Projected changes to ocean food webs and oceanic fisheries

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Projected changes to ocean food webs and oceanic
fisheries
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Based on..
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Outline

• Food webs for tuna
• Differences in food webs among provinces of the
  Pacific Ocean
• Effects of CC on provinces and their food webs
• Sensitivity of tuna habitats to oceanic variables
• Effects of climate change on tuna stocks

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(No Transcript)
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Tuna food web
Food webs are complex
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Five oceanic provinces
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Five oceanic provinces

• Warm pool

Normal El Niño
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Five oceanic provinces

• North and South Gyres (case 3) and equatorial
  divergence (case 4)

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Impact of climate change

• Surface area of the provinces

? of rich equatorial divergence ? of poorer gyres
and warm pool
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Impact of climate change
present
future

• Exchanges between deep rich water and surface
  poorer waters

? of nutrients reaching the surface where
photosynthesis can occur
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3. The impact of climate change

• Effect on phytoplankton and zooplankton

? of phytoplankton and zooplankton
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3. The impact of climate change

• Effect on micronekton

Image Valerie Allain, SPC
? of micronekton
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Now, turning to tuna
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Tuna habitat temperature

• Each tuna species has evolved with a preferred
  range in temperature

Species Temperature (C)
Skipjack 20-29
Yellowfin 20-30
Bigeye 13-27
Albacore 15-21
Sth. bluefin 17-20

• Impacts vertical horizontal distribution
  (habitat and food) reproduction location and
  timing

Range of sea surface temperature with substantial
catches
Source Sund et al. (1981)
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Tuna habitat oxygen
Sensitive to combined effects of SST
O2
Less tolerant to low values
Estimated lower lethal oxygen
Skipjack Albacore Yellowfin Bigeye
Species Fork length (cm) Lower lethal O2 levels (ml l-1)
Skipjack 50 1.87
Albacore 50 1.23
Yellowfin 50 1.14
Bigeye 50 0.40
Most tolerant to low values
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Tuna habitat oxygen

0
0 m
100 m
Well oxygenated
Low oxygen
500 m
Typical vertical O2 profile
Change in subsurface may have more impact on low
oxygen tolerant species
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Better understanding of oceanography better
expected projections
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Skipjack tuna
Samoa 7
Samoa 10
Unexploited
Fishing effort x 1.5
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Albacore projection
2000
2000
Adult biomass
Larval density
2050
2050
No change in O2
Sensative to O2 hence distribution changes
With modelled O2
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Conclusions

• There is still uncertainty about impacts of
  climate change
• Fishing has a strong impact and will continue to
  be a major driver of stocks

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Conclusions

• Improved resolutions of SEAPODYM model are needed
  to update these preliminary results
• Better projections of key ocean variables for
  tuna can be achieved using an ensemble of models