Evaluate link between increased YFT catches environmental change

Project N.1.d - Evaluate link between increased YFT catches environmental change

Program(s) in charge: Ecosystem & Bycatch Program - Stock Assessment Program

Click to see projects by theme, goal or target:

Theme 5. Interactions among the environment, ecosystem, and fisheries
Goal N. Improve our understanding of the interactions among environmental drivers, climate, and fisheries
Target N.1. Conduct spatiotemporal analyses to better understand the effect of key environmental drivers on the short-term fluctuations of abundance of tunas and prioritized bycatch species

Objectives: Evaluate whether changes in the environment may have contributed to the increases in YFT catches by the purse seine fleet. Specifically, investigate the occurrence of trends/shifts in YFT suitable habitat and corresponding effects in catch rates and length composition, that may explain recent increases in catch.
Background: In 2022 and 2023 YFT catches far exceeded previous years’ catches. It was hypothesized that this may be the result of changing fishing behavior due to the implementation of the new management measures (Resolution C-21-04) intended to address conservation concerns related to bigeye tuna (BET). The measures included the establishment of annual BET catch thresholds on individual purse- seine vessel, which may have been an incentive to avoid BET and target YFT
instead. Preliminary analyses do not support the switching in target (SAC-15-INF-K), and other factors, such as oceanographic conditions, may have played a role. The productivity of the EPO YFT tuna stock shows large annual variability, likely related to environmental conditions. Availability of YFT may also change in different oceanographic regimes. Concomitant to management changes, recent years have also experienced strong ENSO regime fluctuations, including La Niña and El Niño conditions of different magnitude. Thus, potential changes in the
environment need to be investigated to determine whether it played a role in the increase in YFT catches, either due to increase in biomass or changes in availability.
Relevance for management: The implication for management of the different hypotheses that explain the increase in YFT catches (e.g., target switching, environmental effects in productivity or availability) are very different, thus determining their plausibility will be necessary to provide sound scientific advice for management.
Duration: 12 months
Workplan and status: 2024: Produce maps of YFT catches, catch rates and length compositions over various key environmental variables, individually and in combination, for periods before and after the implementation of C-21-04 ,and examine shifts in YFT suitable habitat.
Deliverables: SAC-15 INF-L