Study reveals that fish retreat to deeper water to escape heat

Researchers from James Cook University have investigated how environmental conditions can be linked to dispersal and movement patterns in coral reef fish. Their study showed that fish retreat to deeper water to escape warm water temperatures, a finding that throws light on what to expect if predictions of ocean warming come to pass.

The redthroat emperor fish was the chosen species for the investigation, with researchers electronically tagging 60 individuals at Heron Island in the southern Great Barrier Reef. The fish were equipped with transmitters that identified them individually and signaled their depth to an array of receivers around the island. Using these transmitters, the researchers monitored the fish for up to a year and found that individuals occurred more often on the reef slope during days of cooler temperatures, suggesting a thermal tolerance threshold may exist. Results indicate that individuals responded to elevated temperatures by moving away from the reef slope to deeper adjacent habitats, thus shifting their position in the water column to remain at a preferred temperature.

To reach this conclusion, the team first considered alternative reasons that could potentially lead to the fish swimming to deeper regions, including air pressure, rainfall, wind and moon phases, but discovered the only significant correlation was with temperature.

One of the reasons that this study could prove significant in the future is due to redthroat emperor fish being commercially important in the region, with members of the species caught regularly by commercial and recreational anglers via handlines, rod-and-line, traps and demersal otter trawling – mainly stern trawling but also semi-pelagic trawling. They are the second most favoured fish for the reef line fishery behind the coral trout. With the significant depth shift exhibited by this species in response to increased temperatures, fishers may begin to catch less and less. Identifying key environmental drivers that affect the distribution of these reef fish is important, and may allow managers to predict the effect of these changes on exploited species.

“If it’s not around in the shallows in the future then fishers will have to redirect their efforts and it may be significantly harder to catch them,” says lead researcher Dr Leanne Currey.

The teams next phase of research will be to investigate whether the fish could adapt physiologically to warmer sea temperatures, as it appeared other species could, and also whether they can disperse into cooler habitats, by either shifting their distribution deeper or towards higher latitudes.  There is already some evidence of this, with some redthroat emperor recently being caught off Perth, far from their normal habitat further up the West Australian coast.


Header Photo Credit: Matt Salmond 

Journal Reference:

  1. Leanne M. Currey, Michelle R. Heupel, Colin A. Simpfendorfer, Ashley J. Williams. Assessing environmental correlates of fish movement on a coral reef. Coral Reefs, 2015; DOI: 10.1007/s00338-015-1318-7

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