Intra-guild predation and cannibalism in pelagic predators: implications for the dynamics, assessment and management of Pacific tuna populations Tim Essington, University of Washington Mary Hunsicker, University of Washington Mark Maunder, IATTC Robert Olson, IATTC Jim Kitchell, University of Wisconsin Enric Cortes, SEFSC Top-Down Control in Marine Ecosystems • Abundant examples: – Sea otter – sea urchin – kelp forest trophic cascade – Northern prawn in the N. Atlantic – Clupeids in the Baltic Sea – Capelin in the Barents Sea Could this be important in tuna fisheries? • Maybe not? – Fishing primarily targets high-trophic level species Could this be important in tuna fisheries? • Maybe not? – Fishing primarily targets high-trophic level species • But then again… – All fish are sometimes small – Juvenile tunas are not uncommon in billfish and tuna stomach contents Upper Food Web of the Central Pacific 5 Sperm Whales Blue Marlin Lamnids Other Billfish Swordfish Carcharhinids 4 Blue Shark Yellowfin Albacore Bigeye Mahi mahi Large Squid Skipjack Small scombrids Baleen Whales 3 Trophic Level Small Squid Mesopelagic Fish Epipelagic Fish 2 Hypothesis: production of skipjack, yellowfin and bigeye tuna stocks has been enhanced through depletion of their predators If true, fisheries management plans for billfishes and tunas should not be treated independently Our Goal • Is it biologically plausible for marlins, sharks, and large-bodied tunas to exert strong top-down control on juvenile tunas? Our Approach • Survey literature – Develop database of apex predator food habits • How does predation vary with species, body size, season, location of capture? • Partition natural mortality into mortality due to each predator – Evaluate the range of predation effects Timeline: Year 1 • September 2006: first workshop meeting at IATTC • October 2006: Nearly completed database of published food habits data • March 2007: Compile raw data from contemporary and historical sources • We Need Your Help! http://fish.washington.edu/tunapred/ http://fish.washington.edu/tunapred/ Yellowfin, Skipjack Yellowfin, Bigeye, Skipjack Study locations: Tunas Blue, Striped Marlins Striped, White, Black Marlins; Blue Marlin Swordfish Study Locations: Billfishes Striped, Blue, Black Marlins Albacore Albacore, Bluefin Porpoises, Sailfish Albacore, Bluefin, Sailfish Mahi Mahi Albacore Study Locations: Other species Enric Cortes : Shark food habits “Best Data” Timeline: Year 1 • September 2006: first workshop meeting at IATTC • October 2006: Nearly completed database of published food habits data • March 2007: Compile raw data from contemporary and historical sources • We Need Your Help! • May 2007: Initial estimates of feeding rates on juvenile tunas Goal: statistically standardize feeding habits in much the same way we standardize CPUE in an assessment Timeline Year 2 • Complete food habits analysis • Couple food habits with bioenergetics models and assessments (where possible) to get distribution of predation “catches” • Incorporate these into skipjack, yellowfin, and bigeye age-based analyses to partition natural mortality among predators • Run “scenarios” that explore the impact of predation on tuna productivities Thank you! http://fish.washington.edu/tunapred/ .