Bigeye tuna dive deep with the moon

Karen EvansCSIRO scientist, Karen Evans. The bigeye tagging project had 66 conventional tags and 17 archival tags returned, from fish that had travelled 10 to 8000 nautical miles from where they were tagged.

CSIRO scientist Karen Evans has tagged tunas, billfish, sharks, seabirds, seals and whales from the tropics to Antarctica. She develops and deploys tags, and analyses their data to see how top order predators move, feed and behave.

Karen led the data analysis and reporting of a project that tagged 161 bigeye tuna with archival tags and 269 with conventional (spaghetti) tags in the Coral Sea. Sixty-six conventional tags and 17 archival tags were returned, from fish that had travelled 10 to 8000 nautical miles from where they were tagged. Daily fish positions estimated from archived light levels and sea surface temperatures (from satellite) showed most bigeye did not migrate long distances. They spent their days 250–500 metres deep, their nights at less than 200 m, and tolerated waters from 2.5–22°C. Some fish dived deeper on the full moon. This wide-ranging diving behaviour enables bigeye to locate scarce prey while minimising competition with other tuna species.

Bigeye tuna are caught by pole and line and purse seine fisheries in the western and central Pacific Ocean, and in Australia's Eastern Tuna and Billfish Fishery. Tagging studies help scientists account for fluctuations in fishery catch records in order to better estimate fish stocks. Spikes in catches can be due to shifts in fish diving behaviour and their availability to fishing gear, rather than rises or falls in overall numbers. For example, when bigeye dive deeper on the full moon their habitat range overlaps with night-time sets of fishing gear resulting in higher catches.

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