Climate change may have a previously unforeseen impact on coral reefs: it may make some fish more aggressive. At least that is the finding of a new study to be published in the Jan 2010 edition of the Proceedings of the Royal Society B. The research team, led by Dr. Peter A. Biro from the School of Biological, Earth and Environmental Sciences at the University New South Wales, found that certain coral reef fish changed their behavior dramatically when water temperatures was increased only a few degrees.

It is now well understood in Zoology that many animals express what we might call personalities, or variation in traits like boldness and activity level. In this study, the researchers found that these personality traits not only varied between individual damselfish, but also changed with water temperature. The evidence for this came from two experiments: an observational experiment of juvenile Pomacentrus moluccensis (left) and a manipulative one of juvenile Pomacentrus bankanensis, both collected from the Great Barrier Reef.

In both experiments, scientists looked at key PTs, or personality traits, that varied from fish to fish housed individually in separate aquaria. They focused on ones previously used by similar studies: activity, measured by the cumulative distance moved during a 3 min observation period or number of times the fish crossed a midpoint in the aquaria, boldness, determined by response to a simulated predatory threat of dipping a net in the water, and aggressiveness, determined by reaction to the introduction of a smaller individual of the same species into the tank.

In the first, purely observational experiment, water temperatures fluctuated naturally according to ambient air temperatures in the open-air lab from 24°C to 28°C across the 11 day study period. They found that, on average, individual activity rates increased fivefold with only a 3°C in temperature, and the trend held true for aggression. However, as you can see from the figure on the right, there was a lot of variation among individuals.  Some individuals had virtually the same levels of activity at all temperatures, but most increased activity substantially. Individuals who stayed the same tended to be very sedentary or active across the range of temperatures. Depending on the individual, activity increased with temperature from as little as 1.06 to as much as 30-fold. Aggressiveness varied, too, with increases ranging from 1.6 to 11-fold, with the average being about 4-fold. Boldness, too, increased with temperature (the figure shows latency to emerge from shelter after threat, or how long it took them to come back out after being scared by the net). On average, individual boldness increased 2.5-fold as a function of temperature, varying from 1.5- to 8-fold depending on the individual.

In the second study, they decided to experimentally manipulate water temperatures using a temperature-controlled setup. Water temperature was maintained at an average of 29.0°C for the first half of the experiment, and an average of 26.2°C for the second half of the 13 day experiment.  The experiment had similar results as the first, supporting the argument that small changes in temperature (3°C or less) have dramatic effects on fish behavior.

"Our results also suggest that temperature variations are much more significant than we thought in the way they affect the behaviours of individual animals," said the lead author Peter Biro in the study's press release. "We observed that most of the individuals in our experiments were very responsive to changes in temperature, dramatically increasing their levels of activity, boldness and aggressiveness as a function of increases of only a few degrees of temperature," the authors write in their conclusions.

"If we had ignored temperature in these studies, and had measured different individuals at slightly different temperatures, our estimates of the ‘personality’ of any given individual would have been strongly affected by the temperature at which we measured its behaviour. For example, a generally shy individual who was observed under slightly warmer conditions could have appeared to be as bold as, or even bolder than, a generally bold individual who was observed under slightly cooler conditions," they elaborate.

The consequences of such changes in behavior are huge. Since fish are ectotherms, it makes sense that temperature changes might affect personality traits. "Individual variations in activity and boldness can affect food acquisition, encounter rates with predators and even the likelihood of an individual being captured by sampling or harvesting gear," explains Biro.

However, this means that changes in ocean temperatures might be even more detrimental than previously thought. The oceans store more than 90 percent of the heat in the Earth’s climate system and act as a temporary buffer against the effects of climate change, but they do not do so without any consequences. Recently, ocean temperature rises have accelerated, and the predicted rises in water temperatures are increases. Studies like this one show that not only will this change alter food webs and species that are particularly sensitive to water chemistry, it will affect fish that we might otherwise think should be OK, as the change that will occur in ocean temperature still fall into their normal temperature ranges.  In other words, increased ocean temps will have previously unexpected effects on fish that might otherwise be unaffected by climate change.

"Our results suggest the possibility for increasingly disrupted patterns of behaviour in fishes and other ectotherms under a scenario of climate warming, with increases in mean-level activity, boldness and aggressiveness of a magnitude that has been shown to result in greater vulnerability to predation, and decreased survival in other fish populations," the authors concluded. What this will mean for coral reefs is uncertain, but it doesn't look good.

Biro, P.A., Beckmann, C.&J.A. Stamps (2010). Small within-day increases in temperature affects boldness and alters personality in coral reef fish. Proc. R. Soc. B 277, 71-77.