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How Water Temperature Affects Development of Young Salmon

Temperature is one of the most critical factors in salmonid incubation. Salmon at any stage need cold, clean, oxygenated water to survive and optimal temperatures vary depending on the species and life stage. The cooler the water, the slower the development, while warmer waters cause development to speed up. We can only estimate the growth rate of a wild salmon because the natural environment is always in flux, but we can fairly accurately predict the growth rate of salmon held in a controlled environment, like an aquarium, by closely monitoring the water temperature. That’s how hatcheries keep track of the development of their young salmon. Classrooms can do this too with a Classroom Incubation Permit (CIP) from the Alaska Department of Fish and Game. Without a permit, it’s illegal to move eggs or fish from one location to another.

The following questions and answers explain how Salmon in the Classroom hold lessons in both natural history and math.

When can you expect to see the first signs of development?

How are your eggs doing? You won’t be able to see much at first. If you could look inside the eggs with a microscope you would see cells multiplying but the first distinct features you will see with the naked eye are two black dots, which are the salmon eyes just beginning to form. This is called the eyed stage. You can predict when each stage is likely to occur by knowing the stages of the salmon lifecycle and monitoring the water temperature of the tank.

How can you predict the stages of the lifecycle?

That’s a great question and a fun math exercise for you and your students. But first, let’s review a little background information. The rate of development (or growth) of your salmon eggs is determined by the water temperature in the aquarium. The warmer the water temperature, the faster the eggs will grow. The reverse is also true: the colder the water temperature, the slower the eggs will grow. Unlike your students’ growth rate, this is something you can control—just like they do at the fish hatcheries.

Salmon require cold, clean, oxygenated water to survive. If the water’s too cold, development will slow and the aquarium may even freeze. If the water’s too warm (over 15° Celsius), the salmon development speeds up too fast. This can cause defects and death and also increases the biological waste in your tank requiring additional water changes to keep the tank clean.

Rapid water temperature changes or broad fluctuations in temperature can be harmful too. Avoid major fluctuations in temperature and make your changes gradually—by no more than 1° or 2° Celsius per day.

What is an ATU and how is it calculated?

Okay, now that you know how temperature affects salmon growth, you can begin to calculate the growth rate and determine if your salmon are developing properly by measuring the temperature in your tank daily. This daily measurement can be added up over a period of days to describe the cumulative effect of temperature over time, which is noted in a unit of measurement known as an Accumulated Thermal Unit (ATU).

Example: If your tank is at 1° Celsius all day then your thermal unit (TU) for that given day is 1. If your tank is at 2° Celsius the next day, then your TU for that given day is 2. To calculate your ATUs, simply add up each daily TU. So far your ATUs would be 3 in our example. [Note: You can calculate ATUs in Fahrenheit (see table below), but it is much easier to calculate the ATUs in Celsius. The formula to convert Fahrenheit temperatures to Celsius is: C = (F-32) x 5/9.

Your calculated ATU will now enable you to determine the growth rate of your salmon and when to expect each stage of development. The table below describes the expected number of ATUs for important embryonic developmental stages in coho salmon:

Stage ATUs (°C) ATUs (°F)
To Eyed Stage 220 396
To Hatch 400–500 720–900
To Emergence 700–800 1260–1800

Your students can help you keep track of TUs on a calendar or on a daily log (see resources below). Either way, be sure to track your TUs daily (it’s okay to estimate the weekend ATUs) so you can see if your salmon are developing properly.

More Resources for Classrooms