swordfish PFRP Home > Trophodynamics Projects List

Examining Latitudinal Variation in Food Webs leading to Top Predators in the Pacific Ocean

Progress reports (PDF): FY 2009, FY 2008, FY 2007

(1) Compare the trophic pathways supporting large pelagic fishes from tropical and temperate waters of the Pacific Ocean using existing data on stomach contents of tunas and billfish from these regions
(2) Provide qualitative models of the resulting trophic pathways and identify species and interactions that are key to the system's structure and dynamics

A trophodynamics study (PFRP Project #659559) has demonstrated significant heterogeneity in trophic pathways across the tropical Pacific, associated with either the prevailing mesoscale oceanography or the seabed topography. These results have the potential to not only identify shifts in ecosystem structure (climate change), but also may help to define stock structure of widely distributed tropical tunas. A similar, but more localized study off eastern Australia (Young et al. 2004) is also beginning to reveal differences not only within the region but also in comparison with the study in the tropical Pacific (Olson et al. 2005). Given the proximity of the two studies geographically and the potential migration of tunas between the regions, project researchers will conduct a study that compares the trophodynamics of the pelagic ecosystems of these regions using largely existing data sets. The analysis would combine statistical comparisons with qualitative models to determine similarities or otherwise of the two regions. Such comparisons may offer insight into the ecosystem impacts of potential climate change expressed as ocean warming.

Researchers will compare stomach contents of top predators from tropical and temperate waters of the western, central, and eastern Pacific Ocean to examine latitudinal differences in the trophic pathways of these regions. The results will be used to develop qualitative models (Dambacher et al. 2002) of the trophic flows within each region.

Compilation of data sets
Researchers will examine three recent data sets of predator prey matrices: one from the western tropical Pacific Ocean (10°N to 20°S), one from temperate waters from the same region (20° to 35° S), and one from the eastern Pacific Ocean (20° N to 20° S). Preliminary assessment of these data sets showed existing trophic data for the majority of pelagic fish species found in the three areas, a total of more than 100 taxa). In particular, researchers found ~10 predator species [target species, including yellowfin and bigeye tunas; bycatch species, including dolphin fish, wahoo, skipjack and albacore] for which detailed trophic data was available from all three regions. Other species for which trophic data was available from at least two of the three regions included swordfish and species of Carangidae (jacks).

There are also a series of global environmental data sets accessible from CSIRO Hobart which can be accessed for the relevant temporal and spatial scales. To establish in detail which data to use in the analyses, a PI meeting in Hobart is proposed for September 2006. Because the eventual qualitative models will require an understanding of lower trophic level complexity for the different regions, researchers will also consider available data and literature sources that will be required to fill in these lower trophic levels.

Analysis will initially consist of a series of comparisons using classification and regression tree (CART) analysis, to identify significant groupings of food web structure and the main environmental correlates that can be used to distinguish them. Classification and regression trees are ideally suited for the analysis of complex ecological data (De'ath and Fabricius 2000).

Food web data sets from the different regions will be individually analyzed in terms of their predator-prey links. This will be done in multiple ways, one of which will be to consider only if the species is or is not consumed by a predator, and to ignore its relative importance to the predator's diet. Here analysis of network structure is purely qualitative, and link values from prey to predator equal +1 or 0. Another analysis will consider the proportion, by wet weight, which a prey species represents in the predator's diet, thus weighting more prevalent prey and minimizing the effect of rarely consumed species. Alternately, a blending of the two considers the qualitative network with rare prey items excluded. Analysis of network structure by each of these different means will be used to identify commonalities and differences between the food webs of the different regions.

A novel aspect of this work is the inclusion of additional information into the aggregation algorithm, and thus researchers will explore the influence factors associated with life history, allometry, and habitat (e.g. SST) on system structure. For instance, short-lived species can be expected to respond relatively fast to environmental perturbations, and thus their prey and predators can be expected to experience indirect effects of a perturbation more quickly than prey and predators associated with long-lived species. Comparing foodwebs thus aggregated from each of the different regions can identify structural differences related to responsiveness of the system to environmental change. Similarly, incorporating information on ocean habitat in the analysis of network structure can potentially reveal important patterns of association that are spatially structured.

Year 1 funding for this 2-year project estimated to be available mid-2006.

Dambacher, J.M., H.W. Li and P.A. Rossignol. 2002. Relevance of community structure in assessing indeterminacy of ecological predictions. Ecology 83(5):1372-1385.
De'ath, G., and K.E. Fabricius. 2000. Classification and regression trees: a powerful yet simple technique for ecological data analysis. Ecology 81 (11): 3178-3192.
Olson, R., J. Young, V. Allain, and F. Galván-Magaña. 2005. OFCCP workshop on the application of stable isotopes in pelagic ecosystems, La Paz, B.C.S., Mexico, 31 May-1 June 2004. GLOBEC Newsletter 11 (1): 42-44.
Young, J. W., Hobday, A. J. , Dambacher, J. D. (2004) Determining ecological effects of longline fishing in the Eastern Tuna and Billfish Fishery. FRDC Research Project, 2004/063.

Project Investigators:

Dr. Jock W. Young
Dr. Jeffrey M. Dambacher
CSIRO Marine & Atmospheric Research
P.O. Box 1538
Hobart, Tasmania 7001
Phone (61-3) 62-325360
FAX (61-3) 62-325012
email: jeffrey.dambacher@csiro.au

Dr. Robert Olson
Inter-American Tropical Tuna Commission
8604 La Jolla Shores Drive
La Jolla, CA 92037-1508 USA
Phone (858) 546-7160
FAX (858) 546-7133
email: rolson@iattc.org

Dr. Valerie Allain
Oceanic Fisheries Programme
Secretariat of the Pacific Community
98848 Noumea cedex
Phone (687) 26-20-00/26-54-18
FAX (687) 26-38-18
email: valeriea@spc.int

rainbow horizontal bar

This page updated August 31, 2009