It depends on how old and big they are.
Over the past year we have talked extensively about the value of life history diversity in steelhead, and in particular, that a broader range in spawn timing – which results in a broader range of emergence for juveniles – can help ameliorate density-related impacts by spreading steelhead of various ages and sizes into different habitats.
The concept is this: juveniles that emerge earlier will grow to a large enough size that they will occupy different habitat by the time later spawned juveniles emerge. Research on pink salmon found that this type of variation in emergence doubled the capacity of a small habitat system in Alaska.
There is evidence of a similar pattern in steelhead. A study in 1988 by Chandler and Bjornn (Ted Bjornn was one of the foremost steelhead scientists in the Pacific Northwest) looked at what happens when you have early- and late-emerging juvenile steelhead, and whether differences in emergence timing can increase the capacity of a stream to support juvenile steelhead.
The authors conducted an experiment in artificial stream channels by spawning early and late emerging steelhead, the timing of which was about a month apart. They then collected the juveniles fr
om the two groups and placed them into the artificial channels and measured growth, habitat use, density dependence and behavioral interactions.
Two of their results were very interesting, though not unexpected. First, there was an advantage to emerging early. By the end of the experiment, early emerging juveniles outnumbered later emerging juveniles about 2 to 1. Further, the early-emerging juveniles were generally longer and heavier. This finding is not surprising. It makes sense that emerging earlier conveys some benefits. Those fish get a longer period to grow and they also have a size advantage when competing with smaller, later-emerging fish. Basically, early fish have what is referred to as “prior residence,” or as the authors labeled it, “settlers rights” to the territories. Why? Because early emerging fish got there first and it is easier to defend a territory than it is to acquire a new one.
Second, and most importantly, at the end of the experiment juvenile densities were highest when early and late fish were stocked together compared to when either was stocked alone. How is this possible? Because different sized fish occupied different parts of the stream channel. Essentially, even small differences in size results in slightly different habitat preferences. As early emerged fish grew larger and became better swimmers, they moved from the shallowest channel margins out to slightly deeper water.
The bottom line is that differences in spawn timing ripple through the life histories of their offspring, which has implications for the capacity of the habitat. As we see here, more fish can occupy a given amount of habitat if they are diverse in size, because diversity in size – which is due to differences in spawn timing – results in fish competing for slightly different habitats.
The principle of juvenile organisms of the same age class but of different “emergence” dates preferring or requiring different habitats is not exclusive to steelhead, of course. In humans, think of kids in school. Schools are delineated by grade, which is basically a reflection of emergence timing for us. Metaphorically then, the shallow channel margins in streams are like pre-school, slightly deeper and faster water is like grade school. The main-stem rivers and even swifter waters are high school, and the migration to the ocean is college.
These findings, and others, underscore why it is important to recover the breadth of spawn timing in steelhead. It not only increases diversity, but it also increases the potential productivity of the population.