Habitat preference and diverse migration in threespine sticklebacks, Gasterosteus aculeatus and G. nipponicus.

Threespine sticklebacks of the genus Gasterosteus, are small teleost fish that are widely distributed across the northern hemisphere. The fish is believed to have two major types of life history, freshwater resident and anadromous; however little is known about their migration ecology. Comprehensive research on the migratory history, habitat use and relative composition of migratory types was conducted by analysing the otolith strontium and calcium concentrations collected in various environments of northern Japan. The present study first demonstrated that approximately 90% of morphologically anadromous sticklebacks had estuarine resident migration pattern, consistently living in brackish water and/or marine environments through their life cycle without any time spent in freshwater. The dominant occurrence of the estuarine resident was temporally and spatially consistent with their general migration ecology. The estuarine resident is thought to be the ancestral migrations of G. aculeatus and G. nipponicus, which thereafter gradually immigrated into freshwater habitats and settled in the anadromous form in both species and finally became the freshwater resident G. aculeatus. Thus, this study provides novel insights into the evolutionary migration of these fish, as well as a new discovery regarding the dominant migratory history and habitat use in threespine sticklebacks.