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.
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