# Migratory history of the threespine stickleback Gasterosteus aculeatus in western Ireland

## TL;DR

This study reveals that all three-spine sticklebacks in western Ireland live in marine or brackish water their whole lives, without migrating to freshwater.

## Contribution

The first confirmation that Irish coastal sticklebacks are exclusively estuarine residents, not anadromous.

## Key findings

- Otolith Sr:Ca values showed two distinct migration patterns: freshwater and estuarine residents.
- Freshwater residents had low Sr:Ca values (0.41–0.58 × 10−3), while estuarine residents had high values (1.82–4.26 × 10−3).
- No anadromous sticklebacks were found in the study area, challenging typical migration assumptions.

## Abstract

Microchemical analysis of trace elements in otoliths and bio-mineralised earstones of teleost fishes is an emerging approach to analysing the environmental migratoryand life histories of fish species. The migration history of the three-spine stickleback (Gasterosteus aculeatus) collected in western Ireland was examined using calcium (Ca) and strontium (Sr) concentrations in otoliths. The otolith Sr:Ca values fluctuated with the habitat. The habitat use and migration history of G. aculeatus can be categorised into two types, as determined by the mean value and life history transect of the otolith Sr:Ca; that is, freshwater and estuarine residents, whereas there were no anadromous sticklebacks which is believed to be a typical migration pattern in the species. The otolith Sr:Ca profiles of the freshwater resident fishes exhibited constantly low Sr:Ca values, averaging 0.41–0.58 × 10−3 from the core towards the edge. However, the otolith Sr:Ca profiles of the estuarine resident fishes exhibited constantly high Sr:Ca values from the core towards the edge without a clear transition point from low to high Sr:Ca values, as found in the anadromous fish, averaging 1.82–4.26 × 10−3. The present study is the first published confirmation that 100 % of sticklebacks living in coastal habitats in Ireland > have an estuarine resident migratory pattern, constantly residing in marine environments or brackish water throughout their lifespan and not in freshwater environments in Ireland.