Pacific bluefin tuna recaptured

A 300 kg Pacific bluefin tuna that featured in a joint-venture research project conducted off the South Island west coast has been recaptured two years later. The project, funded by University of Auckland, supported by Stanford University and co-ordinated by Blue Water Marine Research, involved placing satellite tags on 23 of the adult tuna, which aggregate in August and September each year around massive schools of spawning hoki and the trawlers that harvest them. Where these fish spend the rest of the year is still unknown. All 23 of the tags reported, showing that these fish successfully survived release after being captured on rod and reel or hand lines.

The tags accumulate various data including the time of sunrise and sunset, ambient water temperature and the swimming depth of each fish. The accumulated information is summarised into short time periods and stored. After a predetermined interval the tags are programmed to detach from the host fish and float to the surface. Orbiting Argos satellites then receive the summarised data downloads direct from the tag until the battery life runs out, which is generally after about ten days. The tethers that anchor the tag to the fish remain behind and carry serial numbers much like conventional tags, in case the tuna is recaptured at a later date.

This particular tuna was caught on the Nelson charter vessel “Cova Rose” on 19 August 2008 after being fought on rod and reel with 60kg line for 90 minutes by angler Dave Wallace. It was tagged and released 35 nautical miles west of West Port. Its satellite tag detached and began transmitting information three and a half months later on 29 November 2008. At that time the fish was 800 nautical miles off East Cape, and around 1100 nautical miles in a straight line from where it was tagged.

On the night of 18 August 2010 it was recaptured, ironically by the same boat, “Cova Rose”, and the skipper Tony Roache immediately realised the significance of the tether hanging off the tuna. It had remained perfectly embedded for two years, and a quick phone call to John Holdsworth in Tutukaka soon revealed the identity of the fish and its release details. The recapture is a significant one fore a number of reasons.
 This is the longest time elapsed between the initial release and the later recapture for this species in the South Pacific,
 The fish had returned to the same area just 22 nautical miles southwest of where it was tagged,
 It demonstrated the effectiveness of the attachment procedure for the satellite tags.
Researchers have been concerned by the failure of the tags to remain on the tuna for periods of more than six months. This recovery suggests that the tag heads are very effective and the tag area had healed well. The premature release of some tags may be related to some other factor, such as other large tuna biting them and pulling them off the host fish.

Pacific bluefin tuna grow to in excess of 400 kg in weight. Most of those tagged to date have ranged between 200 kg and 300 kg, with a few larger fish as well. The only known spawning grounds are between Japan and the Philippines. One objective of the research is to see if Pacific bluefin move between the west coast hoki grounds and the known spawning grounds, and if so, whether this is a seasonal migration or something else. Whether this tuna travelled to the distant spawning grounds or remained in the South Pacific over the last two years is one of the questions that continue to fascinate researchers, but its recapture is one more link in a slowly emerging picture.

Locations of the 300 kg Pacific bluefin tuna estimated from satellite tag data. This fish was recaptured last week close to where it was released two years earlier (image by George Shillinger).

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