The last couple weeks we’ve started our mornings by standing around the truck in sub -freezing temperatures, pleading with the sun to shine a little brighter. This week, it was back to shorts, t-shirts, and hot breezes. Cruel joke, Mother Nature. Not only is it going to make winter feel a little more bitter, but the trout are now confused about whether it’s time to spawn.
My post last week described some of mating rituals brook trout go through before and during spawning. But, I forgot to mention a key part- water temperature! There’s no one set temperature that triggers spawning, but we know that as temperatures drop below 50°F the probability of spawning gets much higher.
Two weeks ago water temperatures in my Loyalsock Creek study streams were between 43-47 °F. We were seeing more fish in riffles, regular and moderate rains were keeping stream flows steady, and fewer fish were swimming away when we approached. Spawning seemed close.
Then, three days of 70°F+ air temperatures and, more importantly, overnight lows above 50°F, and the streams are now back to around 53°F. Reset the clock.
This temperature swing may not seem like much. In fact, it happened frequently during the summer when air temperatures soared into the 90s and forced stream temperatures close to brook trout lethal limits. However, while streams are still fairly cool, in the fall trout are less prepared for quick changes in stream temperature. In the summer, trout are already in deep pools where water is a little cooler. Plus, fish are conserving energy and limiting oxygen demands by moving less. Put another way, trout in the summer are like sunbathers on the beach. They might be hot, but they are lazily lounging and not exerting much energy. In the fall, they become marathon runners and need more oxygen, more energy, and do best when temperatures are lower.
This creates the potential for a perfect storm during the fall. Trout need a lot of energy and oxygen to move and spawn. But, they are not consuming a lot of calories because fewer bugs are hatching and emerging and trout are focused on spawning. Warmer water also carries less dissolved oxygen, and trout are seeking shallower riffles and runs which are prone to heating faster than deeper pools. In short, this means that sudden increases in fall water temperature have the potential to be very stressful despite being well below lethal limits.
And, that’s only the adults. Trout eggs require oxygen-rich, cool water. If trout spawn before a sudden temperature increase, eggs can quickly suffocate from a decline in oxygen or loss of adequate stream flow. If this happens enough, it could cause complete collapse of an entire year class and could quickly cause an entire population to become extirpated from a watershed.
This highlights the importance of another aspect of climate change that is often overlooked. Yes, there is projected to be an increase in stream temperature in the future. But, equally important is the increased variability and unpredictability in weather conditions. Centuries ago weather patterns were much more stable, making temperature a reliable cue that trout could use when deciding the best time to start spawning. Now that weather patterns are more unpredictable, environmental cues are giving false information about the suitability of habitat conditions. Trout can’t predict the future (and the unreliability of 10-day weather forecasts says humans aren’t great at it, either). It took many generations of evolution for trout to use stream temperature as a spawning indicator. So, it’s unlikely that they will quickly learn that they need to wait a few more weeks before water temperatures are safe and stable.
And, water temperature isn’t the only wild card in climate change. There is a fairly narrow range of stream flows that will deliver enough oxygen to eggs while not washing them downstream. And, unfortunately, precipitation is also going to become harder to predict in the future. Just this week Loyalsock Creek suffered devastating flooding that washed away several bridges and many homes. Any trout eggs that were already in the streams likely perished in the high flows. In this, increases in stream temperatures a few days earlier may have saved eggs, assuming the adults survived.
Timing is everything, and climate change is busting the clocks for many trout populations.