Nicholas Klomp

Understanding the causal mechanisms that determine recruitment success is critical to the effective conservation of wild fish populations. Although recruitment strength is likely determined during early life when mortality is greatest, few studies have documented age-specific mortality rates for fish during this period. We investigated age-specific mortality of individual cohorts of two species of riverine fish from yolksac larvae to juveniles, assaying for the presence of a “critical period”: A time when mortality is unusually high. Early life stages of carp gudgeons (Hypseleotris spp.) and unspecked hardyhead (Craterocephalus stercusmuscarum fulvus)—two fishes that differ in fecundity, egg size and overlap between endogenous and exogenous feeding—were collected every second day for four months. We fitted survivorship curves to 22 carp gudgeon and 15 unspecked hardyhead four-day cohorts and tested several mortality functions. Mortality rates declined with age for carp gudgeon, with mean instantaneous mortality rates (-Z) ranging from 1.40–0.03. In contrast, mortality rates for unspecked hardyhead were constant across the larval period, with a mean -Z of 0.15. There was strong evidence of a critical period for carp gudgeon larvae from hatch until 6 days old, and no evidence of a critical period for unspecked hardyhead. Total larval mortality for carp gudgeon and unspecked hardyhead up to 24 days of age was estimated to be 97.8 and 94.3%, respectively. We hypothesise that life history strategy may play an important role in shaping overall mortality and the pattern of mortality during early life in these two fishes.

Aim  In subalpine and alpine environments, range shifts of species to higher altitudes are predicted to occur in response to reductions in the snow cover from climate change. However, the distribution of key resources may constrain the range of animal populations and prevent any upward migration. This study examined the local resource constraints on the upper range limit of a large, native herbivore in Australia, the common wombat Vombatus ursinus.Location  The subalpine zone of the Snowy Mountains, Australia.Methods  Logistic regression analyses of snow and habitat predictors were conducted on the presence/absence of wombat signs recorded along an altitudinal gradient during winter and summer, using parametric and nonparametric methods.Results  Wombats responded strongly to the altitudinal gradient, but snow cover alone did not fully explain their upper range limit. Wombat occurrence in the subalpine zone was influenced by local habitat features in combination with maximum snow depth. More rugged, high‐relief terrain was important to wombats in winter, allowing individuals access to a wider range of altitudes, snow depths and shelter sites. During summer, high soil bulk density was an important predictor of occurrence, and in both seasons, occurrence declined in response to a higher cover of burnt grass.Main conclusions  These models demonstrate that local habitat factors play a role even where there are strong regulating environmental factors. For wombats, this may limit future range expansion into the alpine zone despite the potential for an increase in abundance at their present range limit. These findings show the need for local ecological studies to be conducted in parallel with broad scale climate modelling if we are to understand shifts in species distributions as the climate rapidly changes.