Science Friday: Warm water’s influence on the speed of upstream migrating steelhead
The dog days of summer are fully upon us. The predicted forecast for adult summer steelhead returning to the Columbia and Snake River basins is now, unfortunately, shaping up to be worse than expected (and it was already very low). However, it can be difficult to determine how accurate this forecast actually is because at this point in time the Columbia River is running warm. In fact, it’s not an exaggeration to say, from a fish perspective anyway, that it’s running hot.
Water temperatures at Bonneville Dam have been consistently over 70°F and reaching upwards of 74°F for the past week or two. Why is this important relative to steelhead migration upstream? Because such warm temperatures can literally affect the swimming speed of the fish on their journey back to fresh water.
One reason that managers are not yet certain how poorly this year’s run of summer steelhead will fare is because of those very high water temperatures. While steelhead are relatively tolerant of warmer water temps compared to many species in the family salmonidae, temperatures over 70°F can have a suite of negative impacts – including potentially delaying their upstream migration. If the temperatures are delaying upstream migration through the dams, then we might not have a full understanding of the true run sizes until the temperatures subside into a more optimal zone for the fish.
A study by David Salinger and James Anderson in 2006 measured upstream migration rates of steelhead in the Columbia. The goal of this analysis was to understand how much, if at all, elevated water temperatures altered the pace of steelhead migrating upstream. To accomplish this the authors relied on passive integrated transponder (PIT) tags, which are placed within the body cavities of fish and are about the size of a grain of rice. The dams have PIT readers, which allowed scientists to track how quickly the fish moved upstream. In total, they evaluated the speed of upstream movement for over 3,200 steelhead from 1998-2002.
The results reveal several key findings.
First, over 93% of the steelhead detections at Bonneville Dam (the lower-most dam on the Columbia) occurred when water temperatures were between 65°F -72°F.
Second, there was extensive variation the time it took steelhead to cross Bonneville Dam and reach Lower Granite Dam on the Snake River. The distance between the dams is about 277 miles. The median time it took for steelhead to cover that distance ranged from 32 to 47 days, depending on the year. However, some individuals raced through the 277 miles in only 9 days, while others took as long as 160 days. This is a remarkably broad range in terms of how quickly steelhead traversed the six dams between Bonneville and Lower Granite.
Lastly, the speed at which steelhead ascended the path up to Lower Granite was strongly associated with water temperature. Specifically, the speed of migration decreased as water temperature increased, presumably because the elevated temperatures increased metabolic demands and left less energy for swimming.
All in all then, warm water temperatures can and do impact the swimming speed of steelhead, which in turn influences how quickly they are able to ascend the Columbia and Snake Rivers.
The warmer-than-average water temperatures are just another challenge for steelhead this summer. Already at low abundance, the warmer water is likely to prompt steelhead to find thermal refuge as they migrate — to seek out places where they can reduce exposure to warmer temperatures and simultaneously reduce their metabolic demands. This can make them more vulnerable to predation and disease, and if temperatures remain elevated for long periods of time the steelhead may also begin to experience other effects, such as reduced viability of their eggs.
It will be another couple of months before we have a solid grasp on this year’s summer run sizes, and a better idea of just how much the water temperatures influenced their rate of migration. Until then, lower water temperatures would really help more steelhead make their upstream journey.