Projected changes to ocean food webs and oceanic

Projected changes to ocean food webs and oceanic fisheries Solomon Islands Government

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 • 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 • Reduced exchange between deep, nutrient-rich water and surface waters future

Impact of climate change • Effect on phytoplankton and zooplankton Present present 2035 2050 2100

Impact of climate change Image: Valerie Allain, SPC • Effect on micronekton

Now, turning to tuna!

Tuna habitat – temperature • Each tuna species has evolved with a preferred range in temperature • This preference affects: § vertical & horizontal distribution of species § where and when tuna spawn Range of sea surface temperature with substantial catches Species Temperature (°C) Skipjack 20 -29 Yellowfin 20 -30 Bigeye 13 -27 Source: Sund et al. (1981)

Tuna habitat – oxygen Estimated lower lethal oxygen concentration Species Length (cm) Lower lethal O 2 levels (ml l-1) Skipjack 50 1. 87 Yellowfin 50 1. 14 Bigeye 50 0. 40 Less tolerant to low O 2 levels Skipjack Yellowfin Bigeye Most tolerant to low O 2 levels

Tuna habitat – oxygen + 0 0 m 100 m Well oxygenated Skipjack Yellowfin 500 m Low oxygen Bigeye Typical vertical O 2 profile

Skipjack tuna Solomon Is = -4 Solomon Is = -3 Unexploited Fishing effort x 1. 5

Bigeye projection 2050 2000 Adult biomass Larval density 2000 2050 Good fishing grounds could be displaced further eastward

Conclusions • There is still uncertainty about impacts of climate change on tuna • 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 Resolution 2° Resolution 1° • Better projections can be achieved using an ensemble of models Resolution 0. 25 °