Food-web inferences of stable isotope spatial patterns in copepods and yellowfin tuna in the pelagic eastern Pacific Ocean

Evaluating the impacts of climate and fishing on oceanic ecosystems requires an improved understanding of the trophodynamics of pelagic food webs. Our approach was to examine broad-scale spatial relationships among the stable N isotope values of copepods and yellowfin tuna (Thunnus albacares), and t...

Full description

Saved in:
Bibliographic Details
Published in:Progress in oceanography 2010-07, Vol.86 (1-2), p.124-138
Main Authors: Olson, Robert J, Popp, Brian N, Graham, Brittany S, Lopez-Ibarra, Gladis A, Galvan-Magana, Felipe, Lennert-Cody, Cleridy E, Bocanegra-Castillo, Noemi, Wallsgrove, Natalie J, Gier, Elizabeth, Alatorre-Ramirez, Vanessa, Ballance, Lisa T, Fry, Brian
Format: Article
Language:English
Subjects:
Amino acids
Copepoda
Ecosystems
Estimates
Fishing
Foods
Isotopes
Marine
Mathematical models
Movement
Thunnus albacares
Tunas
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:Evaluating the impacts of climate and fishing on oceanic ecosystems requires an improved understanding of the trophodynamics of pelagic food webs. Our approach was to examine broad-scale spatial relationships among the stable N isotope values of copepods and yellowfin tuna (Thunnus albacares), and to quantify yellowfin tuna trophic status in the food web based on stable-isotope and stomach-contents analyses. Using a generalized additive model fitted to abundance-weighted-average delta 15N values of several omnivorous copepod species, we examined isotopic spatial relationships among yellowfin tuna and copepods. We found a broad-scale, uniform gradient in delta 15N values of copepods increasing from south to north in a region encompassing the eastern Pacific warm pool and parts of several current systems. Over the same region, a similar trend was observed for the delta 15N values in the white muscle of yellowfin tuna caught by the purse-seine fishery, implying limited movement behavior. Assuming the omnivorous copepods represent a proxy for the delta 15N values at the base of the food web, the isotopic difference between these two taxa, " YFT-COP," was interpreted as a trophic-position offset. Yellowfin tuna trophic-position estimates based on their bulk delta 15N values were not significantly different than independent estimates based on stomach contents, but are sensitive to errors in the trophic enrichment factor and the trophic position of copepods. An apparent inshore-offshore, east to west gradient in yellowfin tuna trophic position was corroborated using compound-specific isotope analysis of amino acids conducted on a subset of samples. The gradient was not explained by the distribution of yellowfin tuna of different sizes, by seasonal variability at the base of the food web, or by known ambit distances (i.e. movements). Yellowfin tuna stomach contents did not show a regular inshore-offshore gradient in trophic position during 2003-2005, but the trophic-position estimates based on both methods had similar scales of variability. We conclude that trophic status of yellowfin tuna increased significantly from east to west over the study area based on the spatial pattern of YFT-COP values and the difference between the delta 15N values of glutamic acid and glycine, "trophic" and "source" amino acids, respectively. These results provide improved depictions of trophic links and biomass flows for food-web models, effective tools to evaluate climate and fishin
ISSN:0079-6611
DOI:10.1016/j.pocean.2010.04.026