TUNA trophic structure in the Pacific Ocean. PFRP funded project

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1 TUNA trophic structure in the Pacific Ocean Valerie ALLAIN SPC Noumea Brian POPP UH Honolulu Felipe GALVAN CICIMAR La Paz Brittany GRAHAM UH Honolulu Bob OLSON IATTC San Diego Brian FRY LSU Baton Rouge PFRP funded project

2 OBJECTIVE 1 Define the trophic structure of the pelagic ecosystems in the different areas of the tropical Pacific ocean WARM POOL COLD TONGUE WARM POOL warm water low primary productivity TUNA MAXIMUM BIOMASS COLD TONGUE strong divergent equatorial upwelling high primary production

3 SAMPLING OF STOMACH AND MUSCLE SAMPLES IN THE WESTERN, CENTRAL AND EASTERN PACIFIC PURSE SEINE samples 89 cm 76 cm 63 cm 101 cm 58 cm 59 cm 105 cm 100 cm LONGLINE samples

4 PREY DIVERSITY in stomach contents Stomach content of a caught by a LL in French Polynesia

5 PS 460 stomachs (64% empty) 37 preys 18 families 1.4 preys per predator from PS 6.2 preys per predator from LL 75 stomachs (20% empty) 21 preys 13 families 3.6 preys per predator 1032 stomachs (72% empty) 37 preys 16 families 2.2 preys per predator 106 stomachs (64% empty) 13 preys 7 families 2.3 preys per predator 199 stomachs (51% empty) 22 preys 9 families 2.0 preys per predator 212 stomachs (29% empty) 133 preys 63 families 10.3 preys per predator from LL 1.5 preys per predator from PS 270 stomachs (5% empty) 142 preys 64 families 6.9 preys per predator 212 stomachs (4% empty) 125 preys 59 families 6.8 preys per predator LL

6 0% Meso 0% Bathy 0% Bathy 1% MICRO YFT 27% Meso 11% 16% PS 9% 0% - PAPUA - %W - n=17 YFT - PAPUA - %W - n=75 - PAPUA - %W - n=47 PAPUA 2% 25% Meso 4% 2% surface M 1% 14% Bathy 3% 17% surface M 7% Bathy 14% deep M 1% 4% 2% 3% Meso 29% NC FP Meso 4% 1% 7% surface M 4% - NC - %W - n=40 surface M 3% - POLY - %W - n=18 Bathy 1% Bathy 1% 9% 30% Meso 6% 2% 8% 5% Meso 4% YFT - NC - %W - n=109 Bathy 2% YFT - POLY - %W - n=68 surface M 2% 12% 23% surface M 10% deep M 2% surface M 8% - POLY - %W - n=52 Meso 19% Bathy 5% - NC - %W - n=56 Bathy 1% 5% 3% 6% 3% LL 10% 11% 25% Meso 20%

7 GEAR EFFECT on the stomach content PS LL % of empty stomachs high low diversity in prey items low high number of different prey items per stomach low high species categories in the diet few many Samples from different gears give a different picture of the diet as described by stomach content surface 200 m YFT PS LL LL and PS catch fish -at different times in the 24h-cycle -at different depths in the tuna vertical habitat -of different sizes in the tuna population 400 m 24 hour-cycle COMPARE SAMPLES COLLECTED BY SAME GEARS

8 COMPARING THE PREY COMPOSITION OF THE DIET Vertical distribution and migration of forage species NIGHT DAY surface EPI oceanic reef/fad MIGRANT MIGRANT HIGHLY MIGRANT 200 m 400 m

9 COMPARING EASTERN AND WESTERN PACIFIC TUNA DIET Epipelagic oceanic Epipelagic FAD/reef associated Mesopelagic Migrant Mesopelagic Meso 0% 0% YFT Bathy 1% Bathy 0% WESTERN MICRO 27% Meso 11% 16% 9% 0% 14% - Eastern B - %W - n=63 Epi - ocean 6% 25% YFT - Eastern B - %W - n=191 Meso 1% Epi - ocean 17% - Eastern B - %W - n=34 Epi - ocean 2% EASTERN B 21% 25% 22%

10 WESTERN PACIFIC EASTERN PACIFIC surface EPI oceanic reef/fad MIGRANT MIGRANTHIGHLY MIGRANT EPI oceanic reef/fad MIGRANT MIGRANTHIGHLY MIGRANT 50 m 100 m 200 m 400 m

11 Vertical distribution and migration of forage species NIGHT DAY WESTERN PACIFIC EASTERN PACIFIC surface EPI oceanic reef/fad MIGRANT MIGRANTHIGHLY MIGRANT EPI oceanic reef/fad MIGRANT MIGRANTHIGHLY MIGRANT 50 m 100 m 200 m 400 m

12 TROPHIC STRUCTURE AND ISOTOPES DESCRIPTION OF THE DIET STOMACH CONTENT STABLE ISOTOPE Probably less than 12h Bias linked to the sampling gear (24h-cycle; habitat) About 3 months No bias linked to gear Possible to compare all areas To compare the trophic structure using isotope values Comparison of the relative position of the tuna species To avoid spatial differences calculation of the difference in isotope values between the species (YFT as the reference species)

13 RELATIVE ISOTOPE VALUES OF THE 3 TUNA SPECIES d15n d15n YFT or d15n d15n YFT S S MICRO NC S S A S 0.5 S S S EASTERN-CENTRAL PACIFIC PAPUA FP B S WESTERN EQUATORIAL PACIFIC WESTERN- CENTRAL SOUTH PACIFIC D

14 MORE TO BE DONE -Better delimitation of the areas (based on isotopes, on stomach content) -Study the gear effect on the diet description: LL vs PS FADset vs DOLPHINset vs UNASSOCIATEDset isotope and mixing model -Incorporate other species (mahi mahi, wahoo, marlins, sharks) -More information needed on the prey distribution in the Eastern and Western Pacific