Predation, Cannibalism, and the Dynamics of Tuna Populations

(continued on page 2) Introduction The unmistakable trend toward ecosystem approaches to fisheries management is documented in a wide range of official declarations, national and international guidelines, and national-level legislation. The central motivation for this movement is the view that a more holistic ecological perspective on fisheries can lead to more robust management strategies. Ecosystem-based fisheries management includes consideration of the impacts of ecosystem processes on fisheries and the unintended alteration of ecosystems by fisheries. The latter interest has been fueled by the rapidly accumulating evidence that selective removal of marine life can induce profound restructuring of marine food webs. This weight of evidence is remarkable considering that until very recently strong trophic interactions were thought to be rare in marine ecosystems. The challenge for fisheries scientists is to be able to identify and predict these strong species interactions and to consider their management implications. Pelagic longline fisheries for tunas, sharks, and marlins are unique compared to other fisheries in the world in that they catch a relatively narrow component of the food web. While these piscivorous fishes constitute the apex predators in these ecosystems, many of these species also prey upon members of their own guild. Might these same types of food-web interactions be occurring among species targeted in the high-seas pelagic fisheries? These high-seas pelagic fisheries target tuna species that are dominant components of the predator guild in pelagic ecosystems but also capture high-trophic-level marlins and sharks. Given the restricted range of the food web that is captured, one might reasonably anticipate little in the way of trophic interactions that could potentially corrupt conventional single-species management approaches. However a closer examination of the trophic linkages within the apex-predator guild reveals the potential for complex interactions among and within species in the form of intra-guild predation and cannibalism. Large-bodied marlins commonly consume skipjack and yellowfin tuna; pelagic sharks are widely viewed as opportunistic top predators; and skipjack, yellowfin, and bigeye tunas all cannibalize juveniles with some regularity. Consequently even species considered CONTENTS A Graph-Theoretic Approach to Analyzing Food Webs Leading to Top Predators in Three Regions of the Pacific Ocean............................ 4 Upcoming Events........................................................ 5 Supply, Demand, and Distribution of Pelagic Seafood on O‘ahu: Select Results from the PFRP Seafood-Distribution Project ........................... 13 Hawai‘i Tuna Tagging Project Two (HTTP2) ................. 18 PNAS Publication for Graduate Student ...................... 20 Predation, Cannibalism, and the Dynamics of Tuna Populations