Science Friday: Steelhead fry party when the lights go off — but will they wake up high and dry?
In our last Science Friday post we took a look, in part, at the feeding behavior of bass. This week we are moving down in the feeding column to focus on catfish.
Just kidding. It’s all steelhead all the time here.
Today we review a paper by David Hines and several colleagues. The study focuses on the behavior of newly emerged juvenile steelhead, otherwise known as fry, and discusses the implications of the findings for how vineyards manage their water use.
The Hines study was conducted in a couple of streams that drain into the Pacific Ocean just north of the San Francisco, including Austin (tributary to Russian River), Lagunitas and Salmon Creeks. The goal was to understand movements of steelhead and coho fry, but for our purposes we will look only at their results for steelhead.
The researchers uncovered some interesting things.
First, time of day influenced habitat use by fry. Steelhead fry occupied shallow water near stream margins both during the day and at night. However, the proportion of fry within a foot or two of the channel’s edge increased substantially as ambient light declined, from 6% of the fry during the day to 44% at night.
Second, the proportion of fry in 4” of water or less similarly increased inversely with ambient light, from 26% during the day to 76% at night.
The phenomenon of fry moving into shallower water at night has been found in other studies, and I have observed the same in my field surveys. In fact, it is not uncommon for me to dip my head into the shallows at night and see hundreds of shining fry as I sweep the beam of my waterproof light across the river. The fry simply love the shallow areas when it’s dark. The combination of my spotlight and the shining fish makes it look like they are enjoying a throw-down party.
My inference is that steelhead fry rely heavily on shallow water habitat when conditions are favorable for them to do so because the shallows are one of the safest places for them to be. Bigger fish can’t get into those areas to feed on the fry. Herons and other winged or furred predators don’t have much chance of catching fry in the dark. It’s not perfectly safe however. Sculpin can make it into those shallow areas too. Still, one predator is better than several. The bottom line is, steelhead fry are poor swimmers and the best way for them to make a living is to move to different places over the course of each 24 hours where risk of predation is minimized and chances for acquiring food are maximized.
While this behavior is an important evolutionary adaptation, according to the Hines study it also poses risks. This is because reliance on shallow channel margins puts the fry at increased risk of being stranded if there are reductions in stream flow — such as are common at certain times of year in wine country, where vineyards divert water from steelhead streams for various uses, including frost protection. Such actions can temporarily reduce stream flow by more than 50%.
There are a number of vineyards in the study area that divert water for frost protection. Application of water to grape vines to prevent frost damage typically happens in the middle of the night. Hines et al surmised this has implications for steelhead fry, which favor the shallows under cover of darkness. Nighttime diversions for frost protection may increase the risk for steelhead fry of becoming stranded or unable to move or respire.
At the end of the day — literally and figuratively – juvenile steelhead rely heavily on shallow water habitats early in life. This matters because the future of steelhead populations — and our fishing opportunities — depend on survival of those fry.
This is another reason that TU and Wild Steelheaders United are working with wine grape growers and other agricultural operators from Washington’s Methow Valley to California’s Russian River watershed, to restore habitat and fish passage and reduce the impacts of stream diversions on steelhead and salmon. Go here to watch short videos of some of these projects, and here and here to read about some of them on the Trout Unlimited website.