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The Role
of Oceanography on Bigeye Tuna Aggregation and Vulnerability in the
Hawaii Longline Fishery from Satellite, Moored, and Shipboard Time Series
Progress
Reports (PDF): FY
2005, FY 2003,
FY 2002, FY
2001, FY 2000
Project Overview
Bigeye tuna (Thunnus obesus) is the principal deepwater target
species of the longline fishery in the Pacific Ocean yielding total annual
catches exceeding 150,000 tons (Hampton et al., 19961; Hampton
et al., 19982). Stock assessment models require the use of
longline fishery catch-per-unit-effort (CPUE) as an index of bigeye abundance.
Fishery-dependent CPUE does not necessarily represent the abundance of
stock, but rather the "catchability" of the stock. Catchability, in turn,
is dependent to a considerable extent upon variable oceanographic conditions.
Oceanographic variability can significantly affect the depth of the thermocline
and the most likely depth of occurence of bigeye tunas as well since the
preferred foraging habitat of bigeye tunas is the 8-15° C water at
or near the base of the thermocline (Hanamoto, 19873; Holland
et al., 19904; Boggs, 19925; Brill, 19946;
Boggs et al., 19987; Musyl and Brill, 19988).
The goals
of this research are to:
- examine
closely the relationships between bigeye tuna CPUE from the Hawaii-based
longline fishery and oceanographic features observed using moored,
shipboard, and satellite time series of the vertical and spatial structure
of the upper ocean;
- utilize
those relationships to develop methods to improve stock assessment
estimates based on standardized logbook CPUE using remotely-sensed
observations of sea surface height (altimeter), sea surface temperature
(AVHRR), ocean color (SeaWiFS), and surface winds (scatterometer).
Proposed
activities include:
- 1.
Moored Observations
- While
there are currently three oceanographic moorings collecting vertical
thermal structure data in the upper ocean which are in general proximity
to the bigeye fishing grounds of the central Pacific there are some
limitations. These moorings are either located outside of the main
fishing grounds, not adequately equipped to sample the vertical
thermal structure, and/or not equipped to measure velocity or shear
of the upper 500 m. Because of the lack of adequate time series
information in a region of high bigeye tuna CPUE, project researchers
propose to establish a mooring to measure the vertical structure
of temperature and current velocity in the bigeye tuna fishing grounds
located south and west of the main Hawaiian Islands. Optimization
based on CPUE, variability of CPUE, proximity to TOPEX/JASON satellites
crossover paths, variability of oceanographic structure, and linkages
to other components of the Pelagic Fisheries Research Program was
used to determine a preferred mooring location about 185 km southwest
of the island of Niihau at 20°36.0'N, 161°34.2'W. The
mooring will be designed to focus on describing the habitat of bigeye
tuna by providing continuous vertical profiles of temperature and
velocity in the upper 500-600 m. Time-depth recorders will be deployed
on longlines during research cruises and commercial longline trips
to provide longline depth information concurrent with mooring data.
- 2.
Shipboard Sampling
- This
will be conducted to: 1) identify and further characterize the area
spatially represented by the mooring and provide three-dimensional
ground-truthing to remotely-sensed surface measurements through
hydrographic surveys in the region around the mooring; 2) conduct
experiments to ascertain the influence of prevailing oceanographic
conditions on longline gear performance; 3) obtain information on
the biological community that constitutes the bigeye forage base.
Shipboard sampling will include closely spaced (10 km) CTD casts
along 200 km transect lines centered on the mooring location. The
biological community will be sampled with a large dual-warp midwater
trawl (140m2 fishing area at the mouth) targeting the
depth strata identified as bigeye habitat from hydrographic surveys.
- 3.
Satellite Data
- Comparisons
between both mooring and cruise data and the altimetric anomalies
will be used to develop an algorithm to map estimates of the vertical
thermal structure over the fishing grounds every month. Project
researchers hypothesize that when the vertical thermal structure
shoals the bigeye habitat contracts toward the surface and the bigeye
density increases. Conversely, when the vertical structure deepens
then bigeye habitat expands and bigeye density decreases. Researchers
propose to use estimates of the change in vertical thermal structure
(estimated from altimetry) to correct or standardize commercial
longline CPUE for changes in vertical habitat following the approach
of Hinton and Nakano (1996)9 so that changes in CPUE
reflect changes in bigeye abundance and not changes in the vertical
extent of habitat.
-
Funding
for this project awarded in October 1999.
Footnotes:
(1) Hampton, J., A. Lewis, and P. Williams, 1996: Estimates
of western central Pacific Ocean bigeye tuna catch and population parameters.
Working paper presented to the World Meeting on Bigeye Tuna, La Jolla,
California, 11-15 Nov. 1996.
(2) Hampton, J., K. Bigelowe, and M. Labelle, 1998: Effect
of longline fishing depth, water temperature and dissolved oxygen on
bigeye tuna (Thunnus obesus) abundance indices. Standing Committee
Tuna and Billfish Working Group Report.
(3) Hanamoto, E., 1987: Effect of oceanographic environment
on bigeye tuna distribution. Bull. Jap. Soc. Fish. Oceanogr.,
51:203-216.
(4) Holland, K.N., R.W. Brill, and R.K.C. Chang, 1990: Horizontal
and vertical movements of yellowfin and bigeye tuna associated with
fish aggregating devices. Fish. Bull., 88, 493-507.
(5) Boggs, C., 1992: Depth, capture time, and hooked longetivity
of longline-caught pelagic fish: Timing bites of fish with chips. Fish.
Bull., 90, 642-658.
(6) Brill, R.W., 1994: A review of temperature and oxygen
tolerance studies of tunas pertinent to fisheries oceanography, movement
models and stock assessments. Fish. Oceanogr., 3, 204-216.
(7) Boggs, C.H., M. Musyl, R. Brill, D. Curran, M. Laurs,
and J. Gunn, 1998: Bigeye tuna crepuscular diving data from an archival
tag indicates geoposition and lunar influences. (MS in prep.)
(8) Musyl, M. and C. Boggs, 1998: Integration of temperature-depth
recorders (TDRs) with catch to assess environmental and gear factors
important in the commercial landing of eight pelagic fish species, representing
three families (Carcharhinidae, Istiophoridae, Scombridae), in the Hawaiian
Longline Fishery with notes on their ecology: "How deep is deep?" To
be submitted to J. of Fish Biol.
(9) Hinton, M.G. and H. Nakano, 1996: Standardizing catch
and effort statistics using physiological, ecological, or behavioral
constraints and environmental data, with an application to blue marlin
(Makaira nigricans) catch and effort data from Japanese fisheries
in the Pacific. Bull. Int. Am. Trop. Tuna Comm., 21(4): 171-200.
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