Spatiotemporal dynamics of the Pacific chub mackerel revealed by standardized abundance indices

Standardization of catch per unit effort is an effective way to not only extract temporal abundance trends but also understand distribution shifts of fish resources. Chub mackerel in the Northwest Pacific has recently been of great concern to fishermen and fisheries managers because the total catch has been increasing, and fishing grounds have expanded from the Japanese exclusive economic zone eastward to high-sea areas. Understanding the spatiotemporal dynamics of chub mackerel is urgently needed for evaluating the current status (i.e., abundance and distribution) and considering future exploitation. Here we constructed two recruitment indices from pelagic trawl survey data of the Pacific chub mackerel. For recruitment indices of age-0 fish, we integrated an algorithm for automatic area stratification into the delta-GLM that combines two separate generalized linear models (for encounter probability and positive catch rates) to simultaneously investigate changes in relative abundance and distribution. Recruitment indices were high in the recent years of 2013, 2016, and 2017, especially at around 160 °E longitude within the high-sea areas, suggesting simultaneous changes in abundance and distribution. Furthermore, we found that our approach had better predictive ability, especially for positive catch rates, than that of a generalized additive model by cross validation, and the obtained standardized indices mitigated potential bias in stock assessment estimates. Our integrated approach will be useful for understanding the spatiotemporal dynamics of fishes and standardizing abundance indices for stock assessment and management.