Honors College Thesis
 

Cool hideaways: use of temperature refuges by juvenile coho salmon in the West Fork Smith River

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  • The West Fork Smith River, a 69 km2 watershed in the Coast Range of Oregon, is prone to short periods of very high water temperature in mid-summer due to a combination of human and natural influences. In the summers of 2003 and 2004 more than 400 juvenile coho salmon regularly packed themselves into a 4m2 area of cooler water near Crane Creek. In the same watershed, other groups of coho salmon and trout were monitored in three other cold refuges where they avoided main channel water temperatures that reached as high as 25oC. In 2003 we mapped cold refuges and performed snorkel surveys and fish counts at several sites. In 2004 we chose four main refuges as study sites. We installed temperature loggers in each refuge and nearby main channel. We measured (length, weight) and PIT-tagged juvenile coho salmon at each site in early July and recaptured and measured fish in September. We tracked fish movement throughout the summer using a portable PIT-tag reader and snorkel surveyed to count fish numbers in the refuges and nearby pools at various water temperatures and to observe fish behavior. However, of the 178 coho salmon tagged only 50 were recaptured. Local fidelity to refuge sites, in association with peak temperatures in mid summer best describes juvenile coho use of coldwater refuges at West Fork Smith River. The number of coho salmon in refuges increased with main-channel temperatures from 18ºC to 25ºC. Coho salmon and other fish congregated in multiple smaller pockets of cold water or “micro refuges” not detected during the initial refuge mapping. Even so, the total number of detected coldwater refuges was less than fifteen areas in over 15 kilometers and total refuge surface area was only 0.00002% of the mainstem surface area searched. At the four study sites differences in peak temperatures, refuge morphology and presence or absence of large trout resulted in variable patterns of use by juvenile coho salmon.
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