#47: Part 6, Mechanisms of change in fish populations: Interacting effects of flow and temperature
Dan Isaak
Climate-Aquatics Blog #47:
Part 6, Mechanisms of change in fish populations: Interacting effects of flow & temperature
This onion has some layers to it…
Hi Everyone,
Over the last several blogs we’ve thought about how climate induced changes in temperature might affect fish survival and growth (Blogs: 45, 46) separately from how flow changes affect the volume of habitat and food availability (Blogs: 44). In reality, however, these changes often occur simultaneously and populations are constantly adjusting their BIDE processes to deal with this variability. So today, we’ll look at two recent studies that examined the combined effects of temperature and flow on growth of individuals in fish populations to get a sense of the resulting complexities.
The first study by Xu and colleagues focuses on a brook trout population in Massachusetts (graphic 1). Electrofishing rodeos were conducted 4 times (once during each season) each year from 2000 – 2007 and captured fish were tagged and measured. When some of these fish were captured during subsequent rodeos, their difference in size from the previous capture was used to estimate growth. Stream temperature and flow were monitored continuously over the course of the study, and as you might imagine, varied considerably within and among years. When the growth rates are averaged across years by season, a strong signal emerges wherein rates are highest during spring and low during other seasons (upper right panel in graphic 1). Notice, however, the error bars associated with those estimates. Those bars represent the amount of inter-annual variation in growth rates that occurred for that season during the 8 years of study. Examined in more detail (lower right panel in graphic 1), that variation relates to the temperature and flow conditions that occurred within a year relative to other years. That is, growth was fast during the spring if it was a relatively warm one, but growth was slow if it was a relatively cold one. Interestingly, this relationship isn’t consistent for all seasons because warmer temperatures in the summer and fall meant less growth. A similar dynamic occurs with regards to inter-annual variation in flows. Higher flows were good for growth during spring, summer, and fall, but bad during the winter (3 lines associated with each season in the graphic).
The second study by Crozier and colleagues describes variation in the size of juvenile Chinook salmon across 16 years in 13 different populations from central Idaho streams (graphic 2). Juvenile Chinook were electrofished, captured, and measured each summer and fish lengths were then related to inter-annual variation in temperature and fish density. Although the investigators didn’t address flow per se, the use of density in this case was a better choice because the huge variation in salmon numbers during the study (almost 3 orders of magnitude) gave a better measure of the variation in the number of mouths to feed and the likely intensity of competitive interactions. Results suggest that juvenile Chinook length was affected by temperature in all of the populations but the nature of the effect was strongly mediated by fish density. When densities were low, warmer temperatures resulted in bigger fish, but just the opposite occurred when densities were high. This pattern was consistent across each of the 13 study populations and suggests competitive interactions may become a more important determinant of fish size in these populations as the climate warms in future decades.
Both studies suggest there are some interesting layers to this climate onion once you start the peeling to look at details within and among populations. But although the effects of climate will be context specific to be sure, the fact that some form of bioclimatic relationship appears in many populations when we look for it also means there are some general relationships at play. Better describing those relationships simply requires more data from more populations, so it’s encouraging that much of what’s needed can be obtained from trend monitoring data on temperature (Blog #3), flow (Blog #21), and fish abundance (Blog #41) and that these data are out there. One key piece we may sometimes be missing, however, is information about food availability. There’s a big literature that describes the importance of food ration size on fish growth and survval and it’s possible that some of the variation in fish-climate relationships that today’s studies describe among seasons, years, and populations might be resolved if we knew more about fish food. That’s a challenge in and of itself, and perhaps moreso now if climate change is altering nutrient availability in streams—a possibility that we’ll touch on next time.
Until then, best regards,
Dan
Now Tweeting at Dan Isaak@DanIsaak