Science Friday: Turn up the A/C
Summer-time is here. That means hot weather and hot water, two things that don’t mix well with a cold-water fish like steelhead.
As we suffer through the largest heat wave of the summer, we wanted to review a piece of research that looked at how adult steelhead alter their behavior and use micro-habitats to cope with warm weather and water.
Claudio Baigun published a study in 2003 that examined habitat use by wild summer steelhead in Steamboat Creek, a tributary to Oregon’s North Umpqua River. Specifically, he wanted to determine whether steelhead were seeking out and using certain types of habitat during the hot summer months. During the two years of data collection more than a quarter of the wild summer population was estimated to be holding in Steamboat Creek, clearly demonstrating the importance of this small tributary to the Umpqua’s wild steelhead. Despite the large numbers of fish present in Steamboat Creek they did not use a large proportion of the available habitat, rather, they focused strongly on using one type of habitat: deep pools.
How did they come to this finding? First, researchers measured and characterized the habitat found in 38 deep pools, or those pools having at least 0.8 meter of depth — a bit under 3’ deep. Strikingly, while such pools accounted for only 4% of the total habitat in Steamboat Creek, they were the only types of habitat used by adult steelhead. Think about that, hundreds of steelhead in a fairly large creek relying on only 4% of the available habitat.
Second, the author further separated these deep pools into “cold” and “warm” categories. He found that steelhead strongly favored the cold pools, which were defined as having a temperature below 19°C (66°F). Not all pools were simply “cooler.” Some were warmer than 19°C at the surface, but were more than 2°C colder near the bottom. They were what we call thermally stratified, which happens if there is limited mixing of the warm and cold water. Colder water is denser than warmer water, so if springs or seeps come in somewhere along the bottom of the pool, the cold water tends to remain there. And that is exactly where steelhead in those pools were holding.
Interestingly, steelhead were not observed in cold non-pool areas near tributary junctions where one might expect to find them, indicating temperature was not the only factor driving habitat selection. Fortunately for anglers, steelhead in larger rivers don’t have to be quite as selective about their holding water because there tends to be more cover and diversity in water temperature.
As steelhead fishermen and women we know that some pools are better than others. That should not come as a total surprise. However, we also know that no single factor explains why fish use a certain place. That could explain that while pool selection was strongly correlated with temperature and depth, fewer than half of the deep cold pools were used in either year of the study. Why? By further examining the data, Baigun was able to conclude that steelhead were also keying in on coarse substrate, abundant shade and favored long pools over short ones.
The research underscores the important nature of these relatively rare micro-habitats that enable wild steelhead to survive warm low water conditions for many months. This has a number of implications for conservation. For example, it suggests that most of the steelhead pack into a few places. That can make them particularly susceptible to predation and angling. It also can be used to help direct restoration efforts so that they can have the maximum benefit to wild steelhead and in turn, the future of angling opportunity. Lastly, it highlights that steelhead will increasingly need these types of cold-water habitats to persist through climate change and the types of heat waves we are now experiencing.
A bill that would designate 100,000 acres surrounding the headwaters of Steamboat Creek as a wild steelhead sanctuary is currently making its way through congress. This designation would afford additional protections to this important tributary and the resting pools necessary for wild steelhead to survive the summer and reproduce, all while maintaining public access. Thank you to all who have helped make this a priority for our legislators.