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Investigation of Aggregation Behavior of FAD-Associated Small Yellowfin Tuna and Size Dependant Vertical Stratification

See also Instrumented Buoys as Autonomous Observatories of Pelagic Ecosystems.
See also Trophic Ecology and Structure-Associated Aggregation Behavior in Bigeye and Yellowfin Tuna in Hawaiian Waters


Progress reports (PDF): FY 2006, FY 2005

Project Overview
The purpose of this project is to obtain an improved understanding of the biology of aggregation behavior and structure-associated behavior in smaller size classes of yellowfin tuna. This project is a direct extension and expansion of the ongoing instrumented buoys project that has been gathering detailed information about the aggregation dynamics of bigeye and yellowfin tuna at fish aggregating devices (FADs) and the movements of tunas within a network of FADs. For this project, the researchers will adapt techniques successfully developed from the instrumented buoys project for use with smaller size classes of yellowfin tuna that are frequently the dominant component of the FAD assemblages.

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.

Methods
Project researchers will utilize methods and protocols currently used in their ongoing studies. Individually coded sonic transmitters will be surgically implanted into the gut cavity of yellowfin tuna that are <50 cm FL. All fish will be captured and released at FADs around the Hawaiian Islands. Vemco V8 transmitters will be used with data detected and logged by Vemco VR2 data logging receivers mounted at 20 meter depths on the FAD mooring chains. The range of the V8 transmitters will probably be less than the V16 transmitters currently in use but the schools of small fish occur sufficiently close to the FAD mooring that V8 transmitters will be detected by the data logger and will be able to provide high accuracy residency and visitation data.

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.

Year 1 funding for this 2-year project to be available mid 2004.

Dr. Kim Holland
Hawaii Institute of Marine Biology (HIMB)
University of Hawaii at Manoa
Coconut Island
P.O. Box 1346
Kaneohe, Hawaii 96744 USA
Phone (808) 236-7410/533-4110
FAX (808) 236-7443
email: kholland@hawaii.edu

Mr. David Itano
University of Hawaii
Joint Institute for Marine & Atmospheric Research
Pelagic Fisheries Research Program
1000 Pope Road, MSB 312
Honolulu, Hawaii 96822 USA
Phone (808) 956-4108
FAX (808) 956-4104
email: dgi@hawaii.edu