If you’ve been following the information stream on climate change, you have likely heard of global warming. And, if you happen to be interested in fish or marine life (like yours truly), you also probably know that this means many of the world’s water systems are projected to increase in temperature . Even at face value, this seems like a doom-and-gloom story, but as an environmental scientist, I had to sit and think about what this really means. If there are so many other changes going on right now – pollution, ocean acidification, mass extinctions – why should we care about temperature? What exactly happens to ecosystems when temperatures rise?

As I’ve come to learn through my current research, temperature is one of life’s master variables. It affects where organisms can go, what cues they use to feed, how fast they can move, how well their bodies function, whether they can survive, and in some cases (such as sea turtles [1]), even what sex they are. In fact, we can divide most organisms into two broad categories based on how they control their body temperature. Many animals, such as mammals (including humans) and birds, are endotherms. This means that we can maintain our body temperature regardless of what is going on around us. You may have also heard of endotherms referred to as “warm-blooded” animals. Being an endotherm is like having a cool superpower – barring any other obstacles, you can undertake your daily activities across a very wide range of temperatures. Alternatively, ectotherms (often referred to as “cold-blooded” organisms) rely heavily on their environment to control their body temperature and have to behave accordingly. This is why you may observe lizards basking on a rock, for instance; the heat from the sun is an excellent way to increase their body temperature [2]. Reptiles and most fish are ectothermic, occupying habitats that have the most appropriate temperature for their bodily functions. It is important to note here that this optimal temperature will depend on the species; some fishes operate better at warmer temperatures than cooler temperatures, for example. In the context of climate change, all of this information suggests that certain organisms, particularly ectotherms, will have more difficulty adapting to global warming than others. How that vulnerability will manifest is something I have spent the last year exploring. 

Here in California, salmon are a major organism of concern. These fishes face numerous human-driven threats. Such issues are complex, but put simply, they include a combination of: overfishing, habitat loss, disease, and subpar genetics in the populations (because we now depend mostly on hatcheries to stock the rivers) (e.g. [3]). Salmon are also ectothermic, cold-water organisms, and because they rely heavily on the environment to regulate body temperature, they are very likely to be affected by warming waterways. In fact, studies have shown that outmigrating salmon (those swimming out to the ocean after hatching in upstream habitats) tend to have a lower probability of surviving when water temperatures are warm [4] (Note: for more information on salmon and their life cycles in the California, check this out: https://wildlife.ca.gov/Conservation/Fishes/Chinook-Salmon).  However, through research conducted at UC Davis, we have learned that salmon in the lab are capable of surviving at warmer temperatures; in fact, they could do so in water that was 15°C warmer than the optimal temperatures for which they are suited. In that case, could temperature alone lead to low survival? Perhaps correlation does not necessarily equal causation, and we have to consider what else is going on in the ecosystem. 

Another key threat to salmon is predation. There are many different predator species in the Sacramento-San Joaquin River Delta, an expansive inland river and estuary system that is home to millions of salmon in California each year. Now, if we consider that different organisms possess different optimal temperatures, it is almost certain that predators and salmon will not respond in the same way to the same temperature changes. For instance, we could expect salmon to perform better (I’ll go into what this means below) at cooler temperatures, but perhaps a key predator species would perform better at warmer temperatures. This might lead to the outcome that has been previously reported: salmon have decreased survivorship at warmer temperatures. These were the hypotheses and predictions that led me to my first experiment, starting in the spring of 2019. 

With the support of the Delta Science Fellowship, the Fangue lab at UC Davis, and my collaborators (Ken Zillig and Cyril Michel), I set out to determine how temperature influenced the risk of predation in juvenile Chinook salmon. The predator in question was the largemouth bass, chosen because of its role as a major predator of these fish in the Delta [5] – a single bass can eat multiple juvenile Chinook salmon in a day. In contrast to their salmon prey, however, largemouth bass are known to prefer warmer waters. Consequently, we wanted to see if these bass would outperform salmon at warmer temperatures in a series of tests, and whether the opposite would be true in colder water. These tests pertained the animals’ physiology, or how their bodies functioned. In our case, we were first interested in how much oxygen the salmon and bass were able to consume at a given temperature. Like in humans, oxygen fuels energy creation in fish. The more oxygen they could consume, the more energy they would have to escape, reproduce, and find food, and presumably be better able to survive in the wild. Across five different temperatures, we individually swam at least fifty fish in sealed swim tunnels, which acted like underwater treadmills to keep the fish moving. We inserted probes into each tunnel to measure how much oxygen was being consumed over time. This was repeated for both bass and salmon (yes – that’s a LOT of swimming). We determined the optimal temperature for each species based on the temperature at which they were able to consume the most oxygen. 

However, the ability to consume oxygen is not the only important indicator of how well an animal is able to escape a predator. Our second test determined how well the bass and salmon were able to ‘burst’ (i.e. exhibit a quick increase in speed) at those same five test temperatures. To measure this, we built two burst tunnels, a smaller one for the salmon and larger for the bass. Both were long rectangles of clear plexiglass lined with laser beams at defined distances (note that these lasers did not affect the fish in any way). We would place a fish in a small chamber at one end, and then startle it with a small pinch to cause the fish to burst through the rest of the tunnel. Each time a fish broke through a laser beam as it went, a computer program was able to record the time and distance, and thus the speed of the burst. We would repeat this until the fish was exhausted, which gave us an indicator of both how quickly and how many times that fish would be able to burst at a given temperature. Presumably, both species would be able to burst faster, and more times, at their given optimal temperatures. Taken together, these tests allowed us to see how the bass and salmon would do aerobically (for activities requiring oxygen, as in test 1), and then anaerobically (for activities not requiring oxygen, as in test 2), depending on the temperature. That would be analogous to seeing how well a human could do long-distance running versus sprinting, and how this would change depending on whether that person was adjusted to doing these activities in hot or cold weather. 

After several months of watching fish swim and burst for hours on end, we came to an exciting preliminary conclusion: temperature did play a role in how well each species was able to perform! While I won’t give too much away just yet, it seems that predators like bass are better suited for warmer water. They consumed more oxygen and were able to burst faster and more frequently at the higher end of our test temperatures. Salmon are more versatile than we expected, but still performed optimally in many ways at cooler temperatures. Could this suggest that predation may be happening more at higher temperatures? With such an interesting result under our belts, we have a few more months of fish physiology and a few more tests to do before we will reveal the final outcome. Stay tuned at the Newsroom for the future unveiling.

Alexandra McInturf is a fifth-year PhD candidate in Animal Behavior at UC Davis. She is conducting this research as a Delta Science Fellow and a member of the Fangue lab. She credits her other research interest, on shark behavior, for helping inspire the predation component of this project. While she has had tremendous support in the lab, she would like to recognize Ken Zillig as her partner in crime and resident fish physiology expert, without whom this project would not be possible.

References: 

[1] McCoy, C. J., Vogt, R. C., & Censky, E. J. (1983). Temperature-controlled sex determination in the sea turtle Lepidochelys olivacea. Journal of Herpetology, 17(4), 404-406

[2] Bauwens, D., Hertz, P. E., & Castilla, A. M. (1996). Thermoregulation in a lacertid lizard: the relative contributions of distinct behavioral mechanisms. Ecology, 77(6), 1818-1830

[3] Katz, J., Moyle, P. B., Quiñones, R. M., Israel, J., & Purdy, S. (2013). Impending extinction of salmon, steelhead, and trout (Salmonidae) in California. Environmental Biology of Fishes, 96(10-11), 1169-1186.

[4] Johnson, R. C., Windell, S., Brandes, P. L., Conrad, J. L., Ferguson, J., Goertler, P. A., … & Swart, B. G. (2017). Science advancements key to increasing management value of life stage monitoring networks for endangered Sacramento River Winter-run Chinook salmon in California. San Francisco Estuary and Watershed Science, 15(3).

[5] Michel, C. J., Smith, J. M., Demetras, N. J., Huff, D. D., & Hayes, S. A. (2018). Non-native fish predator density and molecular-based diet estimates suggest differing impacts of predator species on juvenile salmon in the San Joaquin River, California. San Francisco Estuary and Watershed Science, 16(4).

[Edited by Maggie Creamer]