#46: Part 5, Mechanisms of change in fish populations: Exceedance of thermal thresholds
Dan Isaak
Hi Everyone,
As the drought and record breaking temperatures continue across many parts of the western U.S., they provide a natural segue to our next blog topic describing thermal mechanisms of change in fish populations. And unfortunately, the timing is too good as we just witnessed a thermal “event” wherein high temperatures killed a bunch of adult Chinook salmon in an Oregon river. The fish had just migrated back from the ocean and were staging near their natal headwater streams prior to spawning when they were caught by a big stream temperature spike associated with a recent heat wave. Although it’s impossible to attribute any 1 such event to climate change per se, an increased frequency of these events is one of the tell-tale signs we’d expect to see as global warming slowly changes the environment. Near the margins of fish distributions, in both space and time, there will simply be more times when previously suitable habitats aren’t, due to temperatures, or flows, or other stream characteristics exceeding biological tolerances. Those changes have obvious implications for the BIDE processes of fish populations and will lead to changes in abundance or a species distribution if repeated through time. Usually the biological responses aren’t as dramatic (& traumatic) as seeing big, beautiful fish keel over in a stream but are more subtle and virtually invisible (unless we’re doing good long-term biomonitoring (blogs 37, 38, 39)) because it’s a gradual, multigenerational process. These dramatic events, however, serve as stark reminders of the changes that are going on around us.
So what exactly happens when a fish gets too hot and dies? Many of us are familiar with the basic phenomenon having innocently fried a few fish due to poor temperature regulation in our childhood aquaria. And apparently it’s an experience that marks one’s psyche as more than a few of those kids have grown up to make it their life’s work to invent clever new ways of cooking fish in laboratories. As such, there’s a long and storied literature on the topic and today we’ll focus on one especially comprehensive set of basic and applied research for sockeye salmon in the Fraser River of British Columbia. The sockeye runs in the Fraser are some of the world’s largest, consisting of 100,000s of returning adults each year, and there are significant commercial fisheries associated with these populations. There are also good long-term temperature and flow monitoring records for the Fraser dating back 60+ years that show the expected long-term trends from climate change. So when it became the case in recent years that significant portions of upriver migrating adults were “disappearing” during especially warm years and not arriving at the spawning grounds, there was more than a little interest in finding out why.
Many studies concerning the thermal ecology of Fraser River sockeye have been done over the last decade or so (graphic 1) but the paper by Farrell and colleagues “Pacific salmon in hot water: Applying aerobic scope models and biotelemetry to predict the success of spawning migrations” (graphic 2; study hyperlinked here: http://faculty.forestry.ubc.ca/hinch/Farrell%20et%20al%202008%20PBZ.pdf) provides a nice synthesis of the physiological processes and relates them to field evidence to develop a mechanistic hypothesis explaining the disappearances. The basic idea, as explored in detail by Farrell in a subsequent paper (hyperlinked here: http://people.landfood.ubc.ca/anthony.farrell/pubs/p290-Farrell_et_al_2009b.pdf), is that the aerobic scope for activity, which is the difference between basal and maximum metabolic rates (graphic 3) declines to nil beyond some species-specific, optimal temperature. This occurs because basal oxygen demand increases exponentially with temperature but cardiac pumping capacity reaches a plateau and then collapses. No aerobic scope leads to anaerobic metabolism, exhaustion, and death if experienced over a sufficiently long period. Farrell and colleagues hypothesize, therefore, that some upriver migrating sockeye, during especially warm years in the Fraser, experience a collapse of aerobic scope caused by chronic exposure to high temperatures and significant mortality—sometimes involving 10,000s of fish—ensues. The technical terminology for what happens inside the fish is hypoxic bradycardia, but in real terms, it’s akin to dying of a broken heart.
So what about those fish in Oregon or those long lost fish souls from our childhood aquaria? Well, given that aerobic scope is strongly temperature dependent, fish don’t have to be doing anything nearly as exhausting or inspiring as swimming hundreds of miles for their aerobic scope to collapse. They just have to be in water that’s too warm for too long. If that occurs and there’s no place nearby for them to cool off, their hearts will also break.
The work from the Fraser helps us understand this process through what could be a generalizable mechanism but to broaden our understanding & address the topic proactively, it would be ideal to have stream temperature monitoring data (Blog# 3) from those locations where future thermal events are most likely to occur. In many cases, we already have a good sense of which stream reaches and fish populations are most vulnerable because they have already shown occasional heat stress and/or are where climate velocities are fastest (graphic 4; Blog# 36). By establishing temperature monitoring networks there now, it would be possible to compare temperatures to aerobic scope curves prior to a thermal event happening and determine how close a stream is to a biological threshold. Or, in the unfortunate case of doing a post-mortem analysis, we’d be poised for a rigorous assessment that enables us to learn from our fishy friend’s demise.
Until next time, best regards,
Dan