Science Friday: Surviving heat, drought and ponded streams
It is that time of year again. Heat wave after heat wave.
As summer progresses stream flows will continue to decline all across steelhead country, and in some cases, smaller tributaries will go dry. In other cases, streams won’t be completely dewatered; instead, they will become ponded. This occurs when flows diminish so much that the only remaining surface water is found in a few disconnected pools.
Fish occupying these pools are, literally, stranded.
Such streams may not seem all that important during summer. However, research indicates that in some watersheds juvenile steelhead do indeed depend on habitat in intermittent streams and in fact can survive an entire summer in them, as long as the water temperatures don’t get too warm or dissolved oxygen levels too low.
Which brings us to this week’s Science Friday topic: a study by C. Woelfle-Erskine and co-authors at University of California, Berkeley (https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecs2.1645). The authors looked specifically at the survival of juvenile steelhead in streams that became ponded during summer.
The results are not entirely surprising to those familiar with steelhead habitat conditions in California and other dry regions of the steelhead-world. They are nonetheless interesting because they highlight just how resilient steelhead can be – if they have the necessary habitat.
In this study the authors examined juvenile steelhead in two tributaries to Salmon Creek, a small coastal stream in Sonoma County, California. Their goal was to examine the survival of juvenile steelhead as stream flow declined and the creek became ponded, and, to determine why certain pools were more favorable for steelhead survival than others.
First, the authors found that dissolved oxygen and pool volume decreased sharply as riffles went dry and pools became disconnected. However, water temperatures did not exceed 65F, which is getting warm, but is not yet lethal for O. mykiss.
Second, pools with the most steelhead in June tended to have more juveniles remaining at the end of summer. Third, survival in these conditions was not easy. Juvenile steelhead were able to persist for weeks in large pools with low dissolved oxygen levels, including some pools where Dissolved Oxygen in at least part of the water column reached sublethal or lethal levels.
This is not to suggest that low dissolved oxygen levels are not important or of concern in managing to conserve wild steelhead. Maintaining water quality is critical, but the results underscore that some fish can persist even the toughest of times.
Last, pool volume, depth and surface area generally had a positive effect on survival of juvenile steelhead, indicating that larger pools with adequate depth are the best places to hunker down when streams become disconnected.
The results suggest that juvenile steelhead survival depends on the quality of the pool habitat during the dry season and during times of drought—at least in streams that are intermittent. As anglers, we focus less on these habitats, but as this paper (and others) indicate, they play an important role in providing viable habitat for young steelhead. And nearly every intermediate to large watershed from Alaska to southern California has some intermittent streams—even in the rainforest of Washington’s Olympic Peninsula.
So, next time you see a drying-up stream and observe some juvenile steelhead in one of its pools, it’s okay to be concerned—but better to remember that the most current science suggests young O. mykiss can survive in much of this type of habitat, especially if we continue to do the kind of work Trout Unlimited does all across the United States: building partnerships with landowners and resource agencies to secure domestic and agricultural water supplies while reducing stream diversions in the summer and fall.
Given their relative ubiquity, and high value to steelhead, Wild Steelheaders United has spent a lot of time and energy trying to maintain and improve protections for intermittent streams in steelhead watersheds. You can help this effort by signing our Credo and donating to the cause. And get involved in your local watershed to help ensure we manage the habitat in ways that benefit steelhead.