Although I originally just set out to describe some of my newly published research (“Costs of living for juvenile salmon in an increasingly warming and invaded world”), it’s probably not an accident that I find myself part of an unintentional series on effects of interacting stressors (e.g., temperature, disease, or pollution) on organisms. With all of the global change going on – from climate to ocean acidification – it’s no wonder multiple environmental stressors are on our minds. Despite the stressful (pun intended) topic, I’m not one to turn down good company, such as Nate’s recent post on climate-driven stress on PNW yellow-cedar and Sarah & Kristen’s report on stress-related decline of Colorado aspen. I’m glad to be able to offer an account from my more fishy (no pun intended) view.
This research began two years ago (as part of my Master’s thesis) when I set out to test effects of multiple stressors on juvenile Chinook salmon (Oncorhynchus tshawytscha). Our lab was investigating impacts of non-native predators, such as smallmouth bass (Micropterus dolomieu), walleye (Sander vitreus), and catfish (Ictalurus punctatus), in the Pacific Northwest.
Invasion and widespread establishment of these fish (fish typically classified as warmwater species) into historic salmon areas was starting to raise eyebrows even in the much-abused Columbia River and its tributaries. As part of our lab effort and with significant help from partners at the US Geological Survey, I decided to examine the separate and interactive effects of temperature and predation – stressors which are ubiquitous and at the same time occurring in novel combinations as a result of non-native invasions and climate change. I was also interested in looking beyond the effects of stress on mortality and delving into what happens to physiology, growth, and behavior.
Research questions (like multiple stressors) can lead us to ironic places. When I started my Master’s I was sure about 3 things: I didn’t want to do lab experiments, I didn’t want to study salmon, and I was definitely not interested in behavior. The universe must have a sense of humor, because a year later I was cranking up the temperature of experimental stream channels and recording behavior of juvenile Chinook into a voice recorder over 2-day trials.
Four weeks, five trials, and 600 behavioral surveys later, the experiment was done. Over the next year, I assimilated the data on behavior, growth, physiological stress indicators, and mortality, in response to cool (15°C), warm (20°C), predator (1 bass), and warm+predator treatments.
The results were a mixture of the surprising and the intuitive. The combined stressors did not affect vulnerability to predation or other mortality. I did, however, see significant interactive effects on salmon behavior, growth, and physiology. Juvenile salmon showed stronger anti-predator responses in warmer temperatures, by spending less time swimming, and more time shoaling near the surface.
Growth was reduced with each added stressor (see figure below), while physiological responses appeared to be maxed out by the combination of predation and temperature. And even though this experiment was done in a lab, the results illustrate how multiple stressors likely play out in the “real world”: with more variable outcomes, reduced growth, and decreased physiological capacity to respond to additional stressors (such as disease or pollutants).
In discussing this research over the years, I’ve had many people suggest: “Well, doesn’t stress just toughen animals up? Survival of the fittest and all that…they’ll just adapt!” It’s an argument I’ve contemplated myself (and heard more often than I’d like) – a kind of Darwinistic version of “What doesn’t kill you makes you stronger”. This argument begs the question, however, do we really understand evolution well enough to bet on it? And while I enjoy a good philosophical debate as much as the next person, the research being amassed on effects of stress are fairly clear – that for salmon at least, what doesn’t kill them doesn’t really make them stronger.
I dislike ending on a blue note, however, and in this case think it’s not useful. Where I prefer to focus is that these results suggest that efforts to restore fish habitat – through policies like riparian zoning to fight temperature rise, barrier removals to open up headwater areas, and non-native/predator removals – are probably not wasted.
Also, as we begin to appreciate the role of environmental stress and incorporate appropriate strategies into species management and recovery plans, we may see new progress around historically stubborn conservation issues.
You can link to the original article which came out in the September 2012 issue of Canadian Journal of Aquatic and Fishery Sciences here