Projected changes to ocean food webs and oceanic

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 pool

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 + O 2 Less tolerant to low values Estimated lower lethal oxygen Species Fork length Lower lethal O 2 (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 0 m 100 m Well oxygenated Albacore 500 m Skipjack Yellowfin Low oxygen Bigeye Typical vertical O 2 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 O 2 With modelled O 2 2050 Sensative to O 2 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 Resolution 2° • Improved resolutions of SEAPODYM model are needed to update these preliminary results Resolution 1° • Better projections of key ocean variables for tuna can be achieved using an ensemble of models Resolution 0. 25 °