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Hawaiian shrimp and prawn biology_ and their role in

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					  Hawaiian shrimp and prawn
biology, and their role in vertical
           carbon flux
           Yasha Podeswa
     What are shrimp and prawns?
• Both crustaceans from the Order
  Decapoda
• Shrimp:
   – Suborder Pleocyemata, Infraorder
      Caridea
• Prawns:
   – Suborder Dendrobranchiata
  Why study pelagic Hawaiian shrimp
            and prawns?
• Biology of most local shrimp and prawn species
  undescribed
   – Morphometrics (size/shape), sex ratios, fecundity
     (reproductive rate of an organism/population),
     diet/position in food web
• Little commercial harvest, but may be harvested in the
  future
   – Pre-harvest data important but often unavailable
   – Understand the “baseline”
• Locally very abundant, could play a large role in vertical
  carbon flux through active vertical migrations
           Goals of my study
• Describe the biology of all
  pelagic shrimp and prawns in
  the open ocean near Hawaii,
  including surface, mesopelagic,
  and migratory populations
• Estimate the contribution of
  migratory shrimp and prawns to
  vertical carbon flux
 Why has nobody studied this before?
• Fish generally well studied
• Zooplankton generally well
  studied
• Micronekton generally not
  well studied
  – Small but bigger than
    plankton, free swimmers
  – Avoid plankton nets, too
    small to be caught by most
    fishing nets
                             Methods
• Used 3 different sampling gears
  with different mesh sizes                              0
                                                                   Nightime oblique tows (1hr)

    – Cobb trawl (10 mm)                                15

    – Isaacs-Kidd Midwater Trawl (5 mm)                 30
                                                        45
    – Hokkaido University Frame Trawl (3




                                           Depth (m)
                                                        60                                            HUFT
      mm)                                               75                                            IKMT
                                                        90                                            Cobb Trawl
• 58 casts, sampled at day and night,                  105
  sampling both surface scattering                     120

  layer (roughly top 100 m) and deep                   135
                                                       150
  scattering layer (roughly top 500 m)
                                                             Daytime horizontal tows (1hr at depth)
                                                        0


                                                       150
                                           Depth (m)

                                                       300                                            HUFT
                                                                                                      IKMT
                                                       450                                            Cobb Trawl


                                                       600


                                                       750
                          Methods
• Classify all shrimp and prawns in
  the samples
   – Many samples over 50% shrimp
     and prawns by volume
   – 32 species identified from 8
     families
• Measure carapace length, total
  length, wet weight and dry weight
• Sex individuals based on appendix
  masculina (for shrimp) and petasma
  (for prawns)
• Determine fecundity through
  counting eggs and oocites at
  various stages of development
• Determine diet/food web location
  through gut content analysis and
  stable isotope analysis
         Stable isotope analysis
• Gut content analysis
  – Underestimates easily digested prey
  – Often very difficult to identify prey to species level
  – Only contains very recent meals
  – Hard to get quantitative data
• Stable isotope analysis
  – Quantitative data
  – No bias towards hard to digest prey
  – Integrates diet over a longer timescale
         Stable isotope analysis
• “You are what you eat”
  – Predator should have similar isotopic makeup of
    prey
• Measure the amount of heavy isotopes of
  carbon (13C) and nitrogen (15N) in muscle
  tissue, calculate the ratio of the heavy isotope
  to the lighter, more common isotope

                13C 12C
     d13C=   (C13    12C
                           sample



                         standard
                                    )
                                    1   1000
         Stable isotope analysis
• d13C and d15N measured
  through mass
  spectrometry
• The predator’s d15N
  signature will generally
  be about 3.2 ‰ greater
  than its prey’s
• The predator’s d13C
  signature will be very
  similar to the prey’s,
  only about 1 ‰ greater
         Stable isotope analysis
• Why the enrichment?
  – Lighter isotopes (12C and 14N) are preferentially
    metabolised
  – Heavier isotopes (13C and 15N) are retained, and are
    thus enriched in the predator’s tissues
  – Small enrichments from prey to predator in d13C allow
    the source of primary production to be easily
    identified
  – Larger enrichments from prey to predator in d15N
    allow trophic levels to be distinguished more
    accurately
        Stable isotope analysis
• Simple food chains can
  be identified visually
• Complex food webs
  can be estimated using
  mixing models
             Vertical carbon flux
• The oceans have absorbed about 48% of total
  fossil fuel and cement manufacturing CO2
  emissions since the beginning of the industrial
  revolution
• The ocean’s carbon reservoir dwarf’s the carbon
  reservoir in the atmosphere
  –   Over 1,000 Gt C in surface waters
  –   Roughly 38,000 Gt C in mid and deep waters
  –   Roughly 78,000,000 Gt C in ocean sediments
  –   Roughly 600 Gt C in the pre-industrail atmosphere
              Vertical carbon flux
• The ocean is density stratified
   – Not well mixed outside of the surface layer (upper few
     meters to upper few hundred meters)
• Mixed layer shallow in the tropics, simple turbulence
  not responsible for much vertical carbon flux
• Carbon is sequestered in the ocean primarily through
  the “solubility pump” and the “biological pump”
                Vertical carbon flux
• Solubility pump
    – CO2 more soluble in cold water than warm water
    – More dissolved CO2 in cold, high latitude waters than warm, low
      latitude waters
    – Deep water forms at high latitudes, thus pumping high CO2 water from
      the surface to the deep ocean
• Not very significant in the tropics, not a down welling region
                Vertical carbon flux
• Biological pump
   – CO2 fixed into organic C
     through photosynthesis
   – Sinks as “marine snow,”
     aggregates of dead or
     dying phytoplankton,
     dead or dying
     zooplankton, faeces
     and mucus
   – The carcases of larger
     animals (such as whale
     carcasses) can also
     contribute to vertical
     carbon flux
   – Active vertical
     migration can also be
     important
             Vertical migration
• Virtually all
  zooplankton and
  micronekton in the
  oceans perform diel
  vertical migrations
  – Spend the night at
    the surface, and the
    day at depth
  – Some also perform
    seasonal vertical
    migrations
            Vertical migration
• Driving force behind vertical migrations still
  somewhat debated, but generally seen to be
  based on the following factors:
  – Surface food quality and quantity
  – Visual predator avoidance
  – Metabolic gains
  – Avoidance of UV damage
        Active carbon transport
• Some carbon is transported
  downwards through formation of
  faeces at depth
  – Short gut clearance times for many
    zooplankton make this of minimal
    significance, but for larger shrimp
    and prawns it could be more
    significant
• Much carbon is also transported
  through respiration of CO2 and
  excretion of DOC
           Active carbon transport
• Previous studies tend to
  focus on smaller migrators,
  especially krill
   – Generally make up the bulk
     of migratory biomass, but
     pelagic shrimp and prawns
     much more abundant in this
     ecosystem
   – Krill are generally shallow
     migrators (top 100 m),
     shrimp and prawns migrate
     much deeper, could
     contribute much more to
     vertical carbon flux if present
     in similar biomass
        Active carbon transport
• Previous studies focusing on krill and other
  zooplankton migrators in the upper 100 m have
  shown them to contribute significantly to vertical
  carbon flux, ranging from 6 – 34% of POC flux
• Fluxes due to the less abundant micronekton
  have generally been ignored, but may be very
  locally significant in this ecosystem due to high
  relative biomass, deeper migrations, and longer
  gut clearance times
        Active carbon transport
• My preliminary findings show that many
  Hawaiian shrimp and prawns migrate to deep
  depths (to around 500 m), and only a fraction
  of the population migrates to the surface each
  night
  – Thus spend numerous days/hours at depth for
    every day/hour at the surface, facilitating carbon
    transport
              Wrapping up . . .
• Understanding the role of pelagic shrimp and
  prawns in the ecosystem through studying their
  biology, diet, and their contribution to vertical
  carbon flux should aid in any management
  decisions in the future, especially should this
  become a commercial fishery

				
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posted:12/6/2012
language:English
pages:23