Juvenile steelhead doing what it takes to become a smolt: You grow in summer, I’ll grow in winter
Another Friday, another blog post on the science of steelhead. We love this day of the week!
Our topic this week is the growth of juvenile steelhead. Did you know that the growth rate of steelhead can vary depending on the season of the year? This variation in growth rate can be considerable.
There are a number of reasons that juvenile steelhead grow at different rates during the year. Two recent papers highlight this biological complexity and address some of the potential causes.
The first study, published by Jamie Thompson and Dave Beauchamp (thompson2016), took place in the Skagit River watershed and used a bioenergetics model to look at growth patterns in three tributaries with very different water temperature regimes. In general, juvenile steelhead tend to grow best when water temperatures are somewhere between 50-65°F. That is their metabolic wheelhouse where they have lots of potential for metabolizing food and converting it to energy and growth. Thompson and Beauchamp found that growth potential in the tributaries was greatest when water temperatures were 45-65°F, which means that fish grew fastest in summer and early fall.
While fish grew faster and put on mass during that wheelhouse period, the cooler temperatures of winter were adversarial to growth. Once temperatures dropped down into the 30°F’s and 40°F’s growth rates slowed and fish lost body mass. In other words, the strategy was for fish to put on growth and fat during the good times so that they were able to hang on through the lean times, almost like a rainy day fund.
Temperature was only one factor influencing growth rate, however. The model also found that the wheelhouse for growth could be expanded if feeding rate and the quality of the food were increased. This was beneficial because it improved survival to the smolt stage, which is what we want because smolts are essential to producing adult steelhead.
Although more food is important, it also depends on the quality. For example, salmon eggs contain very high energy levels, particularly fat, compared to insects. And among insects, terrestrial varieties – like beetles and grasshoppers – have higher energy levels than aquatic nymphs and winged adults. This means that increasing high quality food inputs, such as escaping more salmon and restoring riparian areas where terrestrial insects live and can fall into the streams, could improve the growing seasons for juvenile steelhead. And that would help rivers produce more smolts.
The second study was conducted by Ian Tattam and three co-authors (tattam2016). It took place in the John Day River basin, a tributary to the mid-Columbia River in Oregon. Rather than focus on multiple streams, the authors looked at growth rates of juvenile steelhead in three reaches of a single tributary to determine if growth changed across the year and if periods of fastest growth differed among reaches.
The John Day is a much different basin than the Skagit. It is semi-arid and temperatures in the study creek exceeded 77°F during summer and dropped down to freezing in the winter. The extremes in temperature varied among reaches though. Some were cooler in summer and warmer in winter, while others were really warm in summer and really cold in winter.
This study found the period of greatest growth was in spring. That is very common in streams. The water temperatures start warming, day length increases and bugs become more active. Spring is a time of bounty for juvenile steelhead.
During other seasons, growth of fish varied between reaches. In some reaches fish grew more in summer, while in another reach fish grew faster in winter. This presumably occurred because of really warm water temperatures in summer, which likely limited growth potential. For example, the reach with the warmest summer temperatures had the warmest winter temperatures, and that is where fish grew fastest during winter than summer. The opposite was true in the reach with the more moderate summer temperatures.
The results of both studies highlight the complexity in understanding how steelhead grow and respond to water temperature and food supply. What happens in one watershed does not necessarily happen in another, and what happens within a creek may differ from one mile to the next.
It is this diversity that is so important to sustaining steelhead populations. Although difficult to monitor, more research like this is necessary to ensure that we can manage and restore habitat conditions in a way that maximizes growth and survival of juvenile steelhead. This is because water temperatures and food supply are likely to shift as climate change progresses. Steelhead are incredibly adaptive and can persist, but helping them to do so requires understanding what types of restoration will be most effective. For that, we rely on studies like these.