Predation, vegetation and habitat-specific survival of terrestrial eggs of a diadromous fish, Galaxias maculatus (Jenyns, 1842)

Abstract

Mortality from predation during the early life-stages of most teleost fishes can be extreme, and many species have developed specialized spawning strategies to reduce predation. In the diadromous fish Galaxias maculatus there is terrestrial development of eggs which restricts aquatic predation, but exposes them to terrestrial predators and a more extreme physical environment. We hypothesised that exotic slugs ( Milax gagates, Deroceras panormitanum and D. reticulatum) and mice ( Mus musculus) reduce the survival of developing eggs and tested this using laboratory and field predation experiments and sampling. We also tested the effects of riparian vegetation composition and density on egg survival. We found that exotic slugs ( M. gagates and D. panormitanum) reduced the survival of eggs in laboratory experiments, but that neither slugs nor mice affected egg survival in field experiments. Egg densities were positively associated with the stem density of riparian vegetation and the thickness of the aerial root-mat. Egg survival was also positively associated with stem density and aerial root-mat thickness, but was not density-dependent. Although predation by mice and slugs did not appear to be a major cause of egg mortality in our study locations, mortality dynamics could be different in areas with greater densities of predators. Abiotic factors are important in egg survival and these are heavily modified by the height and density of riparian vegetation. It is likely that G. maculatus egg survival, therefore, co-varies with the composition of riparian vegetation.