Retroactive Determination of Movement at Age in Relation to Water Mass in Patagonian Toothfish Through Laser-Based Elemental Analysis of Otoliths

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9614756 Jones Laser-based Inductively-Coupled Plasma Mass Spectrometry LA-ICPMS) is a new technique, which is being applied for the first time in the Southern Ocean, to examine movement-at-age in Patagonian Toothfish (Dissostichus eleginoides). The chemical structure of growth increments in the otoliths of fish reflects the composition of water passing across the gills; the increments also give a chronological record of the life history of individual fish before capture and are used for aging. Elemental analysis using LA-ICPMS allows accurate sampling of a suite of elements in the otoliths, giving a record of changes in the water mass experienced by the fish. This can be related retroactively to age and real time using the increments. The project is an international collaboration between scientists from the United States, United Kingdom, France, Chile and Australia. The central collaboration is between scientists at Old Dominion University and the British Antarctic Survey, bringing together the most extensive experience currently available of the technique, with that of Antarctic life systems. Scientists from France, Chile, Australia and the United States are providing samples of Toothfish otoliths from the Southern Ocean and southern South America. The project consists of four inter-linked modules, addressing the overall question: what are the patterns of movement of Patagonian Toothfish over its life history in relation to variability of water mass in time and space? The objectives are to show that the LA-ICPMS technique can discriminate between Patagonian Toothfish from different geographical regions, develop a validated aging method, and characterize the variability of the chemical signal with age and between years. Other objectives are to elucidate differential spawning sites, and examine age-specific movement retrospectively in individual fish within the Southern Ocean, and within the Patagonia-Scotia Arc region. NSF will fund only module 3 (Does eleme ntal analysis indicate movement of Patagonian toothfish between the Atlantic, Inian, and Pacific sectors of the Southern Ocean?), while modules 1, 2, and 4 will be funded by other governments. The first module consists of a preliminary test for elemental variability in otoliths, using LA-ICPMS. The second is the development of a reliable aging method with fully validated criteria for interpretation of growth increments, using marginal increment analysis. The third module consists of analysing, with LAICPMS, a suite of elements present across otolith sections of fish taken from Macquarie Island, the Kerguelen Islands and South Georgia in the Southern Ocean. Discrimination of element concentrations at the otolith edge between regions will confirm that the technique can detect geographic separation in Patagonian Toothfish. Discrimination by year class of concentrations at the nucleus will indicate that spawning sites are separate. Comparison between known-age sites in the otoliths will test for changes in the elemental profile with age corresponding to movement of fish between water masses. Comparison between the same known-age sites of fish from different age classes will test for variability between years. The fourth module examines elemental concentrations on a finer scale, comparing consecutive year sites across otoliths sampled from northern and southern Chile, the Falkland Islands and South Georgia. The results of the project will be of direct benefit to population and ecosystem ecologists, oceanographers, and fisheries managers working in the Southern Ocean and outside. They will indicate where movement occurs in time and space, and where populations are isolated potentially by oceanographic barriers. The validation of aging criteria will be a major contribution to accurate aging of Patagonian Toothfish. Better knowledge of age and stock structure, and mixing levels, is important for the use of fisheries yield models; better understanding of the role playe d by environmental processes will allow better prediction of recruitment and growth. The chemical structure of the otolith may be used as a retroactive environmental record, giving data that may enhance understanding of the role of inter-annual variability in structuring marine ecosystems, and allow detection of trends associated with climate-induced change.