Big Waves Are Costly for Fish Trying to Stay Put

by Dominique Roche, Sandra Binning, and Mark Taylor

Dominique Roche and Sandra Binning completed their Ph.D.s in the Department of Ecology, Evolution, and Genetics at the Australian National University in 2013. They are now postdoctoral fellows at the University of Neuchâtel in Switzerland. Mark Taylor received his Ph.D. from the Department of Biology at Carleton University in Canada and currently works for Parks Canada in Banff, Alberta. The three authors visited FHL in August 2011 for the five-week biennial Fish Swimming graduate field course (Figure 1). Here they share the results of their work, which was recently published in the Journal of Experimental Biology with co-authors and course instructors Jacob Johansen, Paolo Domenici, and John Steffensen.

Complex water flows, such as waves and eddies, are common in nature and play an important role in shaping aquatic communities. In shallow marine habitats, waves are ubiquitous disturbances that influence the abundance and distribution of fishes depending on their ability to move in or withstand these physical forces (Liao 2007, Webb et al. 2010). However, the harsh realities of living in naturally moving water are generally ignored when researchers study fish swimming costs and behavior in the laboratory. For example swim tunnels, which are used to quantify swimming performance and energetics in the lab, traditionally produce very uniform flow at constant speeds. While these types of flows are easy to reproduce and describe, they are a far cry from the constantly fluctuating and often turbulent flows that most fishes experience on a daily basis. Therefore, laboratory measures of swimming performance might profoundly underestimate the true costs of swimming in nature.

Changes in water flow speed create what is referred to as "unsteady flow." Swimming in unsteady flow is not only a challenge for fishes moving between areas but also for fishes that want to avoid being swept away. There are many reasons why fish would want to remain stationary relative to the sea floor. For example, to defend a territory or a nest, or to remain close to a potential mate or a food patch. For these reasons, we were interested in measuring the energetic costs of swimming against unsteady water flow that mimicked oscillating wave patterns in a fish’s natural habitat. We also wanted to compare these values to the cost of swimming against steady, uniform flow that most researchers recreate in laboratory experiments. This comparison was appealing to us for two reasons. First, our experiment would allow us to better understand how fish deal with the physical challenges they experience in the natural world. Second, wave regimes are expected to intensify in ocean basins as a result of climate change. Therefore, it makes sense to understand how fish are affected by present wave conditions, especially if we aim to predict how they might respond to future climate scenarios.

To answer our question, we chose to work with the shiner surfperch, Cymatogaster aggregata, a very common fish species around Puget Sound (Figure 2). C. aggregata is fascinating for many reasons, namely because it swims with its pectoral fins rather than its tail, or caudal fin. In fishes and other animals like turtles, pectoral fins are analogous to arms in humans.

Of the few studies that have examined the energetic costs of swimming in complex flows, most of them have looked at turbulent flows in rivers and virtually all of them have focused on fishes that use their body and caudal fin for propulsion. However, roughly 15–20% of all living fishes routinely use their pectoral fins for swimming. Most of these so-called "labriform" swimmers inhabit shallow marine or freshwater habitats where wind-driven waves create constant changes in flow velocity. C. aggregata flaps its fins much like a swimming penguin, and this swimming behavior is generally associated with efficient, high speed swimming and life in high water flow habitats. Other labriform swimmers row their fins like paddles, and this behavior is thought to be better suited for maneuvering in more sheltered, low-flow habitats.

The experimental design we used was simple. We compared the energetics, swimming performance, fin beats, and body movements of C. aggregata swimming in steady versus unsteady water flow (Figure 3). Our unsteady flow treatments mimicked a unidirectional wave surge (i.e. sinusoidal velocity fluctuations around a constant mean) with two different amplitudes of flow speed: small oscillations (half a fish body length per second) and large oscillations (one fish body length per second). We used a computer-generated sine function that determined the rotational speed of the swim tunnel’s propeller to precisely control four key parameters in the different flow conditions: the period, amplitude and wavelength of the water velocity fluctuations, and the mean flow velocity. Each fish was challenged with a traditional critical swimming speed trial until it became fully exhausted and stopped swimming.

We estimated theoretical values of oxygen consumption rate for fish swimming in the two unsteady flow treatments with simulations based on the relationship between swimming speed and oxygen consumption rate in steady flow. The purpose of this exercise was to see if we could use data collected in normal laboratory conditions (i.e. steady flow) and apply simple mathematical principles to predict the energy consumption of fish in a more complex situation, such as unsteady flows.

We found that...you can’t! We predicted that swimming costs would always be larger in oscillating flows but found that the swimming costs of C. aggregata in unsteady flow depended on the magnitude of water velocity fluctuations. In fact, individuals swimming in the small oscillation treatment performed as well as fish in steady flow with no speed oscillations. However, swimming costs for fish in the large oscillations treatment were on average 25% higher than in steady flow and 14% higher than our estimated values. Because speeding up, slowing down, and maintaining stability all require extra energy, it’s likely that swimming costs in large flow oscillations exceeded theoretical estimates due to these extra requirements.

Interestingly, swimming costs in C. aggregata were also influenced by the behavior of fish in the swim tunnel, in particular how well they were able to adjust their pectoral fin beats to the flow environment (see link to the video below). This was a neat and somewhat unexpected finding! Measures of swimming behavior and oxygen consumption rates in large flow oscillations indicated that timing fin beats with changes in water flow saves energy. Some fish appeared to be better than others at adjusting their fin beats to match the flow velocity and this allowed them to conserve energy by increasing the time that they could glide between waves.

In conclusion, our study showed that flow type and individual behavior are important factors that affect the swimming costs of fishes – at least in fishes that use their pectoral fin to swim. Waves, currents, and eddies are common phenomena in aquatic systems and understanding how animals cope with these stressors is a relatively new and exciting area of research. Improving our ability to measure energetic costs in settings that approximate wild conditions is critical not just for understanding how environmental flows affect fish ecology, but also for practical applications such as fisheries and habitat management.

If you’re interested in learning more about this study, you can find the article in the Journal of Experimental Biology (Roche et al. 2014, doi:10.1242/jeb.085811). A video of C. aggregata swimming in unsteady flow is also available on YouTube: https://www.youtube.com/watch?v=IuboHhqIMY0

References:

Liao, J.C. 2007. A review of fish swimming mechanics and behavior in altered flows. Philosophical Transactions of the Royal Society of London Series B Biological Sciences: 362 (1973-1993).

Roche D.G., Taylor M.K., Binning S.A., Johansen J.L., Domenici P., and J.F. Steffensen. 2014. Unsteady flow affects swimming energetics in a labriform fish (Cymatogaster aggregata). Journal of Experimental Biology: 217 (414-422).

Webb P.W., Cotel A., and L.A. Meadows. 2010. Waves and eddies: effects on fish behavior and habitat distribution. In P. Domenici and B.G. Kapoor (Eds.), Fish locomotion: An eco-ethological perspective pp. 1-39. Enfield, NH: Science Publishers.