By John McMillan
We are going old school today, Science Friday style. No, we’re not talking about shooting ourselves with a tranquilizer gun, going streaking, or starting a fraternity to compensate for a mid-life crisis. We’ll leave that to Will Ferrell.
But we are going back in time, to 1971. Today we revisit one of the foundational pieces of research conducted by Ted Bjornn (RIP), a former scientist and professor with University of Idaho and the Idaho Cooperative Fishery Unit. Ted authored numerous studies on salmon, trout and steelhead, and in many cases, we stand on the shoulders of that great work.
Bjornn conducted a study in two Idaho creeks to better understand when and why juvenile steelhead and rainbow trout move within a stream. Juvenile steelhead and trout are often highly mobile. They might be born in one place and then move to another. In fact, we now know they may undertake extensive migrations for over tens of miles to reach different habitats at different times of year. Bjornn’s work to study this phenomenon was seminal.
Basically, Bjornn wanted to evaluate how streamflow, water temperature, food, competitor density and substrate influenced the movement of juvenile fish. His methodology included installing weirs in two streams so he could capture juveniles, and creating artificial stream sections by adding big rocks, to determine the influence of substrate size.
For our purposes, we will focus only on the results for juvenile steelhead and rainbow, which are lumped into one group because they are indistinguishable in appearance as juveniles.
The study produced an intriguing series of results. Can you guess which of the five factors evaluated had the strongest influence on juvenile movement?
Temperature seems like a great candidate. Juvenile O. mykiss, and other species of salmonids, often undertake movements during the transition from summer to fall. The general assumption is that such behavior is prompted by changes in water temperature and stream flow. At this time of year, in the study area, flows get higher and temperatures drop. Fish seek out other areas in preparation for winter. (While the influence of season on flows and water temps varies across steelhead range, the behavior of fish under conditions such as those monitored in the Bjornn study is expected to be consistent from one watershed to the next.)
Indeed, fish started leaving the streams in fall as water temperatures declined, but, in the test troughs he could not induce movement by altering the temperature. Moreover, the fish moved downstream despite the streams having relatively stable flows.
If not temperature, then what about food? There is typically less insect drift in winter than summer. If the fish notice a reduction in macroinvertebrate drift, perhaps that stimulates them to move?
That was not the case either, however. Like little prophets, juvenile Mykiss actually started leaving before the abundance of drift insects declined going into winter.
Well, if not those, then maybe density? As we have examined in previous posts, too many fish in too little space is problematic.
Bing, bing! Bjornn found that, indeed, the higher the juvenile fish densities, the greater the rate of emigration from the overused habitat. Still, it can’t be that simple, can it?
No, it wasn’t.
The strongest correlation between potential factors and juvenile fish movement in this study was with larger cobble substrate, which provides ample cover for small fish. In this study, subyearling Mykiss left the creek that did not have any cobble substrate, but remained in the stream that did. That pattern was consistent in both the stream and artificial trough tests. Those little fish did not want to leave the safety of bigger rocks.
And there was an interaction between factors. Juveniles tended to start entering the substrate when stream temperatures declined to 39°F-42°F.
Ultimately, the Bjornn study revealed that temperature was also important. Colder temperatures slow the metabolism of fish and reduce the ability of juvenile Mykiss to evade predators. When things get really cold, they seek cover in the river’s basement (large cobble substrate). And if their summer home doesn’t have a basement, they move to another stream or piece of habitat that does.
But the primary take-away from this groundbreaking study is that larger substrate is really important for rearing juvenile steelhead and rainbow trout and especially so in winter. It provides cover and refuge during challenging times of the year for fish, which is why it is critical for us to properly manage land use practices that can increase fine sediment levels in streams, which in turn can fill those small spaces between cobble that Mykiss use as a winter hidey-hole.