Potential for top-down control on tropical tunas based on size structure of predator-prey interactions

Marine Ecology Progress Series (2012)

Hunsicker M E, Olson R J, Essington T E, Maunder M N, Duffy L M and Kitchell J F

DOI: 10.3354/meps09494

Vol 445, pp 263-277

Abstract

The ecological consequences of widespread fisheries-induced reductions of large pelagic predators are not fully understood. Tropical tunas are considered a main component of apex predator guilds that include sharks and billfishes, and thus may seem unsusceptible to secondary effects of fishing top predators. However, intra-guild predation can occur because of size-structured interactions. We compiled existing data of apex predator diets to evaluate whether skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares) tunas might be vulnerable to top-down control by large pelagic predators in the eastern tropical Pacific Ocean. We identified potentially important predators on tunas by the frequency, quantity, and size/age of tunas in their diets and considered the degree that predated tunas could have potentially contributed to the reproductive output of the population. Our results indicate that the proportion of predator diets consisting of skipjack and yellowfin tuna was high for sharks and billfishes. These predators also consumed a wide size range of tunas, including sub-adults are capable of making a notable contribution to the reproductive output of tuna populations. Our study suggests that, in the eastern tropical Pacific Ocean, tropical tunas act as mesopredators more so than apex predators. Sharks and billfishes have the potential to play an important role in regulating these tuna populations. This study sets the stage for future efforts to ascertain whether diminished levels of large predators have enhanced the production of tuna stocks, and if the trophic interactions of skipjack and yellowfin tunas should be explicitly accounted for when assessing their population dynamics.