Abstract
Amphidromous fishes engage in migration to overcome the spatial mismatch between adult and larval habitat. The torrentfish (Cheimarrichthys fosteri), an endemic New Zealand amphidromous fish and only member of the Family Cheimarrichthyidae, display distinct spatial sexual segregation, but the migration assumed to overcome this segregation for reproduction is undocumented. We investigated the instream distribution of torrentfish, gaining insight into how barriers to reproduction are overcome, and how sex shapes migratory choices for the individual. We surveyed torrentfish at two spatial scales, examining the effects of sex and sexual maturity on distribution. We found that female torrentfish engage in prolonged upstream migrations, up to 100 km inland, with male torrentfish migrating upstream to a lesser extent. Both sexes return downstream to spawn, with males returning at the onset of maturity, while females may return and migrate back upstream, potentially making multiple downstream spawning migrations. Juvenile upstream migration likely results in competitive release and access to resources, but sets up a conflict between reproduction and transport of larvae into the nursery habitat in a timely manner. Adult downstream migration in torrentfish resolves this conflict and is similar to documented downstream spawning migrations in catadromous species.