Tropicalisation is a process in temperate waters where temperature increases allow the intrusion of previously absent tropical species. This process is happening in the Mediterranean with skipjack tuna Katsuwonus pelamis, an opportunistic tuna species. By using fishery data from recreational fishing tournaments taking place during 51 days in the Balearic Sea (western Mediterranean) from 2000 to 2018, this study produced an estimate of relative skipjack abundance with its population growth rate by using a Bayesian State Space Model. This model suggested that sea surface temperature (SST), moon phase, wave height did not have an effect on the catchability of skipjack. It also suggested North Atlantic Oscillation, SST anomaly from three years prior and albacore Thunnus alalunga abundance did not have an effect on the population growth rate and relative abundance of skipjack. Despite the uncertainty, the estimated average population growth rate is consistent with a stable presence of the skipjack for the last 20 years. Our results indicate that skipjack tuna could have maintained a stable summer population in the Balearic Sea through spawning, migration, or both. This study presents new information on the abundance of skipjack that could be of value to future assessment and management.
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