Science Friday: The habitat that steelhead prefer, and how we use it to estimate capacity of rivers

By John McMillan

How many steelhead can you fit into a given watershed? Put another way, what is the carrying capacity of a given watershed for steelhead? This question, and its answer, are important for steelhead fishery managers, and anglers, as we collectively try to rebuild wild fish runs up and down the West Coast.


To be clear, we are talking about juvenile steelhead here — those young fish that become smolts and represent the future.


The short answer (like so many “answers” to questions of science), is: it depends. It depends on the characteristics of each river and stream. It also depends on the behavior of the fish — and new research is shedding light on why we might have to rethink how we estimate capacity for a species like steelhead.


This is the first in a four-part series dedicated to understanding the nuances of habitat capacity, and why it matters to managing steelhead. Today, we focus on the most fundamental component of capacity, the habitat.


Anglers and scientists share a common comprehension of rivers and steelhead, even if they aren’t aware of it. For example, most steelheaders know steelhead are simply more abundant in some streams. And within those streams, experienced anglers are able – after years on the water – to easily identify the most likely holding locations for steelhead. In essence, anglers basically create mental models of rivers based on their experiences, failures and successes. Scientists do the same, just more quantitatively. They count and track fish and measure habitat characteristics, then use that data to develop statistical relationships between steelhead and their habitat (see this paper for an example). Those relationships are the foundation for estimating how many steelhead a given river can support.


There are a variety of ways to estimate the steelhead capacity of a river. Some methods are relatively simple, relying heavily on a few habitat variables and then estimating relationships with steelhead across all of those habitats in a river. Other methods are more complex and focus on estimating survival of steelhead at different life stages and in different habitats. Nearly all these methods rely on a core set of habitat features that are widely known to be important to steelhead, and to salmonids in general. Here we will review those habitat features and briefly describe how they are believed to influence steelhead.


The first factor is stream size. Larger streams have more space, and all else being equal, more space generally can hold more fish. Think high school basketball field house vs. NBA arena.


Second, as mentioned earlier, some habitat is more suitable for steelhead than others. Steelhead seem to prefer intermediate stream gradients that contain a high frequency of riffles and runs with lots of boulder cover (with cover being a key component of that combination). In addition, juvenile steelhead often undertake extensive seasonal movements within watersheds, so habitat connectivity is also important. To paraphrase the immortal Jeff Spicoli, “All they need are some tasty riffles, some boulders, and they’re fine.”


Third, water temperature is very important. There is an optimal range of temperatures at which steelhead grow best, generally 55°F-68°F, and while steelhead are pretty tolerant of warmer temperatures, they aren’t carp. Nor are they cold-loving bull trout. So the temperature regime matters, and the more often temperatures are in the wheel-house for growth, the better the river is for growing steelhead smolts.


Lastly, food is critical. A stream is basically a cafeteria for teenage steelhead. Unfortunately, food is not always abundant nor easy to come by — not to mention there needs to be enough food to rear juveniles for 2-4 years (on average, at least). In general, more food leads to faster growth, which results in earlier smolting, thereby increasing the rate at which juveniles are converted to smolts.


Scientists take advantage of the patterns in habitat use, like an angler does, to better predict where fish are within a river and how many fish that river can support until they reach density dependence. Density dependence occurs when the size of the population begins to influence per capita growth of the population. In other words, above a certain threshold there are so many fish competing for food and space that there simply isn’t enough habitat for everyone.


The bottom line is that high quality habitat is the key to capacity, especially for a species like steelhead. Sea-run O. mykiss juveniles rear for extended periods in freshwater, which means they often need to survive multiple summers and winters, and be in good enough shape to undergo the arduous process of smoltification and migration to the ocean.


It is worth repeating: good habitat is key to healthy wild steelhead runs. This is why Wild Steelheaders United is working so hard to conserve, restore and reconnect steelhead habitat all across the West.


As we will discuss in future posts, salmonids have evolved behaviors to reduce density dependent effects and to take advantage of underutilized habitat to increase the productivity of the population. While most of the research we will cover is not focused specifically on steelhead, the results have numerous implications for how we think about steelhead capacity.


Wishing you a healthy, and fishy, 2018 — with enough tugs to remind you why wild steelhead are the great freshwater sport fish of western North America.