Chronosequences of strontium in the otoliths of two New Zealand migratory freshwater fish, inanga (Galaxias maculatus) and koaro (G. brevipinnis)

Abstract

Half of the freshwater fish fauna in New Zealand is diadromous (i.e., migrates between the sea and freshwater); unimpeded access to and from the sea is important for the conservation of diadromous fish. The genus Galaxias (Galaxiidae: Osmeriformes) contains five diadromous species that spawn in freshwater, migrate to sea as larvae, and then migrate back into freshwater as 50-55-mm juveniles. Microchemistry of the otolith, a calcified structure in the fish's head, allows an independent test of assumptions about these migrations. Concentric layers of CaCO3 with some Sr as SrCO3 comprise the otolith, creating a chronosequence that reflects a fish's migratory history. More Sr accumulates in the otolith when a fish is in seawater than when it is in freshwater. High-resolution nuclear microscopy was used to measure the molar ratios of Sr/Ca in two galaxiid species. Otoliths of inanga (G. maculatus) caught in freshwater all showed a central zone of 100-200 μm in radius with Sr/Ca of 0.008-0.012, indicating early rearing in the sea. Sr/Ca values decreased to 0.001-0.002 as the fish moved into freshwater. Of six adult koaro (G. brevipinnis) caught in a river with sea access and no lakes, five had migrated to sea but one had not, raising questions about the generalized assumptions of migration.