What fish want

What do fish want? I thought I would try to focus on a seasonally-appropriate question, but also one that I spend a lot of time these days pondering. You might think that it would be simple to figure out what motivates a fish—after all, they probably aren’t influenced by the complex emotions that drive people. But unfortunately, we can’t just ask them (I have often wished for just one hour as a fish—I would learn… and publish… so much!).

In general, animal behavior is driven by a balance between the drive to maintain/gain energy stores, survive, and ultimately reproduce. So when asking why a fish chooses to inhabit a particular area of the river, we start by looking at the environmental factors (such as water temperature and shelter from potential predators) that might be influencing its behavior. In my research, I’m interested in what environmental factors are influencing juvenile steelhead behavior surrounding coolwater refugia (areas that are cooler than the main river, often because of an incoming cold creek). What causes fish to use coolwater refuges? And when the river is really hot, what causes fish to leave the refuge? In asking these questions, I’m hoping to gain an understanding of when these refugia areas are most important to fish.

Juvenile steelhead being measured

A heat image of a coolwater refugia area on the Klamath River (Image from U.S. Bureau of Rec)

Taking a step back… many rivers are getting hotter, both because of climate change (influence of higher air temperatures), and land use practices such as logging and agriculture. Logging the riparian area next to a stream reduces shading,  causing the water to heat up. Irrigation can lead to hotter water temperatures if water is diverted into shallow, slow-flowing irrigation ditches, then returned to the river. The Klamath River in northern California, where I do my research, can reach 80°F in the summer, which feels like bathwater. On cool mornings I would sometimes even get into the water to warm up.

Salmon are a coldwater fish, and prefer a temperature range of about 10-18°C (50-65°F). This means that during the summer, when the river temperatures reach 80°F, they are under serious thermal stress—the high temperatures increase their metabolism, so they struggle to maintain weight. As a result, fish will seek out areas in the river that are cooler, such as areas where tributaries flow into the river (i.e. thermal refugia). You can sometimes see hundreds of juvenile salmon packed into small coolwater refuges. However, fish sometimes leave the coolwater areas, even when the river is hot. Why? This question forms the root of my research.

Juvenile steelhead in a coolwater refugia area

I’m hypothesizing that it’s a trade-off between water temperature and the need to find sufficient food. While the fish gain a thermal benefit by hanging out in the cooler water, the high densities of fish suggest that there may be competition for food, forcing fish to leave for brief periods in search of prey.

To put this dilemma in a human perspective, imagine that it's 120°F outside, and you are in a nice air-conditioned house… with no food. You have the choice to go out and pick some food from the garden, or just stay inside and feel hungry. It’s a trade-off! At what point do you decide to leave? Now imagine that your body temperature changes to that of the outside temperature—so as soon as you walk out the door, your temperature begins to rise. This is what happens for fish, since they are poikilotherms—cold-blooded animals that do not stay in a temperature-constant environment, so their internal temperature varies over a wide range. While they are adapted to function over a certain range of temperatures, water temperatures may be rising more quickly than fish can adapt, leaving them with a shrinking amount of available habitat.

In order to understand when and where coolwater refugia are most important, we need to figure out what specific environmental factors are driving fish’s decisions to enter and leave refugia. To do this, I conducted behavioral field studies on juvenile steelhead at coolwater refugia sites on the Klamath River for the past 3 summers. I put radio tags into steelhead (see video below), and was able to track their movement and body temperature (pretty cool!) over time. So far, I’ve found some interesting effects of water temperature dynamics on the likelihood of fish using thermal refugia… stay tuned! (And to learn more about how I’m researching this, visit my website!)

Put in a larger (non-fishy) context, refugia habitats such as this will become increasingly important as the climate continues to change. Whether it’s a range shift upwards onto mountaintops for a high altitude terrestrial species, or a fish seeking out cooler water, understanding how animals use refugia and when they are crucial to their survival will be an important aspect of preserving some species.

And now, if you’ve made it this far, enjoy the holidays! I, for one, will be enjoying a taste of the fruits of my labor.