Otolith microchemistry to explore population structure for the Japanese anchovy Engraulis japonicus

A population is the basic unit for fisheries management and conservation. Consequently, to achieve effective fisheries management it is necessary to explore population structures and the processes related to population dynamics. In this study, we investigated population structure of the Japanese anchovy Engraulis japonicus, an important fisheries species with several spatial and seasonal spawning units near Taiwan coastal waters. Based on genetic analysis of larval samples, a previous study suggested weak spatial but potential within-location seasonal structuring for this species. However, as larval mortality is usually high, it is unclear if the observed larval population structure would hold true at the adult stage. Here, we explored variation in concentrations of trace elements in the cores of otoliths using the LA-ICP-MS for both larval and adult Japanese anchovies. Specifically, our sampling design involved using 64 larvae collected from Tamsui and Ilan in the spring and from Ilan in autumn, and 54 adults from bycatch samples from Tamsui, Ilan, and Penghu, during 2010-2013. Principal component analysis indicated that otolith core elemental compositions for the larval samples showed significant annual changes, but there were no significant spatial or seasonal patterns in the elemental compositions for either larval or adult samples. Furthermore, classification of larval samples based on the linear discriminant functions resulted in low correct classification rates (33.3%), indicating weak relationships in otolith core elemental compositions between larvae and adults. The lack of spatial or seasonal structuring for the otolith core elemental concentrations may reflect homogeneous habitat conditions, which might partially result from the relative small spatial scale in our study design. Nonetheless, based both on the previous and our studies, the inconspicuous population structure suggests that the Japanese anchovy may be a homogeneous population.