Part of a scientist’s job is to develop, establish, or improve methods of collecting, analyzing, organizing, or presenting data. Some scientists are particularly interested in helping to improve scientific methods. In this research, the scientists thought that a different method of collecting data might improve scientists’ understanding of fish development.

Have you ever thought of a better way to do something? Before you knew for sure that it was better, you would have to try your new way and compare it with an existing way. That is exactly what the scientists did in this research.

Over many years, decades, and thousands of years, organisms adapt to different factors in their physical environment. Temperature is one of the environmental factors to which organisms adapt. Many organisms live part, or all, of their lives in water. Water temperature, therefore, is a variable that affects the life cycle of many of Earth’s organisms.

In this research, the scientists studied the relationship of water temperature to the time it took Chinook salmon eggs to hatch and develop as young fish. The scientists also studied how water temperature affected the rate at which the fish developed. Water temperature can change over time as a result of global climate change, dams, irrigation, and changing land use. The scientists in this study, therefore, wanted to understand how variation in water temperature can affect the development of young Chinook salmon (figure 1).

Water temperature helps regulate aquatic ecosystems. Aquatic scientists, therefore, often measure water temperature. Scientists usually record daily water temperature by calculating the mean or average temperature over a 24-hour period. Scientists also calculate degree days. Degree days describe the total temperature units (TU) delivered. One degree day Celsius is equal to a temperature of 1 °Celsius (C) for 1 day. Temperature units describe the amount of energy or heat available to the fish. You will learn about temperature units and degree days in this article.

Aquatic organisms spend their lives in, on, or near water. For many aquatic organisms, life-cycle phases are tied to water temperature. Salmon eggs, for example, generally hatch when a certain number of degree days, or temperature units, have been accumulated. Scientists have figured out how many temperature units are required for Chinook salmon eggs to hatch and for young salmon to grow and emerge as young fish (figure 2). Scientists have focused their attention on total temperature units. Until now, they have not given a lot of attention to how those temperature units are delivered. Does it make a difference, for example, if the temperature is 3 °C one day and then 7 °C the next, or if it is just a stable 5 °C every day? Over a month, the same number of temperature units would be delivered but they would be delivered in different ways.

The scientists in this study knew that stream and river water temperature varies in at least two ways. First, the temperature varies across each 24-hour period. Second, water temperature varies within and across seasons. The scientists knew that recent human actions were altering the patterns of water temperature where Chinook salmon laid their eggs. Dams, land-use changes, irrigation, and climate change can cause altered water temperatures (figures 3, 4, 5, and 6).

The scientists suspected that fish might respond to more than just total temperature units. They thought that it might matter whether the water temperature was stable or variable. Just knowing the number of total temperature units delivered does not enable scientists to understand how temperature variations affect the life cycle of Chinook salmon. The scientists asked this question: Do daily and seasonal water-temperature variations affect the development of salmon eggs and alevins (a lə vənz), or young salmon, before they become free-swimming fry?

To answer their question, the scientists did something new. The scientists observed Chinook salmon development in fish that were exposed to different water-temperature patterns. The scientists altered water temperature experimentally to explore how water temperature variability affects Chinook salmon development.

The scientists collected adult Chinook salmon near the Roza Dam on the Yakima River in June and July 2009 and took them to a hatchery (figures 7 and 8).

The salmon spawned (laid eggs) while in the hatchery, and the scientists collected the fertilized eggs (figure 9).

The scientists put 150 eggs into each of 64 chambers (figure 10).

These chambers were set up so that the eggs would hatch and the alevins could develop into fry (figures 11 and 12).

The scientists controlled the water temperature in the chambers using very large aquarium heaters. They created eight different patterns of water temperature (figure 13). The different water-temperature patterns were meant to simulate stable temperatures, daily variation in water temperature, seasonal variation in water temperature, and daily plus seasonal variation in temperature. In two cases, the scientists also simulated completely unnatural patterns just to see if the fish would respond.

The scientists recorded the water temperature every hour. The temperature patterns were designed so that the chambers with variable temperatures delivered about the same total temperature units every day as those with stable temperatures.

At 11:00 a.m. every day, the total temperature units (TUs) for the past 24 hours were recorded for each chamber and the scientists checked to see if any fish had emerged. The scientists noted the day when most of the fish, 125 of the 150 alevins, emerged as fry. At that point, the scientists calculated the total TUs that had accumulated in each chamber over all the days that the alevins had been developing in that chamber,

The scientists designed temperature patterns that delivered about the same daily total TUs but had different kinds of variability. By using these patterns, the scientists measured the impact of variation in water temperature on the development of Chinook salmon. Note that everything else was held constant in the experiment. Any differences between the TUs that had been delivered when most of the fry emerged, therefore, were likely related to the variability in the water-temperature patterns.

The scientists collected a sample of fry as they emerged from each chamber. The scientists weighed, measured, and determined the fry’s overall condition.

The water-temperature patterns influenced when fry emerged. Eggs and alevins exposed to the most stable water temperature (5 °C and 10 °C) had accumulated a mean of 1,160 (5 °C) and 1,153 (10 °C) TUs by the time most of the fry had emerged. Recall that the scientists used eight chambers for each of the eight temperature patterns. Therefore, the scientists calculated mean accumulated TUs for each of the eight patterns. The scientists expected these numbers to be similar. They expected this similarity because they knew that Chinook salmon eggs require a certain number of TUs to be ready to emerge.

The fry living in the chambers in which the water temperature varied by 5 to 10 °C once per day emerged after accumulating fewer TUs than when the water temperature was held stable (figure 14). The scientists discovered something different for the alevins exposed to water-temperature patterns with extreme daily or extreme seasonal variations. These alevins needed to accumulate more TUs than when the water-temperature was held stable for most of the fish to emerge as fry.

The scientists also discovered that fish exposed to the warmer patterns of water temperature emerged less fully developed than other fish. These fish were four times more likely to still have a visible yolk sac upon emergence.

When scientists consider fish development based only on accumulated temperature units, they may be missing an important influence on fish development. This research shows that the timing of Chinook salmon development is dependent upon more than total accumulated temperature units. It matters how those total temperature units are delivered. Daily and seasonal water-temperature variability have an impact on how many temperature units are required for most of the fish to emerge.

The scientists estimate that variability in water temperature may affect fry emergence by as much as 1 week. Fish, like most wildlife species, have adapted to the availability of
food sources in time. Emerging a few days early or late may cause fry to miss important food resources. If human changes to water temperature patterns affect when fish emerge, fish might emerge at a time with fewer food resources. The possible lack of food resources could affect Chinook salmon survival.

Understanding the relationship between fry emergence and water variability is important now because humans are changing water temperature patterns. Building dams, changing land cover, irrigating, and changing the global climate may have important impacts on the life cycle of Chinook salmon.

In 2014, the National Park Service completed the removal of two large dams on the Elwha River in Olympic National Park,
Washington State (figure 15). For 100 years, the dams blocked all but the last 5 miles of the river. This blockage created reservoirs (large lakes) where a river once was, and stopped natural sediment flow and salmon migration. The dams had dramatically changed the Elwha River ecosystem. Removal of the dams has reopened more than 70 miles of important salmon spawning habitat, restored natural sediment flow, and begun the restoration of the Elwha River ecosystem.