Projected changes to ocean food
webs and oceanic fisheries
Based on……..
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
Image: Marc Taquet, FADIO, IRD/IFREMER
Tuna food web
Food webs are complex
Five oceanic provinces
Five oceanic provinces
• Warm pool
Normal
El Niño
Five oceanic provinces
• North and South Gyres (case 3) and equatorial
divergence (case 4)
Impact of climate change
• Surface
area of the
provinces
↘ of rich equatorial divergence
↗ of poorer gyres and warm
Impact of climate change
present
future
• Exchanges
between
deep rich
water and
surface
poorer
waters
↘ of nutrients reaching the surface where
photosynthesis can occur
3. The impact of climate change
• Effect on phytoplankton and zooplankton
present
2050
2100
2035
↘ of phytoplankton and zooplankton
3. The impact of climate change
Image: Valerie Allain, SPC
• Effect on micronekton
↘ of micronekton
Now, turning to tuna
Tuna habitat – temperature
• Each tuna species has evolved with a preferred
range in temperature
• Impacts vertical &
horizontal distribution
(habitat and food) &
reproduction location
and timing
Species
Skipjack
Yellowfin
Bigeye
Albacore
Sth. bluefin
Temperature
(°C)
20-29
20-30
13-27
15-21
17-20
Range of sea surface temperature
with substantial catches
Source: Sund et al. (1981)
Tuna habitat – oxygen
Sensitive to combined effects of
SST + O2
Less tolerant to
low values
Estimated lower lethal oxygen
Species
Fork length Lower lethal O2
(cm)
levels (ml l-1)
Skipjack
50
1.87
Albacore
50
1.23
Yellowfin
50
1.14
Bigeye
50
0.40
Skipjack
Albacore
Yellowfin
Bigeye
Most tolerant
to low values
Tuna habitat – oxygen
+
0
0m
100 m Well oxygenated
Albacore
500 m
Skipjack
Yellowfin
Low oxygen
Bigeye
Typical
vertical O2
profile
Change in subsurface may have more
impact on low oxygen tolerant species
Better understanding of oceanography =
better expected projections
Skipjack tuna
Samoa +7%
Samoa +10%
Unexploited
Fishing effort x 1.5
Albacore projection
2050
2000
Adult biomass
Larval density
2000
No change
in O2
With modelled O2
2050
Sensative to O2 hence
distribution changes
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
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
Resolution 2°
Resolution 1°
Resolution 0.25 °