Spatial variation in the chemical composition of natal otoliths from a reef fish in the Galápagos Islands

Over the past decade, researchers have used variation in the chemical composition of fish otoliths (earstones) to address a number of ecological questions, such as stock assessment and assessing rates of movement of individuals among habitats or life-history stages. However, these methods have yet to be applied successfully to the study of larval connectivity, a major unanswered question in marine ecology. Accomplishing this task requires a reference collection of pre-pelagic natal otoliths and accurate measurement of the chemical signatures of otoliths cores of recruits. We analyzed the chemical composition of natal otoliths from near-term benthic eggs of a damselfish (Stegastes beebei) from all regions of the Galápagos Archipelago. There was no consistent variation in chemical signature at the largest scale, among regions in the archipelago (separated by ~100 to 150 km). However, we found significant variation among islands within each region (10s of km), and among clutches within islands. In addition, we found that otolith cores of newly settled recruits often differed chemically from natal otoliths (which ultimately make up the otolith core as the fish grows). Variation at smaller spatial scales but not larger ones implies that otolith chemistry will only be usable if larvae disperse short distances in this system, and additional work is required to understand the relationship between the chemistry of natal otoliths and recruit cores. Otolith chemistry has the potential to address questions of connectivity, but our data suggest that these methods should be applied cautiously.