Whale sharks regulate their body temperature
Marine fish often visit the surface to warm up after diving deep. 4 whale sharks "volunteered" to test some time/depth recorders for Michelle Thums of the University of Western Australia and her colleagues. They publish their research paper, Evidence for behavioural thermoregulation by the world's largest fish, in Biology Letters today.
Vertical movement like this has not often been studied, despite its importance for aquatic animals. Air-breathers have an obvious affinity for the surface but it's a surprise to find many gill-breathers curiously similar. Thermal recovery is certainly the most obvious cause for such behaviour.
With negative buoyancy, most pelagic fish species swim towards the surface quite often, but elasmobranchs (eg. sharks and rays) spend long periods at the surface following dives. Rhincodon typus, the whale shark is the world's biggest fish and spends an average of 49% of its time at the surface between its diving stints (of over 1000 metres). Analysing the dives resulted in the discovery of three dive-types.
1. From 0400 to 1600 hours was the commonest dive-type, occupying 44% of the animal's diving behaviour
2. From 1800 to 0600 hours, type 2 dives were almost as common as type 1.
3. From 0300 to 1800 hours, with peaks at 0500 and 1200h, type 3 dive bouts had the longest post-dive surface duration. A shark at Christmas Island was used for a longer period and illustrated more type 3 dives than others over 88 days. This shark travelled to Indonesia during this time, using deeper waters up to 3482m at least! However the other individuals had a maximum of 9% of their diving as type 3. Individual preferences could have had something to do with these dives.
Results supported the idea that thermoregulation took place by using warm surface waters as a heater for the body. Fish physiology demands these higher body temperatures for optimal performance. In warm waters, above 25°C, no relationship exited and it's perhaps necessary to get rid of excess heat at some temperatures by diving deep to cooler water. It wouldn't have been ethical but measuring internal temperatures would have been useful, despite the fact that whale sharks are pure ectotherms. The external sensors should have indicated both ocean temperature and the approximate body temperature, especially after time at depth.
Overall, the research indicated a positive correlation between time spend at the surface and depth of dives at temperatures below 25°C. Other factors such as feeding, especially for a filter feeder, come into play. This animal however, recycles so much water that it would need to replace lost heat energy after a deep feeding "bout." More fish and especially large, non-filter-feeders could be usefully tagged now that this technology has been successfully employed. We look forward to the discovery of more intriguing information on our fish and other species.
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