Is settlement at small spatial scales by diadromous fishes from the Family Galaxiidae passive or active in a small coastal river?

Abstract

Most marine fish and benthic invertebrate species have complex life cycles with sedentary adults and highly dispersive larvae. Mortality is often considerable at the end of the larval phase when species are undergoing settlement, so information about the biophysical mechanisms influencing larvae during this transition between habitats is crucial. For species dispersed passively, settlement is governed by oceanic processes controlling larval transport. However, many fishes have the ability to modify their dispersal behaviourally and patterns of settlement for these species are likely to be quite different. Predicting settlement patterns therefore requires knowledge of the relative importance of larval behaviour v. passive transport. In the present study, diadromous fishes from the Family Galaxiidae were surveyed to examine whether settlement is likely to result from fish settling passively (i.e. driven by environmental conditions) or whether larval behaviour may be important. Although catch rates were weakly negatively correlated with discharge, there was no evidence that fish were settling passively at the scales examined in the study. In comparison, fish appear to be using behaviours similar to selective tidal stream transport to facilitate movement into rivers from the ocean. If galaxiids have the ability to use these behaviours, larval behaviour may affect the distribution of settlers and be an important influence on the population dynamics of these species.