Trophic
Structure and Tuna Movements in the Cold Tongue-Warm Pool Pelagic Ecosystem
of the Equatorial Pacific
Progress
Reports (PDF): FY 2008,
FY 2007, FY
2006, FY 2005, FY
2004, FY 2003
Project Overview
Recent modeling (Lehodey, 2001) suggests that tuna productivity
in the western and central Pacific Ocean is tied to upwelling along the
equator in the central and eastern Pacific. Project researchers propose
to test this hypothesis by combining diet analysis, stable isotopic compositions,
food-web modeling, and stable isotope markers to trace tuna movements
and trophic-level variation in the equatorial Pacific. The hypothesis
predicts that tunas which reside near equatorial upwelling fronts feed
at relatively low trophic levels. Opposite trends are expected in equatorial
regions with little upwelling, such as the warm pool of the western Pacific,
where tunas are expected to feed at higher trophic levels and move extensively,
searching for less abundant prey. It is anticipated that the results of
this project will help define ecosytem linkages leading to tuna production
and the effect of climate variability on the systems. This type of information
is important for both fisheries production and ecosystem modeling of the
equatorial Pacific Ocean.
In the
equatorial Pacific Ocean, upwelling extends westward along the equator
in a cold tongue of water from the coast of South America, eventually
encountering a large pool of warmer water in the western Pacific (the
cold tongue-warm pool system). The eastern cold-tongue system is characterized
by high levels of primary production, and the western warm pool by lower
levels of primary production. The largest proportion of the tuna catch
in the Pacific Ocean originates from the warm pool, even though paradoxically
this is a region of low primary productivity. Tuna movement to upwelling
zones at the fringe of the warm pool may be key in resolving this apparent
discrepancy between algal and tuna production (Lehodey, 2001).
Testing how regional variations in primary productivity relate to production
of tunas in the cold tongue-warm pool system is the the subject of this
project.
To provide
for a wide geographic coverage of the equatorial Pacific cold tongue-warm
pool system, a team of scientists from collaborating organizations (see
PI list below) will sample the tunas, associated pelagic fishes and
mammals and their stomach contents using observers onboard tuna fishing
vessels Pacific-wide. Small samples of plankton in areas where tunas
forage will also be collected by specifically-trained observers on fishing
vessels and opportunistically on otherwise-funded scientific cruises.
The main
objectives of this study are:
- To
define the trophic structure of the pelagic ecosystems in the western,
central and eastern parts of the tropical Pacific Ocean.
•Identify the functional groups involved in the ecosystem. Three
main categories can be identified: predators (tunas, sharks, billfishes,
marine mammals), preys (forage fishes and cephalopods), and plankton
(zooplankton and phytoplankton).
•Determine how energy and matter flow through these groups. Two
types of analyses will be used: 1) stomach content analysis to quantify
prey-predator interactions, and 2) analysis based on stable-isotope
ratios to assess trophic position of the different functional groups
(δ15N analysis) and to trace how different sources
of primary production, related to upwelling and other environmental
factors, are important in supporting these groups (δ13C
analysis). The biodynamic modeling tool, Ecopath with Ecosim (EwE),
will be used to represent the trophic flows between the ecosystem
components of each region. Ecopath will provide a framework
for the construction of mass-balance models of ecosystems and Ecosim
provides a tool to explore hypothesized changes in production by means
of dynamic simulation. By utilizing Ecopath to translate diet
and stable-isotope data into estimates of trophic level for the system
components, researchers will be able to define the trophic structure
of the systems. This EwE-based modeling, the first for the
pelagic western Pacific region, will benefit from recently developed
EwE models for the eastern and central Pacific Ocean regions
(Olson and Galván-Magaña, in press and Cox
et al, submitted).
- To
establish an isotope-derived (upwelling-related) biogeography of the
pelagic tropical Pacific ecosystems.
•Conduct an extensive sampling program over much of the tropical
Pacific Ocean to establish a biogeography of the major ecosystem
linkages to the tunas, and the carbon and nitrogen stable-isotope
composition of the different component groups. Through this isotopic
cartography, researchers will characterize the trophic structures
in different production regimes contrasting low-productivity zones
(the warm pool) vs. high productivity upwelling zones (the cold tongue).
Researchers will use δ15N isotope values to estimate
the trophic level occupied by the tunas, other predators, their forage
prey species, and plankton. They will use δ13C values
to identify different sources of primary production, and to distinguish
rapid production associated with upwelling. The combination of δ15N
and δ13C will serve to map different regions of primary
and secondary production in the Pacific Ocean.
- To
characterize large-scale tuna movements related to upwelling regions
along the equator.
•The biogeography of the food-webs and isotope distinctions between
different geographic areas in the Pacific will form the basis for
identifying natural isotope tags for studying tuna movements.
Isotope ratios of the fish will serve as internal chemical tags that
are characteristic of areas where they are living. Discrepancy between
the isotope ratio of a mobile predator and prey residing in the immediate
area, after accounting for trophic-level enrichment (Peterson and
Fry, 1987), would imply that this predator is not a resident of
the area where captured. A more refined view of tuna movements, one
that estimates the number of days since diet switching and out migration
from a feeding area, might be acquired by comparing tissues and compounds
with different turnover rates, e.g., fast-turnover liver and slow-turnover
muscle tissues. Diet-change experiments on tunas held in captivity
should allow researchers to measure differences in isotope composition
between slow- and fast-turnover tissues when the diet is switched
from a low-isotope to a high/augmented isotope diet. Researchers will
work with the Oceanic Institute (Oahu, Hawaii), to produce doubly
labeled ( δ13C and δ15N) shrimp
and include isotopic analysis of individual fatty acids to obtain
multiple markers of diet switching from the liver, muscle and fatty
acids.
Sampling
and analysis
Using a specific protocol designed for this research study, participating
fishery program observers (through the SPC and IATTC) will gather stomach,
liver and muscle samples from target fishes (tunas) and bycatch species
on tuna fishing boats (purse-seiners and longline) in the western, central
and eastern Pacific Ocean. Prey species (e.g., cephalopods), zooplankton,
phytoplankton, and suspended organic matter will also be collected from
the western and central Pacific. Samples from the western and central
Pacific will be analyzed at SPC Noumea and samples from the eastern
Pacific will be analyzed at CICIMAR, La Paz, Baja California Sur.
Isotope analysis will be conducted on muscle, liver and lipids samples
to determine overall trophic level and whether there is isotopic equilibrium
between these tissues. If there is equilibrium and tissues reflect the
isotope signatures of the local prey after taking into account normal
trophic-level enrichment, then the animals are considered "resident".
For other cases tuna movement between different regions would be inferred.
Researchers will also investigate the transfer of carbon by means of
fatty acid profiles and compound specific stable isotope ratios. Isotopic
analyses of essential fatty acids should reflect their algal source,
and thus provide a natural tag for tracking foraging habits of tuna.
Samples will be analyzed in the stable isotope biogeochemical laboratory
at the University of Hawaii.
Year 1
funding for this 3-year project to be awarded January 2003.
Literature
cited:
•Cox, S.P. et al, submitted. Reconstructing ecosystem dynamics
in the Central Pacific Ocean, 1952-1998.
•Lehodey, P. 2001. The pelagic ecosystem of the tropical Pacific
Ocean: Dynamic spatial modelling and spatial consequences of ENSO. Progress
in Oceanography, 49: 439-468.
•Olson, R. J. and Galván-Magaña, F., in press. Food
habits and consumption rates of common dolphinfish (Coryphaena hippurus)
in the eastern Pacific Ocean. Fishery Bulletin U.S., 100(2):279-298.
•Peterson, B. and Fry B., 1987. Stable isotopes in ecosystem studies.
Annual Review in Ecological Systems, 18: 293-320.
•Popp B., et al, 1998. Effect of phytoplankton cell geometry on
carbon isotopic fractionation. Geochimica and Cosmochimica Acta,
62: 69-77.
•Popp, B., et al, 1999. Controls on the carbon isotopic composition
of Southern Ocean phytoplankton. Global Biogeochemical Cycles,
13:827-843.
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