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Trophic Ecology and Structure-Associated Aggregation Behavior in Bigeye and Yellowfin Tuna in Hawaiian Waters

April 2005: Extension of scope of work: PFADs - Catch composition and aggregation behavior
See also the socio-cultural component of this project extension:
Small Boat Bigeye and Yellowfin Tuna Operations and Regulatory Scenarios in the Main Hawaiian Islands (PIs: Ed Glazier and John Petterson)

See also Investigation of Aggregation Behavior of FAD-Associated Small Yellowfin Tuna and Size Dependant Vertical Stratification, and Instrumented Buoys as Autonomous Observatories of Pelagic Ecosystems.


Progress Reports (PDF): FY 2008, FY 2007, FY 2006, FY 2005, FY 2003, FY 2002, FY 2001

Project Overview
The focus of this project is to elucidate the role of feeding ecology in the aggregation (schooling) behavior of tunas, especially those aggregations found around floating logs, FADs (fish aggregation devices) and seamounts. Not only are tuna aggregations a dominant component of worldwide tuna fisheries but understanding the biology of aggregation phenomena also has direct pertinence to stock assessment and to understanding the ecosystems that support the fishery. From a stock assessment perspective, the contributions of aggregations to the overall distribution of tuna biomass is central to estimating the size of the resource and the movements of the population. The occurence of several different types of tuna aggregation within close proximity to Hawaii, combined with the existence of other pertinent fishery research projects, provides an ideal setting for the research into the interaction between feeding behavior and aggregation behavior.

In order to understand the viability and distribution of a resource, it is critical to understand the physical and biological setting in which it exists. It is also important to understand the impact that the industrial-scale removal of the tuna will have on the balance of the trophic system from which they are removed. Project researchers will examine the aggregation phenomenon using a trophic ecology approach augmented by coordinated sonic tracking and echo-location experiments.

Trophic Biology
Two general approaches will be used: 1) "traditional" examination of stomach contents of captured tunas, and 2) analysis of different tuna tissues for stable isotopes of carbon and nitrogen. These methods provide wide-ranging and complimentary approaches for understanding trophic ecology of tunas. If initial stable-isotope ratio data reveal the anticipated fine-scale differences then project researchers will maintain tuna in captivity to calibrate the field data, elucidate the ontogeny of the differences in isotope profiles and explain their significance. Samples of tuna stomach contents and tissues for biochemical analysis will be obtained from bigeye and yellowfin tunas taken from the various aggregation sites in Hawaiian waters and from offshore longline vessels. The aggregation sites are:

• Offshore, deepwater buoys, NOAA weather buoys and the "Bigeye Buoy" mooring (at 20° 34'N, 161° 36W, see PFRP Project 656196),
• Cross Seamount, which straddles the 700 meter meso-pelagic boundary fauna (MBF) transition depth,
• Nearshore FADs
• Nearshore ko'a (traditional tuna holes). Of particular interest is the productive Hilo ko'a which, because of its proximity to the very active geology of Hawaii Island, may have a forage base with a very distinctive isotope signature.

Sonic Tracking and Echo-integration Studies
Project researchers plan to use acoustic telemetry to make direct intensive observations of the movements, distribution and aggregation of juvenile and adult bigeye tuna aggregated around Cross Seamount and a manmade floating object (the "Bigeye Buoy" mooring). Standard depth-sensitive ultrasonic transmitters (Vemco Inc.) will be attached to tuna to record fine-scale vertical and horizontal movements. The NOAA research vessel, Townsend Cromwell will be used as the tracking/oceanographic platform because of its ability to track tunas while simultaneously recording acoustic assessments of the forage abundance and distribution and oceanographic observations of the vertical structure of temperature, oxygen and current velocity. In the second year researchers propose to measure the behavior of bigeye tuna and their forage in relation to their oceanographic habitat, particularly the persistent cold-core cyclonic eddies now known to form on the lee side of Hawaii Island.

Year 1 funding for this project received in December 2000.