Otolith microchemistry reveals different environmental histories for two endangered fourfinger threadfin species

Many fishes inhabit different environments throughout ontogeny in order to utilize more suitable habitats and meet their growth needs during different developmental stages. Otolith microchemistry analysis is a powerful tool to elucidate the environmental histories and past movement patterns of fishes, providing important insights into the connectivity between various environments utilized by different developmental stages. In this study, we used laser ablation inductively coupled plasma mass spectrometry to analyze the chemical composition of otoliths throughout the entire lifetime of 2 endangered and closely related fourfinger threadfin species, Eleutheronema rhadinum and E. tetradactylum . This is the first study to reconstruct the life history of fourfinger threadfins, and the various element:Ca ratio profiles from otolith core-to-edge analysis suggest differences in environmental histories between the species, especially during their early life stages. We identified 2 contrasting environmental history patterns. Most of the analyzed E. rhadinum individuals were estuarine-dependent, with fish spending their first year in an estuarine environment and then moving to marine coastal systems. In contrast, the majority of E. tetradactylum individuals were coastal-dependent, remaining in the coastal systems throughout their early life history stages. Element:Ca profiles, self-organizing maps and random forest analysis also detected a unique environmental history pattern in early life history stages of the Thai E. rhadinum population that differed from the Chinese E. rhadinum population. There was no evidence of differences between E. tetradactylum populations from Thailand and China. Fourfinger threadfins have experienced drastic population declines over the past decade, and holistic management to increase environmental connectivity for these species is necessary to maintain sustainable fisheries.