Downstream Spawning Migration in the New Zealand Torrentfish: A Mechanism to Enhance Transport of Larvae to Marine Pelagic Rearing Habitats

Amphidromous and catadromous fish dominate many freshwater fish communities throughout the Indo-Pacific region. Both groups of fish exhibit a marine pelagic larval phase followed by a period of extended freshwater residency. In amphidromous fish, spawning occurs in freshwater with larval migration to the sea. In contrast, adult catadromous fish migrate and spawn in the sea. The function of the two life cycles has been subject to prolonged debate. A common feature of both life cycles is the production of small pelagic marine larvae. Producing small pelagic larvae enables (often small-bodied) amphidromous and catadromous fish to maintain high levels of fecundity, but limits the capacity of the larvae to survive extended periods of starvation. The New Zealand torrentfish is an endemic amphidromous freshwater fish that can penetrate over 100 km inland. Such extensive inland migrations pose the question, how do larvae get to their marine pelagic rearing habitat before terminal starvation occurs?

Our extensive sampling around the South Island of New Zealand confirms that primarily female torrentfish undertake extensive upstream migrations (>100 km), with most males remaining in downstream river reaches. Downstream migration by females to spawning sites in riffles close to the sea occurs from late spring. This pattern of upstream migration during growth to maturity, followed by downstream migration to near-marine spawning habitat is a pattern that is repeated in a number of amphidromous fish species, particularly in species that either penetrate long distances inland or inhabit low gradient terrain. This pattern of migration also has close parallels with migrations in various catadromous fish species – rather than spawning in freshwater, the downstream spawning migration continues into near-shore marine habitats. We discuss possible functions for these downstream migrations and the apparent similarities between amphidromous and catadromous life histories in various Indo-Pacific fish species.