Home WORK AREAS Ecosystem Monitoring Conservation forensics help unlock tuna mysteries
Conservation forensics help unlock tuna mysteries
Thursday, 10 October 2013 14:44

otholithTunas are highly mobile fishes that often undertake long-range movements to track food and to reproduce at distant spawning grounds. Information on these movements underpins the effective management of commercially important tuna stocks. In the case of South Pacific albacore (Thunnus alalunga), longstanding questions remain regarding the number and location of spawning areas, the degree of connectivity among larval sources, the migration routes of juveniles and adults and the biophysical factors influencing these processes.

To answer some of these questions for South Pacific albacore, we applied a conservation forensics approach to examine the otoliths (ear bones) from albacore. We collected otoliths from albacore captured across the South Pacific Ocean, in French Polynesia, New Caledonia and New Zealand. We then used a high precision laser to measure the amount of trace elements (i.e. those elements that are present in very low concentrations such as strontium, barium and magnesium) in each otolith. By measuring the concentration of different trace elements at the edge of the otolith, which contains material deposited in the last few weeks before capture, we were able to obtain a clear chemical ‘signature’ that was unique to each of the three locations.

We then measured the concentration of trace elements in the core of each otolith, which contains material deposited in the first few weeks of life. We found that the trace elements from otolith cores sampled from New Caledonia and New Zealand were quite similar, suggesting that albacore from these locations may have originated from the same location. However, trace elements in otolith cores from French Polynesia were distinctly different to the other locations, suggesting albacore from French Polynesia originate from a different location. These results provide important preliminary insights into connections between larval sources and adult populations of South Pacific albacore, and suggest that further application of conservation forensics to albacore will refine our knowledge of movement patterns and population structure for this species.

 

In addition, expanding the use of conservation forensics to other tuna species will broaden our understanding of movement patterns of tuna in general, without the need for tagging fish. Extensive tagging of yellowfin, bigeye and skipjack tuna at tropical latitudes has generated a wealth of information about tuna movement patterns. However, difficulty with tagging programs for these species at sub-tropical and temperate latitudes has limited our knowledge of the broader movement patterns of these species throughout their range. Applying a conservation forensic approach to the otoliths of all commercial tuna species provides the opportunity to attain a stock-wide picture of tuna movement across the Pacific Ocean.

 
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