Recruitment variation in a stream galaxiid fish: multiple influences on fry dynamics in a heterogeneous environment

摘要

1. The relative importance of density-dependent and density-independent processes in explaining fluctuations in natural populations has been widely debated. In particular, the importance of larval supply and whether it may control the type of regulatory processes a population experiences has proved contentious. 2. Using surveys and field experiments conducted in streams in Canterbury, New Zealand, we investigated how variation in the survival of non-migratory Galaxias vulgaris fry was affected by density-dependent and density-independent processes and how this variation influenced recruitment dynamics. 3. Fry populations with high settlement densities experienced a 70-80% reduction in population size from density-related mortality during the first fourteen days after peak settlement but thereafter the influence of density-dependent processes on fry was weak. The impact of environmental conditions on fry populations was dependent on fry size and the magnitude of the perturbation, such that flooding effects on fry survival were most severe when fry were small. 4. In streams not affected by flooding, the size and density of introduced trout (Salmo trutta and Oncorhynchus mykiss) were the most significant factors determining the abundance of eventual recruits. A field experiment manipulating brown trout access to fry populations revealed that trout as small as 110 mm may be capable of greatly reducing and possibly preventing galaxiid recruitment. 5. Overall, the results indicated density-dependent population regulation was only possible at sites with high native fish densities because trout were likely to be suppressing the number of potential recruits at sites with low native fish numbers. Whilst density-dependent processes had a strong effect on fry survival following the period of peak fry abundance, density-independent processes associated with flow and predatory trout influences on fry survival largely determined recruitment variability among galaxiid populations. Focusing conservation efforts on improving habitat to increase fry retention and reducing the impacts of trout on galaxiids would ensure more native fish populations reached their potential abundance.