An updated ecosystem model of the eastern tropical Pacific ocean: analysis of ecological indicators and the potential impacts of FAD fishing on ecosystem dynamics

The Inter-American Tropical Tuna Commission (IATTC) has adopted an ecosystem approach to the management of tuna fisheries in the eastern Pacific Ocean (EPO), as mandated by the Antigua Convention. However, demonstrating ecological sustainability using single-species stock assessments is impractical and cost-prohibitive, given the large number of species with which the EPO tuna fishery interacts, the paucity of catch and biological information for many species of lesser economic or conservation significance, and the fact that these assessment models do not consider the multidimensional predatorprey relationships that can be directly or indirectly impacted by fishing activities. Since 2017, the IATTC has reported time series of catches of non-target species (e.g., sharks, rays, sea turtles, marine mammals, large and forage fishes) and a range of ecological indicators derived from an updated Ecopath with Ecosim (EwE) ecosystem model of the eastern tropical Pacific Ocean (ETP). Together, these data and indicators provide a transparent long-term view of the EPO ecosystem and the potential impacts that may be attributed to the tuna fishery. The ecosystem model can be used in concert with stock assessment models of target species to simulate potential management scenarios that may enable managers to adopt appropriate conservation and management measures that maximize ecological and economic benefits.
This document presents an assessment of the EPO ecosystem using seven ecological indicators that, together, describe changes in the structure and dynamics of the EPO ecosystem during 1970–2017 due to tuna fishing. The results clearly show that the ecosystem structure has changed substantially over the history of the fishery, first due to the increase in industrial fishing from the 1970s, but most markedly since the purse-seine fishery on floating objects—mostly fish-aggregating devices (FADs)—began its dramatic expansion in 1993. The ecosystem model was used to predict the ecological consequences of continued increases in effort in this fishery, and the potential impacts of limiting its effort as a conservation and management measure, primarily to reduce the fishing mortality of skipjack and small bigeye and yellowfin tunas. The model simulations indicate that, even if the rate of effort increase observed in the fishery over the past 10 years is reduced by 50%, the biomass of some target tuna species may be reduced by up to 62%. The model predicts that limiting the number of floating-object and unassociated sets to the 2016-2018 average would maintain the ecosystem structure in its present state and slightly increase the biomass of most target tuna species, but a significant reduction in purse-seine effort (and most likely longline effort as well) would be needed to restore the EPO ecosystem to its state prior to the expansion of the FAD fishery. Updated trophic information, particularly predator stomach contents data and experimental determination of consumption rates, is needed to improve the ecosystem model and the reliability of forecast outputs.