Otolith microchemistry of the amphidromous Galaxias maculatus shows recruitment to coastal rivers from unstructured larval pools

Marine larval development gives amphidromous fishes a powerful ability to disperse, but the low directional swimming ability of small amphidromous juveniles returning to freshwater increases the risk of expatrial dispersal. We used otolith microchemistry to investigate philopatry in Galaxias maculatus, whose juveniles constitute an important but declining commercial and recreational fishery in New Zealand. Using laser ablation inductively coupled plasma mass spectrometry, we analysed the elemental signature of embryonic otoliths from G. maculatus hatchlings from 12 rivers on the east and west coasts of the South Island of New Zealand. We then analysed the core region of the otoliths of recruits from the same cohort as they enter these rivers 6 mo later. The multi-elemental signatures of hatchling otoliths produced a high degree of differentiation among the 12 rivers. Using a multivariate approach, the streams tended to separate into east coast or west coast categories, driven largely by the influence of the relative values of Al and Fe in the otoliths. When the resulting discriminant model was used to classify the multi-elemental signature at the core of recruit otoliths, very few (2.8%) appeared to have returned to their natal river. Connection of G. maculatus populations within ecological time frames seems to be a common occurrence, especially within broad regions of the coast. Our findings imply 'leaky borders' with respect to larval pools, especially within coastlines. We highlight the importance of regional larval pools and the need to think about conservation efforts at both local and regional scales.