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Investigation of Aggregation Behavior of FAD-Associated Small Yellowfin Tuna and Size Dependant Vertical Stratification
Instrumented Buoys as Autonomous Observatories
of Pelagic Ecosystems.
Not only are small tuna a major component of the FAD community, but the results of the ongoing trophic ecology project indicate that a significant shift in feeding ecology occurs in yellowfin and bigeye tuna when they attain a length of 45-50 cm FL. In the trophic analysis study, nitrogen isotope analyses indicate that a shift in feeding ecology is occurring in yellowfin tuna at sizes between 40 and 50 cm FL. The change in isotope signature at 40-50 cm is corroborated by gut content data that show a change in diet at this same size. These findings in the trophic ecology data indicate that the FAD-associated behavior of small yellowfin might be significantly different than that of large individuals. Therefore, to build a complete picture of the influence of FADs on tuna distribution, it is essential that current research is expanded to include the very small size classes. In fact, very little is known about any aspect of the behavior of very small tuna. Project researchers will address the question of whether tuna display size-dependent vertical stratification when they occur in mixed-size aggregation at FADs. This aspect is important because putative size-dependant vertical segregation has been discussed as a possible basis for a management strategy for FAD-based fisheries.
The second year will focus on acquiring vertical distribution data from markedly different size classes of yellowfin tuna when mixed-size schools are present at the FADs. This will be achieved by using pressure sensitive Vemco V16P transmitters in conjunction with the data loggers attached to the FAD mooring lines. In this way, not only will researchers acquire additional residency data of the type acquired in the previous studies but also obtain multi-day data regarding the depth of the tagged animals while they are present at the FAD buoy. Transmitters will be placed in two different size classes of fish (50 cm yellowfin and >75 cm yellowfin) captured on the same day at the same FAD. Researchers believe this is a feasible sampling strategy and should result in data sets that represent quite long residency periods for the two different sizes of tagged fish. The resultant data will be analyzed to determine if there are size related differences in vertical distribution. The anticipated long duration of the data sets should allow robust statistical analyses of the depth data acquired from fish of different sizes while they cohabit the FAD.
funding for this 2-year project to be available mid 2004.
P.O. Box 570
REPUBLIC OF SEYCHELLES
University of Hawaii
Joint Institute for Marine & Atmospheric Research
Pelagic Fisheries Research Program
P.O. Box 1346
Kaneohe, Hawaii 96744 USA
This page updated August 14, 2006