Science Friday: Big fish, big streams; little fish, little streams
A holiday weekend deserves a new Science Friday post. So here we go.
This week we focus on summer steelhead in the John Day River, a large tributary that drains into the middle Columbia River on the Oregon side.
The John Day is a big watershed, covering 8,000 square miles, although the river itself is not that large. To put that in perspective, the state of Vermont is about 9,600 square miles in size. Connecticut is about 5,500 square miles, and Massachusetts is 7,838 square miles.
Think about that. The John Day basin is bigger than the entire state of Massachusetts and almost the size of Vermont.
How many Fenway Parks does the John Day have? Zero. But how many wild summer steelhead does Massachusetts have? Enough said.
Today we review a study led by Justin Mills and colleagues at Oregon State University. Much of the work was funded by Dr. Chris Jordan, a scientist who works the National Marine Fisheries Service (NMFS).
The authors set out to ask a simple question: Does stream size influence the distribution of resident rainbow trout and steelhead in the John Day system?
To accomplish this analysis Mills et al sampled 72 different streams in the basin, ranging from small to large. At each site they collected a couple of juvenile young-of-the-year (YOY) O. mykiss. then extracted otoliths from those fish.
Otoliths are an ear bone in the head of the fish. Scientists can determine from the micro-chemistry of the otolith whether the fish’s mother went to the ocean or remained in freshwater.
The authors thus determined whether each of the sampled fish had a steelhead or a rainbow trout for a mom. They then compared those results to the size of stream where the fish were sampled. So, basically, they wanted to determine if the probability of anadromy changed with stream size.
Their findings are straightforward: the probability of anadromy – being a steelhead – increased as stream size increased.
Conversely, as stream size decreased, the proportion of resident rainbow trout increased.
This makes sense. Larger bodied individuals may not be able to access smaller streams. On the other hand, while smaller bodied fish may access larger streams without problems, a female trout typically matures at a much smaller size than a female steelhead. That means she faces a serious egg deficit, in which case, steelhead – and their thousands of eggs – may simply outcompete a smaller trout – and her few hundred eggs.
Further, larger streams tend to have more variation in stream flow and more energy. Smaller females can only bury their eggs a few inches deep in the gravel, while larger steelhead can bury theirs much deeper. Consequently, high flow events in larger streams may more frequently scour eggs deposited by the little girls.
Regardless of the reason, the findings have important implications for conserving steelhead habitat. While all streams are important to the hydrological processes in rivers, some are more important than others to anadromous O. mykiss. In the John Day, for instance, we can now predict which streams are more likely to have steelhead and thereby focus restoration efforts on those habitats.
Additionally, the Mills study findings can help guide location of steelhead monitoring sites in a watershed. In the John Day as in other river systems, fisheries managers count steelhead redds in a sub-set of streams in order to estimate total how many fish spawn in a given year. The Mills work can help inform managers about which sites are most likely to have steelhead, potentially saving time and energy and increasing the accuracy of run estimates.
Wishing you all have a wonderful long weekend!