Abstract
Amphidromy is a life history relating to an otherwise freshwater organism that spends a short pelagic larval period in the ocean. Frequent long distance dispersal during this larval period has commonly been assumed, due to theorised colonisation and risk bet-hedging benefits. Galaxias spp. and Gobiomorphus spp. are two dominant fish genera in New Zealand that contain amphidromous representatives. The marine larval period is not obligatory for many ‘amphidromous’ members of these genera, and larvae sometimes develop in freshwater lakes (non-diadromous recruitment, or NDR). The prevalence of NDR, and whether all amphidromous Galaxias and Gobiomorphus share common larval strategies, has not been examined. NDR, and thus retention of larvae within freshwater, suggests dispersal may be more limited than commonly assumed. Determining how prevalent freshwater retention is, and whether retention is common to species in both genera, addresses the question of how dispersive we should pelagic larvae to be. I use otolith microchemistry, a powerful method for determining fish movements, to explore the larval history of galaxiids and bullies and reassess the functional significance of amphidromy.
Otolith microchemistry is a relatively new technique that requires refinement and validation. Before attempting to track amphidromous movements, I experimentally validate the ability to reconstruct movements across salinity in two Galaxias species in chapter 2. I then compare the trace element data obtained from alternative otolith analysis techniques, and demonstrate that a more cost-effective method for extracting otolith microchemistry data (drilling, or depth-profiling) generates reliable data (chapter 3).
In chapter 4 I explore rates of NDR in six amphidromous species from two families in lake systems with open access to the sea. Non-diadromous recruitment was apparent in all amphidromous species investigated. A different subset of species developed in each lake, however, which I suggest indicates species-specific larval habitat requirements being met by some lakes and not others. Otolith microchemistry also suggested larval movement was restricted in a marine setting, with Galaxias maculatus, G. brevipinnis and Gobiomorphus huttoni from different river systems exhibiting distinct early larval signatures (chap. 4 and 5).
In chapter 5 I determine how distance from larval source affected population structure and recruitment dynamics of Galaxias brevipinnis in two long river systems. Otolith microchemistry showed that all fish upstream of the lakes were non-diadromous, whereas fish downstream of lakes were diadromous. Density and size structure data suggested larval supply limited the recruitment of fish throughout most of the river catchment, but supply-limitation was alleviated upstream of lakes where NDR occurred.
Overall, I reject the hypothesis that frequent, long distance dispersal occurs during the larval stages of the amphidromous species investigated. Instead, I propose that amphidromy revolves around the benefits of utilising different habitats at different life stages. Fecundity benefits of producing more, small offspring exist when pelagic habitat is available for larvae to develop and favours an amphidromous life history. As with other forms of migration, amphidromy thus revolves around the growth benefits derived from utilising different habitats. I shift focus from dispersal to a complex life-history and stress that localised recruitment patterns mean that habitat requirements of all life stages (including neglected larvae) need to be identified and protected.