All washed up? The effect of floods on cutthroat trout

The activities are as follows:

Mack Creek, a healthy stream located within the old growth forests in Oregon. It has a diversity of habitats because of various rocks and logs. This creates diverse habitats for juvenile and adult trout.

Streams are tough places to live. Fish living in streams have to survive droughts, floods, debris flows, falling trees, and cold and warm temperatures. In Oregon, cutthroat trout make streams their home. Cutthroat trout are sensitive to disturbances in the stream, such as pollution and sediment. This means that when trout are present it is a good sign that the stream is healthy.

Floods are very common disturbances in streams. During floods, water in the stream flows very fast. This extra movement picks up sediment from the bottom of the stream and suspends it in the water. When sediment is floating in the water, it makes it harder for fish to see and breathe, limiting how much food they can find food. Floods may also affect fish reproduction. If floods happen right after fish breed and eggs hatch, young fish that cannot swim strongly may not survive. Although floods can be dangerous for fish, they are also very important for creating new habitat. Floods expand the stream, making it wider and providing more space. Large boulders, smaller rocks, and wood also move into the stream from the powerful behind the moving water. These natural items add to the different types of habitat available. 

A cutthroat trout. It is momentarily unhappy, because it is not in its natural, cold Pacific Northwest stream habitat.

Ivan and Stan are two scientists who are interested in whether floods have a large impact on the survival of young cutthroat trout. They were worried because cutthroat trout reproduce during the spring, towards the end of the winter flood season. During this time juvenile trout,less than one year old, are not good swimmers. The fast water from floods makes it harder for them to survive. After a year, trout becomes mature adults.These two age groups live in different habitats. Adult trout live in pools near the center of streams. Juvenile trout prefer habitat at the edges of streams that have things like rocks and logs where they can hide. Also, water at the edges moves more slowly, which is important for survival of juvenile trout. The edge habitat is good for juvenile trout because it brings them food and does not take too much energy for swimming. In addition, they can avoid getting eaten by the adults in stream pools.

Ivan and Stan work at the H.J. Andrews Long Term Ecological Research site and wanted to know what happens to cutthroat trout after winter floods. They have been collecting data on Mack Creek, one of the streams within their site, since 1987! Major floods occur every 35-50 years. Since flooding can make life difficult for trout, scientists expected trout populations to decrease after major flooding events. 

Fortunately, Ivan and Stan had the data they needed because of the long-term data collection that continues to take place in Mack Creek. To study how floods affect trout populations, Ivan and Stan compared population sizes of trout for many years. They decided to look at the population size of both juvenile and adult trout. They compared the trout population data, measured as the number of trout, with stream discharge, or ameasure of how fast water is flowing in the stream.Stream discharge is higher after flooding events. Stream discharge data for Mack Creek is collected during the winter when floods are most likely to occur. Fish population size is measured during the following summer each year.  

Featured scientists: Ivan Arismendi and Stan Gregory from Oregon State University

Written by: Leilagh Boyle

Flesch–Kincaid Reading Grade Level = 7.5

Additional teacher resource related to this Data Nugget:

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