Seasonal and inter-annual variability in abundance of the main tropical tunas in the EEZ of Côte d’Ivoire (2000-2019)

The seasonal and inter-annual variability in abundance of the main “local tropical tuna resources” in the EEZ of Côte d'Ivoire was analysed with catch and effort data from French and Spanish purse seiners over the period 2000–2019. A seasonal spatio-temporal model developed by Thorson et al. (2020a) was used to estimate abundance indices for the main tropical tunas by commercial category ( = 10 kg, which correspond roughly to maturity stage: immature and mature respectively), and fishing mode (free school sets and FAD sets). Furthermore, we decomposed the abundance time series into intrinsic mode functions using the CEEMDAN algorithm. The decomposition procedure made it possible to filter out the noise in the signal and extract the seasonal and inter-annual components of the abundance indices. A generalized additive model (GAM) was applied to the abundance indices to reveal the influences of environmental factors on species abundance and spatio-temporal distribution. Biological interpretations of the seasonal and inter-annual variability in tropical tuna abundance were made and the possible effects of environmental variables on this abundance discussed. Our results suggest that there are two main fishing seasons in the EEZ of Côte d'Ivoire. It was also found that mature yellowfin tunas are abundant between the first and second quarter of the year while the best season for skipjack occurs between the third and fourth quarter. In addition, we observed a considerable change over time in the seasonal and inter-annual variability of tropical tunas in this area