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Synchronous
Assessment of Bigeye Tuna (Thunnus obesus) and Micronekton Biomass,
Distribution, and Movement Patterns at Cross Seamount, and the Effects
of the Seamount Environment
Progress
Reports (PDF): FY
2010, FY 2009,
FY 2008, FY
2007
Globally,
seamounts play an important role in shaping the distribution of pelagic
species — such as tunas and sharks — and are heavily targeted
by commercial fisheries (Holland et al., in press). In Hawaii, the commercial
offshore handline fishery targets seamount populations of bigeye and
yellowfin tunas, species that are known to aggregate at these environments.
The local handline fishery frequently concentrates its efforts at Cross
seamount, located approximately 290 km south of the island of Oahu.
This fishery takes mostly juvenile and subadult fish, while adult bigeye
and yellowfin tunas around the Hawaiian Islands are an important target
species for offshore longline fisheries. Concerns have been raised about
the effects of the handline fishery removing too many juveniles that
could otherwise recruit to waters away from Cross seamount and help
maintain adult populations in Hawaiian waters. Current estimates from
tag release and recapture methods point to moderate exploitation rates
(10 - 30%, Adam et al. 2003), indicating that the biomass of bigeye
and yellowfin tunas aggregated at Cross seamount should be closely monitored.
Insights into the underlying biological advantage to tunas of aggregating
at seamounts are being gained through analysis of gut contents (Holland
et al., in press) but much is yet to be learned about the behavioral
interactions between the tuna and the mesopelagic micronekton community
that comprises the dominant portion of seamount-associated tuna prey.
Further, little is known about the interaction between prey abundance
and tuna abundance at seamounts and whether or not fluctuations in prey
abundance influence tuna residence times. Bigeye and yellowfin tuna
populations at Cross seamount have already been studied using tag release
and recapture methods that have provided information on characteristics
such as size composition, dispersal patterns, residence times and natural
and fishing related mortality rates (e.g., Adam et al., 2003; Sibert
et al., 2000). While these methods provide useful information on the
residence times and dispersal patterns of individuals, they do not provide
reliable biomass estimates and tell us nothing about the behavioral
interactions between predator and prey or how seamounts influence these
interactions.
In this study, a fishery independent method will be developed to estimate
the biomass, distribution and behavior of bigeye at Cross seamount and
the interaction of bigeye tuna and their prey species. We will use active
bioacoustic surveys in tandem with synchronous acoustic tracks of individual
bigeye tuna. Similar methods have already been successfully applied
to observe the movement patterns and distribution of tuna and their
prey at FADs (e.g., Josse et al., 1998). The acoustic surveys will use
a dual frequency hull-mounted Simrad acoustics system on board the NOAA
ship Oscar Elton Sette. In addition to biomass, the distribution and
abundance of bigeye tuna will be compared to the distribution and abundance
of micronekton, as well as to the physical environment. During some
of these acoustic transects, synchronous active acoustic tracking of
individual tuna will be conducted from a collaborating tracking vessel
thereby linking individual behavior to the assemblage distribution patterns
observed by the acoustic surveys. Besides being fishery independent,
the bioacoustic survey technique is able to provide simultaneous information
on the abundance and distribution of both nekton (such as tuna) and
micronekton by rapidly yielding large amounts of data on a controlled
grid with relatively lower demands on resources. To help in the identification
of acoustic tuna signatures, acoustic tracking and longlining methods
will be utilized. Samples of organisms from the strong acoustic scattering
layers will be taken by net trawls to ground-truth the acoustics data
and help in micronekton composition and biomass estimates. A bioacoustician
with experience in the use of acoustic surveys for tuna biomass estimation
will be participating in the project to aid in the development of the
technique.
OBJECTIVES
1. Estimate bigeye tuna and micronekton biomass at Cross seamount
2. Study the distribution, composition, and movement patterns of micronekton
specific to the seamount environment
3. Study the distribution and movement patterns of bigeye tuna aggregations
and individual bigeye behavior relative to that of the physical dynamics
and micronekton distribution
PROPOSED ACTIVITIES
Proposed activities for this project will consist of two oceanographic
cruises at Cross seamount on board the NOAA ship Oscar Elton Sette,
taking place during two consecutive years. During the proposed cruises,
evenly spaced acoustic transects will be conducted over a predetermined
grid covering the plateau and flanks of Cross seamount, with physical
and biological sampling stations spaced evenly along the transect lines.
During some of the surveys, individual bigeye tuna, equipped with acoustic
tags, will be actively followed by Opah, a tracking vessel provided
by the Hawaii Institute of Marine Biology (HIMB).
Year 1
funding for this 2-year project estimated to be available mid-2006.
References:
•
Adam, M.S., Sibert, J., Itano, D. and Holland, K. (2003) Dynamics of
bigeye (Thunnus obesus) and yellowfin (T. albacares) tuna in Hawaii’s
pelagic fisheries: analysis of tagging data with a bulk transfer model
incorporating size-specific attrition. Fish. Bull. 101:215–228.
• Holland,
K.N & Grubbs, R.D. (In
press) Tunas and Seamounts. In: Seamounts: Biology, fisheries and management
T. Pitcher and T. Morato (eds) Blackwell Press.
•
Josse, E., Bach, P. & Dagorn, L. (1998) Simultaneous observations
of tuna movements and their prey by sonic tracking and acoustic surveys.
Hydrobiologia 371/372:61–69.
•
Sibert, J., Holland, K. & Itano, D. (2000) Exchange rates of yellowfin
and bigeye tunas and fishery interaction between Cross seamount and
near-shore FADs in Hawaii. Aquat. Liv. Res. 13:225– 232.
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