# Strontium and Magnesium in Otoliths Can Trace Schizothorax grahami (Regan, 1904) Life History

## TL;DR

This study shows that strontium and magnesium in fish otoliths can reveal life history patterns and spawning origins of Schizothorax grahami in the Chishui River.

## Contribution

The novel use of Sr and Mg in otoliths to trace life history and spawning origins of an endangered fish species.

## Key findings

- Strontium (Sr) in otolith rings shows consistent distribution patterns across river sections.
- Magnesium (Mg) levels in otolith cores correlate with spawning ground water conditions.
- Mg:Ca ratios in otolith cores differ significantly among samples from different spawning sites.

## Abstract

Life history has always been a key research focus for fish conservationists. However, the connection between life history and the environment has not yet been fully established. By studying the microchemical information of otoliths in Schizothorax grahami from the source section of the Chishui River in China, we found that only strontium (Sr) is regularly distributed in the otolith ring. The values of Sr:Ca and the change frequency significantly differ among the three reaches (intra- and between-class). Mg content in the otoliths is correlated with the water environment of the spawning ground. Meanwhile, the values of Mg:Ca in the otolith core showed significant differences among the samples, indicating that those samples came from different spawning grounds at the early stage of their life history. And, through the characteristics of Mg in the otoliths, the source and life history of S. grahami in the Chishui River could be identified. Therefore, strontium and magnesium in otoliths could be used to trace the life history of S. grahami in the Chishui River.

This research examines the otolith microchemical characteristics of the critically endangered kunming snout trout (Schizothorax grahami) collected from the sources section endemic to the Chishui River, China, a tributary of the upper Yangtze River, and the metal concentrations in the water of fish habitats. Among the analyzed elements, strontium (Sr) exhibited consistent distribution patterns across otolith rings, as observed through face-scan imagery (scanning the polished surface of the otolith cut), with a relatively low coefficient of variation. Statistical analysis using a paired two-tailed t-test revealed significant differences in Sr:Ca ratios among samples from the three river sections. Notably, magnesium (Mg) levels in the otolith core during the early life stages of the fish were notably higher compared to non-core regions. Similarly, Mg concentrations of water were elevated in the spawning grounds relative to non-spawning areas. This suggests that otolith Mg content may be influenced by the specific water conditions of the spawning grounds. Furthermore, Mg:Ca ratios in the otolith core displayed significant variation among samples from U3, L2, and other sites, implying that the fish in these samples originated from distinct spawning locations. These findings demonstrate that strontium and magnesium in otoliths can serve as effective markers for reconstructing the life history of S. grahami in the Chishui River and can contribute to the management of different fish stocks.