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Assessment of the Impacts of Mesoscale Oceanographic Features on the Forage Base for Oceanic Predators

Progress reports (PDF): FY 2008, FY 2007

The main goals of this project are:

1. To assess the impact of each mesoscale feature on the biomass and abundance of the micronekton.
2. To assess the impact of Cross seamount on micronekton community composition.
3. To characterize the micronekton composition in American Samoa.
4. To assess whether each mesoscale feature affects the vertical migration patterns of the micronekton.
5. To compare both acoustic and trawl estimates of biomass in each region to provide acoustic "groundtruthing."

The oceanic mesoscale includes dynamic features such as fronts and eddies that are ubiquitous features of ocean circulation and shallow seamounts that can alter local circulation patterns. These features can contribute significantly to patterning in pelagic ecosystems affecting organismal abundance, distribution, productivity, and trophic structure. These features act to concentrate large fishes such as tuna and cetaceans (Davis et al. 2002; Murphy and Shomura 1972; Olson et al. 1994; Owen 1981; Seki et al. 2002; Sugimoto and Tameishi 1992). Fisherman have long recognized that catches are high at fronts, eddies and seamounts (Olson et al. 1994; Seki et al. 2002; Sugimoto and Tameishi 1992). It is thought that the presence of aggregated food resources is the driving factor behind the concentrations of large nektonic organisms (Davis et al. 2002; Fiedler and Bernard 1987; Murphy and Shomura 1972; Olson et al. 1994; Owen 1981; Sinclair and Stabeno 2002; Sugimoto and Tameishi 1992). Numerous studies have attempted to link variability in catch and the ecology of commercially important species such as tuna and billfish to these features. However, a lack of data on their forage organisms has often led to ambiguous results (Seki et al. 2002). The oceanic micronekton consists of small (~2-20cm) fishes, crustaceans and cephalopods and it is the trophic link between the zooplankton and the top predators in the ecosystem (Brodeur and Yamamura 2005). Several commercially important pelagic fishes such as swordfish, bigeye tuna, and albacore tuna feed directly on micronekton, particularly mesopelagic micronekton (Bertrand et al. 2002; Dagorn et al. 2000; Markaida and Sosa-Nishizaki 1998; Palko et al. 1981; Tsarin 1997). Knowledge of the processes affecting micronekton patchiness/distribution would be of great value in estimating the distribution and yield of large oceanic fish stocks affected by patterning of food supply. In order to understand the effects of mesoscale eddies on the entire pelagic community, the changes in the abundance, biomass, diversity and distribution of the micronekton must be investigated.

Two very different mesoscale oceanographic features are of current interest to the Pacific pelagic fishery community - seamounts and midocean eddies. The Cross Seamount is a site of enhanced bigeye tuna CPUE (Holland et al. 1999) to the southwest of the main Hawaiian islands. It is generally though that the seamount somehow generates higher abundances of micronekton forage for deeper diving tunas. Bigeye tuna do show fuller stomachs at Cross seamount compared to other locations (Grubbs et al. 2002).

The impact of the seamount on the tuna forage base is not clear but they have the potential to affect micronekton biomass, abundance, community composition, and vertical migration patterns. Seamounts can interrupt the local flow field leading to local upwelling or the formation of cyclical flow fields (Taylor columns) which can result in enhanced primary production and enhanced local zooplankton production (Boehlert and Genin 1987; Rogers 1994). However, it is thought that the increased abundance and biomass of micronekton over seamounts are likely the result of these animals being advected over the seamount during the night and then compacting into denser shallower layers when they encounter the top of the seamount during their daytime descent (Fock et al. 2002; Rogers 1994). In addition to compaction during vertical migration, it is possible that increased abundances of micronekton near seamounts are the result of a local fauna, a change in species composition.

Another mesoscale feature which could impact the distribution and composition of tuna forage are midocean eddies. These are known to generate localized upwelling which enhances primary production (Bidigare et al. 2003; Seki et al. 2001). The impact of these features on the micronekton is poorly understood. They are not generally long lived enough to result in local production at this trophic level but they may aggregate tuna food resources. Midocean eddies are a common feature of albacore fishing grounds in the EEZ of American Samoa. The longline fishery in this area has experienced recent and dramatic expansion, targeting albacore tuna in an oceanographically complex region. The primary fishing grounds lie in the path of the westward flowing South Equatorial Current (SEC) and in the south of the eastward flowing, seasonal South Equatorial Counter Current (SECC). From March to April the SECC intensifies and generates shear as it passes the SEC resulting in a seasonal production of midocean eddies which may concentrate micronekton. In the high shear zones of the SECC differing hydrographic conditions and particularly the depth of isolumes (affected by primary productivity) could change the daytime depth of the acoustic scattering layers. So, even though data suggests the highest albacore longline CPUE occurs about 2 months after eddy generation, the causal relationship between the oceanography and catch has not yet been identified (Domokos, unpub. data). Furthermore, the micronekton within the EEZ of American Samoa is poorly sampled. A description of the community for comparison to the acoustic sampling and also to describe the forage for commercially important species in the region is needed. This area lies at an overlap between the central south Pacific and central equatorial Pacific biogeographic provinces each known to contain plankton (Mcgowan 1974) and micronekton (Barnett 1984) assemblages distinct from the other.

Project researchers will investigate the nature and degree of the response of the micronektonic community to these two mesoscale oceanographic features using trawl surveys in conjunction with acoustic surveys. The decision to place the study of both of these features into the same proposal is that previously funded projects have already provided the sample collection (American Samoa oceanographic and tuna tagging studies through PFRP and Cross seamount studies through NMFS-PIFSC and a concurrent collaborative acoustics proposal, lead PI Reka Domokos). During these efforts, trawls were conducted to quantify the abundance, biomass, and community composition of the micronekton alongside acoustic surveys. Cross seamount and control sites were sampled last year and two additional cruises are proposed. Two cruises to American Samoa provided acoustic transects through this region and its complex eddy field. Trawl samples were taken in the high shear edge of an eddy which will allow us to ground truth the comprehensive acoustic data and provide a much needed description of the micronekton from this region.

The work proposed here will allow researchers to characterize the impact of mesoscale features on the micronekton community, an important forage base for deep diving pelagic predators, in a much more comprehensive manner than with the acoustics alone. Analysis of the trawl samples will provide "ground truthing" of the acoustic surveys and allow researchers to assess the composition of acoustic scattering layers. Changes in micronekton biomass and abundance, community composition, and/or vertical distributions could affect the forage base or its accessibility to deep diving pelagic predators and may help to explain their distributions. This project leverages past and present work and stands to make a significant contribution to our knowledge base at a minimum cost. It will also complement published oceanographic, tracking, and diet data to provide a more complete picture of the influence of mesoscale oceanography on commercially important pelagic fish stocks.

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

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Bertrand, A., F. X. Bard, and E. Josse. 2002. Tuna food habits related to the micronekton distribution in French Polynesia. Mar. Biol. 140: 1023-1037.
Bidigare, R. R. and others. 2003. Influence of a cyclonic eddy on microheterotroph biomass and carbon export in the lee of Hawaii. Geophys. Res. Lett. 30: 1318-1321.
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Brodeur, R. D., and O. Yamamura. 2005. Micronekton of the North Pacific, p. 1-115, PICES Scientific Report No. 30.
Dagorn, L., P. Bach, and E. Josse. 2000. Movement patterns of large bigeye tuna (Thunnus obesus) in the open ocean, determined using ultrasonic telemetry. Mar. Biol. 136: 361-371.
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Fock, H., B. Matthiessen, H. Zidowitz, and H. Von Westernhagen. 2002. Diel and habitat-dependent resource utilisation by deep-sea fishes at the Great Meteor seamount: niche overlap and support for the sound scattering layer interception hypothesis. Mar. Ecol. Prog. Ser. 244: 219-233.
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Seki, M. P., R. Lumpkin, and P. Flament. 2002. Hawaii cyclonic eddies and blue Marlin catches: The case study of the 1995 Hawaiian International Billfish Tournament. J. Oceanogr. 58: 739-745.
Seki, M. P., J. J. Polovina, R. E. Brainard, R. R. Bidigare, C. L. Leonard, and D. G. Foley. 2001. Biological enhancement at cyclonic eddies tracked with GOES thermal imagery in Hawaiian waters. Geophys. Res. Lett. 28: 1583-1586.
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Project Investigators:

Dr. Jeffrey Drazen
Dept. of Oceanography
University of Hawaii at Manoa
1000 Pope Road, MSB 606
Honolulu, Hawaii 96821 USA
Phone (808) 956-6567
FAX (808) 956-9516
email: jdrazen@hawaii.edu

Dr. Reka Domokos
National Marine Fisheries Service
Pacific Island Fisheries Science Center
2570 Dole Street
Honolulu, Hawaii 96822 USA
Phone (808) 983-5368
FAX (808) 983-2902
email: Reka.Domokos@noaa.gov


Dr. Jeffrey Polovina
National Marine Fisheries Service
Pacific Island Fisheries Science Center
2570 Dole Street
Honolulu, Hawaii 96822 USA
email: Jeffrey.Polovina@noaa.gov

Dr. Michael Seki
National Marine Fisheries Service
Pacific Island Fisheries Science Center
2570 Dole Street
Honolulu, Hawaii 96822 USA
email: Michael.Seki@noaa.gov

Richard Young
Professor Emeritus
Dept. of Oceanography
University of Hawaii at Manoa
email: ryoung@hawaii.edu
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This page updated August 7, 2008