Tagged tuna graphic by Nancy Hulbirt, SOEST Illustration.

PFRP Biology Projects

Examining Pelagic Food Webs using Multiple Chemical Tracers

Progress Reports: FY 2012, FY 2011, FY 2010

Summary

Middle trophic level organisms (namely macroplankton and micronekton) are the decisive link between primary producers and top predators as well as food items of many commercially important pelagic predators. However, the trophic dynamics of many macroplankton and micronekton are poorly known. Consequently, fishery models and ecosystem-based management decisions would benefit greatly from detailed information on the trophic dynamics of these organisms in the pelagic environment. We propose a three-year project that utilizes multiple chemical analyses (bulk stable isotopes, compound specific isotopes, lipid biomarkers, and mercury concentrations) in conjunction with supplementary stomach content analyses to characterize the pelagic food web, incorporating organisms from primary producers to top predators in waters surrounding the Hawaiian Islands. Results from this project will increase our understanding of trophic connectivity between the forage base and top predators and further elucidate the structure and variability of pelagic food webs, which is necessary information for ecosystem-based managers and ecosystem modelers.

Objectives

The main objective of this research is to use chemical and isotopic signatures and stomach content analyses to construct a comprehensive food web of the pelagic ecosystem around Hawaii. This objective will be met by sampling a diversity of animals, including mesozooplankton, macroplankton, micronekton, and nekton. The samples will be analyzed for bulk stable isotopes, CSIA, lipid biomarkers, and total mercury content. These data will be augmented by stomach content analysis of key species and chemical data already gathered by a variety of researchers.

Specifically, isotopic and chemical analyses and stomach contents of animals will be used to evaluate a) variation in diet between commercially important top predators and amongst major taxonomic and ecological groups of middle trophic level pelagic animals, and b) the trophic connections between epipelagic, mesopelagic, and bathypelagic habitats and communities.

We will interpret observed chemical and isotopic variations amongst predators and mid-trophic level pelagic animals in light of variations in type of prey, depth of forage, and predator size. The first criterion provides the information on non-target species needed to fill critical gaps in ecosystem models. The second criterion is important for understanding complexity in food webs related to ontogenetic shifts in diet and bioaccumulation of mercury. Preliminary data offer evidence that depth-related isotopic and trace metal signals can propagate up the food web, at least to middle trophic levels. The ability to draw concrete conclusions about the trophic connectivity of epi-, meso-, and bathypelagic animal assemblages with depth in the open ocean however requires a better depth gradient. Although results of Choy et al. (2009) suggest that vertical differences in foraging behaviors over the lifetime of a pelagic predator may be directly responsible for total mercury burdens, significant questions still remain. Choy et al. (2009) and others have shown that Hg concentration in fish increase with fish size. Organism size is a variable ecologically linked to numerous other parameters, therefore, such as in the case of mercury levels in fish, a comprehensive evaluation of how size can influence food web complexity and biogeochemical cycling is necessary.

Lastly, we propose to fill critical gaps in knowledge of food habits of key species. Polovina et al. (pers. comm. 2009) recently have shown that catch rates for apex predators have been declining, while catch rates for deeper, lower trophic level fishes (e.g., gempylids, escolars and sickle pomfrets) have been increasing over the past decade, suggesting a trophic cascade potentially related to commercial fishing. We propose to examine the stomach contents of these key species, whose trophic status has not been studied. Some diet work exists on other key mid-water animal groups and we intend to rely predominantly on these published studies (e.g., myctophids (Clarke 1973), ommastrephid squids (Parry 2006), stomiatoids (Clarke 1974), hatchetfishes (Hopkins and Baird 1973)). Due to their small size and the fact that they macerate their food, we will depend upon our chemical analyses to define trophic connections of pelagic crustaceans and smaller cephalopods.

Funding for this project to be available late 2009.

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References

Choy, C.A., Popp, B.N., and J.C. Drazen. 2009. The influence of depth on mercury levels in pelagic fishes and their prey. Proceedings of the National Academy of Sciences USA, in revision.

Clarke, T.A. 1973. Some aspects of the ecology of lanternfishes (Myctophidae) in the Pacific Ocean near Hawaii. Fish Bull 71(2): 401-433.

Clarke, T.A. 1974. Some aspects of the ecology of stomiatoid fishes in the Pacific Ocean near Hawaii. Fish Bull 72(2): 337-351.

Hopkins T.L. and R.C. Baird. 1973. Diet of the hatchetfish Sternoptyx diaphana. Mar Biol 21: 34-46.

Parry M. 2006. Feeding behavior of two ommastrephid squids Ommastrephes bartramii and Sthenoteuthis oualaniensis off Hawaii. Mar Ecol Prog Ser 318: 229-235.

Principal Investigators

Dr. Jeffrey C. Drazen
Dept. of Oceanography
University of Hawaii
1000 Pope Road, MSB 205
Honolulu, Hawaii 96822 USA
Phone (808) 956-6567
FAX (808) 956-9516
email: jdrazen@hawaii.edu

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

Additional Collaborators:

C. Anela Choy, Dept. of Oceanography, UH Manoa
Peter D. Nichols, CSIRO, Australia
Charles F. Phleger, Lipids PMI, Australia
Robert Olson, I-ATTC, La Jolla, USA

 

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