Using an updated ecosystem model of the eastern tropical Pacific Ocean to explore potential impacts of increased fishing effort on floating objects

Griffiths S, Fuller L, Valero J, et al (2021) Using an updated ecosystem model of the eastern tropical Pacific Ocean to explore potential impacts of increased fishing effort on floating objects. In: IATTC - 12th Meeting of the Scientific Advisory Committee. IATTC SAC-12-13, Electronic Meeting, p 39

The Inter-American Tropical Tuna Commission (IATTC) has annually reported on 7 ecological indicators derived from the “ETP7” ecosystem model since 2019 as one of several strategies to facilitate an ecosystem approach to the management of tuna fisheries in the eastern Pacific Ocean (EPO) as mandated by the Antigua Convention. Reporting of bycatch interactions and ecological indicators provide a transparent long-term view of the EPO ecosystem and the potential impacts that may be attributed to the tuna fishery. New bycatch data have been added to the model annually from 2019. However, since 2003 when the ETP7 model was developed, it has not been revised to include new longline bycatch estimates or re-calibrated to time series data that have since become available from stock assessments. The model was restructured to contain multi-stanza delay-difference models for small and large sizes of 10 taxa, and biological parameters of functional groups were updated where possible and the model rebalanced to ensure the model was thermodynamically stable using the new “PREBAL” diagnostic tool in Ecopath software. The Ecosim model was then fit to time series of relative biomass or abundance, fishing mortality, and catch (retained and discards) for 10 groups and catch only for another 16 groups.

Ecological indicator values from the updated ETP7 model, herein called “ETP-21”, complemented results of previous assessments that suggested that the EPO ecosystem structure has changed substantially over the history of the fishery. However, as a result of incorporating new data from the longline fishery, the fishing impacts on the ecosystem appear more pessimistic than in the 2019 assessment with a significant decline in the mean trophic level of the catch from 4.77 in 1991 to 4.65 in 2018, which coincided with an increase in the number of OBJ sets. Under fishing effort scenarios reflecting the possible tuna conservation measures to be put in place on termination of Resolution C-20-06 in 2021, the model predicted declines in the biomass of bigeye, yellowfin and skipjack tunas by 0.67–3% over the simulation period 2018–2024. Small and large sharks were impacted more heavily, declining in biomass by 13.8% and 10.4%, respectively. This decreased the predation mortality on predominant FAD-associated bycatch species (dorado, wahoo, and marlins), which resulted in increases in their biomasses by up to 3.3%. Perpetual increases in purse-seine fishing effort on FADs, coupled with the impacts of the industrial longline and coastal fisheries and a changing climate, is likely to continue to alter the structure and dynamics of the ETP ecosystem. The need for updated trophic information, particularly predator stomach contents data and experimental determination of consumption rates, is discussed to improve the ecosystem model and the reliability of forecast outputs.