Big-eye tuna tagging to fill knowledge gap

Transcript

NEVILLE SMITH: Until this expedition tagging of bigeye tuna has been focussed in the eastern and central Pacific for instance the Line Islands to the south of Hawaii and most of those tagged fish seem to head east. Sort of towards the USA and ultimately out of the Western and Central Pacific Fisheries Commission's area. So we are hoping by tagging fish in the west that we understand what the fish in the western Pacific do. Do they also go east or do they behave differently? There is also the thing called the thermocline which is the average sort of a depth at which the temperature, the water temperature changes significantly. And the average depth of that thermocline is very different in the eastern Pacific versus the western Pacific. It sort of goes from shallow in the east to deep in the west. And bigeye tuna in particular seem to like to spend quite a lot of time below that thermocline in the cooler water. And so that means they have quite different behaviour in the west compared to the east. So this research is trying to fill some of those knowledge gaps because there has been so little tagging of bigeye tuna in the west in the past.

KOROI HAWKINS: Now tagging to me I just think of a tag hanging of the back of a tuna but there is quite a few different tags you are using right?

NS: That is correct yes, there are three types of tags used on this expedition. There is the conventional tag which you kind of mentioned. So kind of looks like a bit of spaghetti maybe ten to fifteen centimetres long with a few details printed on it. They are conventional tags so they are relatively cheap so we try and put lots of them out. They provide limited information compared to some of the modern high tech tags and the other thing with the conventional tag is you need to get it back to get the information. So someone needs to send it back to you at some point. The two types of sort of modern high tech tags that we are using are archival tags and acoustic tags. The archival tags relatively expensive so you only get to put a few out we managed to put a hundred and twenty out during this trip. Which was quite a significant achievement, they provide detailed information on temperature depth and also light intensity and from that light intensity we can use geo-location techniques to track the movement on a day to day basis essentially of the fish. The problem with the archival tags is we still need to get the tag back to get the information. So we also to get some detailed information about the sort of the vertical behaviour on a short term basis of the tuna we use acoustic tags. Again relatively expensive and the other challenge with the acoustic tags is that they need a listening station. Which we sort of deploy at the same time we deploy those acoustic tags. But they do provide very detailed information about movement up and down in the water column and to and from the drifting FADS (Fish Aggregating Devices) they might be associated with. Over a short period of time that information gets transported back to the listening station which then sends it to a satellite and back to the scientist. So the big advantage of that type of approach is you don't need to get the tag back to get the information.

KH: And when all of the data from this gets to you and you get the information you are looking for. What will it mean for management practices and how does it translate to action?

NSWell the science from the exhibition will obviously be used to inform future assessments of bigeye stock status. We are the principal scientific and technical organisation in the Pacific Island region and one of our key roles is to provide that science advice to our members but also as a scientific services provider to the Western and Central Pacific Fisheries Commission. But in terms of what it means for management practices that is the sort of question that I would need to defer to my colleagues at the FFA (Forum Fisheries Agency) who are the authority on such matters. The idea of this, sort of a shift of population from west to east or east to west, I just wanted to mention that you know tuna generally do follow their habitat, which is essentially water of the right temperature with food in it. So as those sort of water masses move around the tuna move with them so it is not always actually less tuna in a particular area that we are observing. What we are observing is actually movement of the water masses and some of the science is trying to help us understand that but we also saw very clearly last year for instance that during periods of El Nino that oceanographic habitats drift to the east and the tuna stocks shifted heavily to the east across several species as a result of that. But we are also now in a mild La Nina event and at that time the tuna shift back to the west which we are actually seeing from the fishery. So just sort of one of those things that this science is starting to underpin for us is to better understand how the tuna movements fit into oceanographic changes especially with respect to things like the El Nino and La Nina cycle.  

KH:That is something linked to climate change isn't in terms of where the tuna will be in 50 years time?

 NS: Yeah the two are sort of linked but they are also decoupled I mean the El Nino, La Nina the southern ocean oscillation is a sort of multi-decadal cycle. It has been ongoing and been observed for years and those sort of water mass shifts as it were are normal. But obviously with climate change it is going to have impacts on how warm the waters are when it shifts and that is going to have impacts on the quality of habitat and those sorts of things that is certainly some of the science that we are also working on here at SPC.