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EMA - Tuna Biology and Behavior
Tuesday, 25 November 2008 13:46

Age and growth of tropical tunas

Otolith reading of daily increments is now routinely carried out, and estimates of age and growth of yellowfin and bigeye tuna, based on these readings combined with available tagging data, and using composite growth models have been presented to SCTB 12. Whilst these estimates can be further refined, they represent a significant inprovement in the description of growth of the two tuna species in the region.


The results indicating that growth of juvenile yellowfin and bigeye tuna is not following the von Bertalanffy growth model is confirmed by independent estimation of growth parameters with the statistical population dynamics model Multifan-CL based on fishing statistics, length frequencies and tagging data.


Movements, migrations and stocks structures

Considerable information on stock structure of yellowfin, skipjack and albacore has been available from tagging experiments, leading to general acceptance of an east-west population structure for skipjack and yellowfin, and hemispheric structure for albacore in the Pacific.


Much less information has been available for bigeye, although a single Pacific-wide stock is often assumed. A genetic study of bigeye stock structure, involving DNA in samples taken from widely dispersed localities across the Pacific, was underatken in cooperation with CSIRO, with funding from AusAID and the University of Hawaii. The study, carried out during 1996/97, did not provide any evidence of genetic differentiation across the Pacific; the null hypothesis of a single, pan-Pacific stock could not be rejected. A single stock hypothesis is thus de facto assumed, despite the quite different oceanographic and fishery conditions.


The SPC, in collaboration with the Commonwealth Scientific  and Industrial Research Organisation (CSIRO Hobart, Australia) is undertaking an archival tagging programme on bigeye tuna in the Coral Sea off northern Australia. In January 2002 , a total of 160 bigeye tuna have been tagged using Wildlife Computers MK7 tags in the area off Cairns that is famous for the large aggregations of tuna that occur there at this time of year. The first recapture of one of these tagged bigeye tuna occurred in November, some six weeks after release.


Dynamics of tuna movements and stocks structure are more or less explicitly considered in populationmodels, allowing to test the assumptions on stock structures of the different species. For instance, spatial stratification by region is used in the Multifan-CL analyses and transfer coefficient estimates between regions provide information on the rate of transfer across each region. With the model SEPoDyM, the movement is explicitly described using advection-diffusion equation over a geographical grid of one degree square. However, there is no integrated statistical estimation and predictions are evaluated independently.

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