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Oceanographic Characterization of the American Samoa Longline Fishing Grounds for Albacore, Thunnus alalunga

Progress Reports (PDF): FY 2008, FY 2007, FY 2006, FY 2005, FY 2004, FY 2003

Project Overview
The American Samoa domestic longline fishery has undergone extraordinary growth, particularly in the fleet composition of large vessels (>50 ft. in length) that have fueled a fivefold increase in fishing effort and landings from 1999 to 2001. Prior to the sudden expansion, most longline fishing around American Samoa were accomplished through a fleet of smaller, 30-ft, open-decked catamarans known as alia.

Albacore tuna (Thunnus alalunga) is the target species in the longline fishery with depth distribution ranging from surface down to at least 380 m. It has been reported that the large sized albacore are broadly found between 80 and 380 m but the center and concentration of their vertical distribution was about 200-260 m (Saito 1973). Much of the incidental catch taken in the domestic American Samoa deep longline fishery is composed of species considered to occupy shallower depths and that are also targeted by local surface troll fishers. These include species such as blue marlin (Makaira mazara), wahoo (Acanthocybium solandri), mahimahi (Coryphaena hippurus), and skipjack tuna (Katsuwonus pelamis). Concerns now abound that the rapidly expanding longline fleet may threaten the local supply of these large pelagics. Negative trends in both the longline and troll catch rates of the shallow species do little to suppress these fears. To address these concerns, a 50-nautical closed fishing area to the large vessels has recently been designated around the islands of American Samoa and a moratorium on the entry of any new longline vessel into the fishery has been proposed until a limited entry program can be implemented.

There has been little study regarding the pelagic habitat in the American Samoa region, much less the spatial and temporal variability of the oceanographic climate. Historical studies conducted on South Pacific longline fisheries and the corresponding environment have focused on foreign fishing activities primarily in waters farther to the west, e.g., west of Fiji. (Yamanaka 1956, Saito 1973). No oceanographic research has been conducted with regards to the influence of oceanographic features on the American Samoan fishery, such as the effects of predominating currents and horizontal shear on gear placement, or effects of geological ocean features on flow regime and water column properties.

The proposed research will:
1) conduct an extensive oceanographic characterization of the pelagic habitat and fishing grounds occupied by the American Samoa longline fishery through the use of satellite oceanographic remote sensing and in situ shipboard surveys;

2) couple the oceanographic assessment with fishery information to develop a functional understanding of the spatial and temporal occupation and movement tendencies of large South Pacific albacore and the role of the environment on longline gear performance and catch. In the latter phase of this project, fishery information will include incorporation of albacore depth distribution and gear performance obtained from commercial longlines instrumented with time-depth-recorders (TDRs) and the set level catch information from the American Samoa fishery logbook program. Products from this research will lead to a better understanding of the pelagic habitat and an improved interpretation of catch rates and patterns, thus providing information necessary to move forward on ecosystem-based fishery management policies and stock assessment efforts.

Proposed Activities

  • Assemble time series of satellite oceanographic products (sea surface temperature, ocean color, sea surface height) for waters of the American Samoa EEZ and immediate vicinity likely to be frequented by domestic longline vessels. Project researchers will take advantage of the recently developed near real-time products from Global Ocean Data Assimilation Experiment (GODAE) which outputs a higher resolution SSH product blended from the sensors aboard the TOPEX/POSEIDON and ERS satellites. The satellite remotely sensed coverage will allow the examination of the seasonal and interannual dynamics of key features (Southern Equatorial Current and eddies) as well as basic physical and biological properties.
  • Conduct two 30-day shipboard surveys aboard the NOAA ship Oscar Elton Sette to better infer subsurface vertical structure of the water column. Sampling will include high resolution CTD casts along transect lines with transect length and sampling resolution designed to accommodate time and space scales necessary to address oceanographic mesoscale perturbations as well as logistical constraints of the field program. Information currents and current shear will be measured with continuous data acquisition from a vessel-mounted 150 kHz acoustic Doppler current profiler (ADCP). Since dissolved oxygen concentration (DO2) is one of the key water properties identified that influences vertical distribution of tunas (Sund et al. 1984, Brill 1994), DO2 will be measured in-situ with a CTD-mounted DO2 sensor and calibrated against independent DO2.
  • Gather information regarding albacore distribution and longline performance. Sources to be examined include:
    - set level fish catch and assemblage patterns obtained from commercial longline logbooks available through the Western Pacific Fisheries Information Network (WestPacFIN),
    - high resolution instrumentation of commercial longlines with TDRs to obtain high detailed profiles of fished depths (and capture depths when applicable),
    - vertical distribution and albacore movement information available from other ongoing studies using popup satellite archival tags (PSATs), although past observations suggests that instrumenting longline-caught albacore with PSATs may pose a considerable challenge.

These biological and fishery data will be merged with the oceanographic assessment to develop a functional understanding of the spatial and temporal occupation and movement tendencies of large South Pacific albacore and to gear characterization and performance in the American Samoa longline fishery.

Year 1 funding for this 2-year project to be awarded December 2002.

Literature cited:
•Brill, R.W.1994. A review of temperature and oxygen tolerance studies of tunas pertinent to fisheries oceanography, movement models and stock assessment. Fish. Oceanogr. 3:204-216.
•Saito,S. 1973. Studies on fishing of albacore, Thunnus alalunga (Bonnaterre) by experimental deep-sea tuna longline. Mem. Fac. Fish., Hokkaido University. 21:107-184.
•Sund, P.N., M. Blackburn, and F. Williams. 1981. Tunas and their environment in the Pacific Ocean: A review. Oceanogr. Mar. Biol. Ann. Rev. 19:443-512.
•Yamanaka, H. 1956. Vertical structure of the ocean and albacore fishing conditions in the vicinity of 10°S in the western South Pacific. Bull. Jap. Soc. Sci. Fish. 21: 1187-1193.

Principal Investigators:

Michael Seki
National Marine Fisheries Service
Honolulu Laboratory
2570 Dole Street
Honolulu, Hawaii 96822 USA
Phone (808) 983-5393
FAX (808) 983-2902
email: Michael.Seki@noaa.gov

Dr. Jeffrey Polovina
National Marine Fisheries Service
Honolulu Laboratory
2570 Dole Street, Room 216
Honolulu, Hawaii 96822 USA
Phone (808) 983-5390
FAX (808) 983-2902

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This page updated August 7, 2008