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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:

  1. 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).

  2. 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.

  3. 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.

Principal Investigators:

Dr. Valerie Allain
Secretariat of the Pacific Community (SPC)
Noumea Cedex
New Caledonia
Phone (687) 26-20-00
FAX (687) 26-38-18
email: valeriea@spc.int

Dr. Robert Olson
Inter-American Tropical Tuna Commission (IATTC)
8604 La Jolla Shores Drive
La Jolla, CA 92037 USA
Phone (858) 546-7160
FAX (858) 546-7133
email: rolson@iattc.org

Dr. Felipe Galván-Magaña
Postal 592
La Paz, Baja California Sur
MEXICO C.P. 23000
Phone (612) 12-25344
FAX (612) 12-25322
email: fgalvan@redipn.ipn.mx


Dr. Brian Popp
University of Hawaii
Dept. of Geology and Geophysics
1680 East-West Road, POST 720B
Honolulu, Hawaii 96822 USA
Phone (808) 956-6206
FAX (808) 956-5512
email: popp@soest.hawaii.edu

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This page updated August 7, 2008