Evaluating habitat suitability and relative abundance of skipjack (Katsuwonus pelamis) in the Western and Central Pacific during various El Niño events

The changes in skipjack (Katsuwonus pelamis) resources in the Western and Central Pacific Ocean (WCPO) region are subject to not only fishing activities but also the profound impacts of El Niño. Previous studies have suggested that there are two different types of El Niño events, which differentially influence currents, upwelling, chlorophyll concentrations, and temperature in the tropical Pacific Ocean. The comprehensive interaction mechanism between the skipjack stock and El Niño events remains unclear. Thus, an understanding of the differences in the effects between the two types of El Niño is urgently needed to protect skipjack resources. This study analyzed fisheries data and oceanographic conditions for the period of 1996-2012 to construct a habitat suitability index (HSI) model and conduct catch per unit effort (CPUE) standardization to a) identify skipjack habitat and determine relative abundance, b) compare habitat suitability and relative abundance between different types of El Niño events, and c) understand the effects of the two different types of El Niño on skipjack. The results demonstrate that 1) the arithmetic mean model (AMM) was found to be the optimal HSI model for skipjack in the WCPO region and for different El Niño events; 2) the factors of time, location and fishing method should be considered in the CPUE standardization before their application in the estimation of relative abundance; 3) Central Pacific El Niño events (CP El Niño) and Eastern Pacific El Niño events (EP El Niño) were found to have negative effects on skipjack, and relatively higher vulnerability values were found to be associated with the simultaneous occurrence of the two types of El Niño (such simultaneous occurrences may account for significant historical El Niño events); 4) in terms of the consequences of the different types of El Niño events, CP El Niño events tend to exert more negative influence on relative abundance than EP El Niño events can do; and 5) spatial factors tend to exert more influence than temporal factors in the relative abundance assessment. This study enhances our understanding of the consequences of different El Niño events. In addition, the findings may help inform researchers and policy makers in future resource assessment and management.