the ocean for as much as 5 years; but like their parents before them instinct, determination, and good fortune will direct them back to Dworshak where they will pass along their legacy to the next generation of Chinook salmon.

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