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MEPS 549:9-25 (2016)  -  DOI:

Finding the way to the top: how the composition of oceanic mid-trophic micronekton groups determines apex predator biomass in the central North Pacific

C. Anela Choy1,*, Colette C. C. Wabnitz2, Mariska Weijerman3, Phoebe A. Woodworth-Jefcoats4,5, Jeffrey J. Polovina4

1Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039, USA
2Institute for the Oceans and Fisheries, University of British Columbia, 2202 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada
3Joint Institute for Marine and Atmospheric Research, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, Hawaii 96822, USA
4NOAA Pacific Islands Fisheries Science Center, 1845 Wasp Boulevard, Honolulu, Hawaii 96818, USA
5Marine Biology Graduate Program, University of Hawaii at Manoa, 2525 Correa Road, Honolulu, Hawaii 96822, USA
*Corresponding author:

ABSTRACT: We updated and expanded a model of the pelagic ecosystem for the area of the central North Pacific occupied by the Hawaii-based longline fishery. Specifically, results from the most recent diet studies were used to expand the representation of the lesser-known non-target fish species (e.g. lancetfish, opah, snake mackerel) and 9 mid-trophic micronekton functional groups. The model framework Ecopath with Ecosim was used to construct an ecosystem energy budget and to examine how changes in the various micronekton groups impact apex predator biomass. Model results indicate that while micronekton fishes represented approximately 54% of micronekton biomass, they accounted for only 28% of the micronekton production. By contrast, crustaceans represented 24% of the biomass and accounted for 44% of production. Simulated ecosystem changes resulting from changes to micronekton groups demonstrated that crustaceans and mollusks are the most important direct trophic pathways to the top of the food web. Other groups appear to comprise relatively inefficient pathways or ‘trophic dead-ends’ that are loosely coupled to the top of the food web (e.g. gelatinous animals), such that biomass declines in these functional groups resulted in increased biomass at the highest trophic levels by increasing energy flow through more efficient pathways. Overall, simulated declines in the micronekton groups resulted in small changes in biomass at the very top of the food web, suggesting that this ecosystem is relatively ecologically resilient with diverse food web pathways. However, further understanding of how sensitive micronekton are to changes in ocean chemistry and temperature resulting from climate change is needed to fully evaluate and predict potential ecosystem changes.

KEY WORDS: Forage species · Hawaii longline fishery · North Pacific Subtropical Gyre · Ecopath with Ecosim · Ecosystem modeling · Climate change · Marine food webs

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Cite this article as: Choy CA, Wabnitz CCC, Weijerman M, Woodworth-Jefcoats PA, Polovina JJ (2016) Finding the way to the top: how the composition of oceanic mid-trophic micronekton groups determines apex predator biomass in the central North Pacific. Mar Ecol Prog Ser 549:9-25.

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