Abstract
The study of the forces shaping diadromous fish migration is dominated by examples drawn from salmonid anadromy. This is in spite of the fact that there are more than 300 species of fish that are known to practice amphidromous and catadromous migrations. A broad suite of often-conflicting demands shape diadromous migrations. Fish may gain growth benefits from outmigration, but be penalized by such movements during reproduction or other critical life stages. In the scientific literature, drivers of amphidromous migrations are typically attributed to the benefits of broad dispersal, which is reinforced by repeated examples of genetic panmixia in amphidromous stocks. By examining migration in the torrentfish (Cheimarrichthys fosteri), an amphidromous fish native to New Zealand, we may gain insights into how these conflicts are resolved, and what the ultimate payoffs are for such movements. This thesis examines amphidromous migrations in C. fosteri and relates them to diadromous migrations broadly, hoping to redefine the drivers of amphidromy and catadromy in fishes.
By tracking instream movements, I was able to quantify the e ects of maturity and sex on migratory behaviour. The species engages in adult downstream migration prior to spawning, in a manner similar to many other amphidromous and catadromous species. Methods for collecting otolith trace element information were tested and then employed in an assessment of stock structure in the species. I found unique trace element signatures associated with individual drainages and regions, reinforcing work in other species indicating that amphidromous larvae may not disperse broadly. Assumptions of genetic panmixia based on life history were tested and rejected. Mitochondrial DNA sequences yielded population level differences in the species across New Zealand.
My work shows that C. fosteri is not panmixic in the larval stage, and that the species displays high levels of phylopatry, down to the individual drainage in some cases. Because of this, I reject the statement that amphidromy is a life history driven by dispersal. Some amphidromous species may disperse broadly, and many species may display genetic panmixia, but studies of amphidromous stocks using markers that generate signals on the scale of the individual indicate regional phylopatry. This lends support to the concept of diadromous migrations as a means to retain high levels of fecundity in depauperate fluvial environments. Amphidromous migrations, both instream and marine, are then driven purely by fecundity benefits, and long distance dispersal is an artifact of the stochasticity of the larval life history.