#45: Part 4, Mechanisms of change in fish populations: Temperature effects on growth & survival
Dan Isaak
Part 4, Mechanisms of change in fish populations: Temperature effects on growth & survival
What the hail’s going on with temperature?
Hi Everyone,
Hope the 4th was good to all. Crazy, record setting heatwave across much of the American West, followed by a crazy sort of cold in some places. They had a foot of hail fall in the desert Southwest if you hadn’t heard (http://www.komonews.com/news/offbeat/Massive-storm-dumps-nearly-2-feet-of-hail-in-New-Mexico-214299611.html)!
Regardless of the extreme, temperature is important—and especially so if you’re an ectotherm constrained to life in a flowing ribbon of water like our favorite critters. So this time we’re doing the first of two blogs highlighting the strong control that temperature has on fish populations. The first paper by Neuheimer & Taggart is a longtime favorite that links fish growth/size-at-age to temperature (hyperlinked here: http://sites.google.com/site/abneuheimer/Neuheimer_Taggart_2007.pdf). The thermal metric they highlighted was the growing degree-day, which is the equivalent of 1˚C over a 24 hour period. A day in which the average temperature was 12˚C would have 12 degree-days, a month in which the average temperature was 12˚C would have 360 degree-days (12˚C x 30 days), etc. The premise is simple, but powerful, because if you know the growing degree-days a fish experiences in its early developmental phases, you can predict the heck out of how long it will be at a certain age (graphic 1). Moreover, this one simple metric has strong explanatory power across many species, freshwater/marine environments, temperate/tropical areas, constant/variable temperatures, and laboratory/field studies as demonstrated by Neuheimer & Taggart. And fish length/size at a young age is a big determinant of survival and/or subsequent life history trajectory.
The degree-day also figures prominently in our next 2 studies, which are companion papers by Coleman & Fausch. In the first of these two papers, the duo conducted a laboratory study to examine how different thermal regimes affected the survival of age-0 cutthroat trout from egg emergence/fry swim-up through the next 6 months (graphic 2; study hyperlinked here: http://warnercnr.colostate.edu/~kurtf/400web/Field%20trip%20assignment/Native%20cutthroat/Coleman%20and%20Fausch%202007%20TAFS%20Lab.pdf). Fish fed similar rations survived very differently, with those in the coldest treatment surviving at less than half the rate of those in intermediate and warm treatments. Warm, in this case, is relative because the study was designed to provide inference about conditions in high-elevation (~10,000 feet) streams of the Colorado Rockies. The “warm” treatment, therefore, consisted of temperatures that averaged 10˚C, which based on personal experience, is cold enough to make your hand start hurting after a few seconds of immersion.
Lots of lab studies have baked fish at various temperatures, but what sets Coleman and Fausch’s baking apart is the companion paper that tested/translated the laboratory results to field settings (graphic 3; study hyperlinked here: http://warnercnr.colostate.edu/~kurtf/400web/Field%20trip%20assignment/Native%20cutthroat/Coleman%20and%20Fausch%202007%20TAFS%20Field.pdf). Through fry surveys and temperature measurements conducted across several streams, they documented a relationship between fry length and degree-days that was very similar to the laboratory relationship. This, combined with earlier research on other aspects of these populations, gave them insight regarding why some streams, despite nearly pristine conditions, rarely supported cutthroat trout recruitment. They concluded that approximately 900 degree days were needed during the growing season for cutthroat trout to consistently reproduce in these streams and support a self-sustaining population. Below that threshold, age-0 cutthroat emerge too late in the summer and grow too slowly to be able to make it through their first winter when things are really cold for a really long time.
It’s a bit ironic to be focusing on how cold temperatures regulate fish populations in a warming world, but there are plenty of places globally (Blog #35) where cold temperatures have served as a barrier to colonization by new species and recruitment bottlenecks for young fish are likely to be a common limiting factor. As such, it’s important to fully understand the processes in these streams and specifically where these bottlenecks occur geographically across a river network as these areas could serve as future climate refuges for the native species we’re often trying to conserve or be the places where non-native invasions become most likely with additional warming (Blog #38). It’s worth noting, also, that the basic pieces for operationalizing these concepts broadly are very obtainable because there are only two main things we need to know: 1) what are the thermal constraints/thresholds associated with juvenile recruitment as per Coleman and Fausch and 2) the geographic locations where these thermal constraints occur. That information can be developed from local stream temperature monitoring (Blogs #’s 3, 24, 26) or accurate model predictions (Blog #’s: 7, 40) to give us the necessary baseline maps. Then we can plug and chug through the stream climate velocity calculations (Blog #36) to see how far/fast those areas with thermal constraints would shift based on various warming scenarios (Blog #23). It’s still an open question whether fish populations will keep apace of these shifts (Blog #42), but we should, at least, be able to accurately predict where key thermal habitats exist for species x, y, or z at times 1 and 2 across riverscapes. That, in turn, will let us identify potential trouble-spots where it might make sense for us to intervene.
So we’ve only scratched the surface of all that’s going on in the busy arena of thermal biology for fishes and next time we’ll highlight a few papers that focus on the hot side of the thermal spectrum. But before closing, I wanted to point readers to two good reviews on this subject, “Burning questions for coldwater stream fishes.” by McCullough & colleagues (hyperlinked here: www.esd.ornl.gov/~zij/mypubs/climate/McCullough09-Burning.pdf). And the more mundanely titled, “Climate change effects on fisheries” by Pörtner and Peck (hyperlinked here: http://www.researchgate.net/publication/47793443_Climate_change_effects_on_fishes_and_fisheries_towards_a_cause-and-effect_understanding/file/9fcfd507e6e0698c40.pdf). The latter focuses more on oceanic fisheries but still contains a lot of good information on thermal mechanisms that regulate fish populations and is by two of the world’s leading metabolic ecologists.
Until next time, best regards,
Dan