Assessment
of the Impacts of Mesoscale Oceanographic Features on the Forage Base
for Oceanic Predators
Progress
reports (PDF): FY
2008, FY 2007
INTRODUCTION
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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•
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