Interacting effects of density and temperature on fish growth rates in freshwater protected populations.

Despite the demonstrated benefits of marine protected areas, there has been relatively little dialogue about freshwater protected areas (FPAs) even though some have been established to protect freshwater species from recreational and commercial fishers. After populations recover from fishing pressure, abundances and densities of formerly fished species increase, and we should therefore expect changes in demographic traits compared to those in exploited populations. To test this, we used capture-mark-recapture data for 10 Galaxias maculatus populations across a density gradient mediated by different degrees of fishery closure. We examined the extent to which density-dependent (DD) and density-independent (DI) effects interact to affect specific growth rates in post-recruit populations. We found that population density, stream temperature and individual size interact to affect growth rates. When population densities were high, compensatory responses of far slower growth rates were strongest, indicating that DD growth is a key mechanism regulating post-recruit populations of G. maculatus. This study emphasizes the importance of understanding DD and DI processes, their interactions, function and effectiveness for freshwater fisheries management. For FPAs to be effective, the extent and quality of target species' habitats must serve as key criteria for protection to alleviate competition for limited resources that underpins DD processes.

Predicting biomass of resident kokopu (Galaxias) populations using local habitat characteristics.

With the global decline of freshwater fishes, quantifying the body size-specific habitat use of vulnerable species is crucial for accurately evaluating population health, identifying the effects of anthropogenic stressors, and directing effective habitat restoration. Populations of New Zealand's endemic kokopu species (Galaxias fasciatus, G. argenteus, and G. postvectis) have declined substantially over the last century in response to anthropogenic stressors, including habitat loss, migratory barriers, and invasive species. Despite well-understood habitat associations, key within-habitat features underpinning the reach-scale biomass of small and large kokopu remain unclear. Here, we investigated whether the total biomass of large (> 90 mm) size classes of each kokopu species and the composite biomass of all small (≤ 90 mm) kokopu were associated with components of the physical environment that provided refuge and prey resources across fifty-seven 50-m stream reaches. Because kokopu are nocturnal, populations were sampled by removal at night using headlamps and hand-nets until reaches were visually depleted. Based on Akaike's information criterion, greater large banded kokopu biomass was most parsimoniously explained by greater pool volume and forest cover, greater large giant kokopu biomass by greater bank cover and pool volume, and greater large shortjaw kokopu biomass by greater substrate size and pool volume. In contrast, greater composite small kokopu biomass was best explained by smaller substrate size, reduced bank cover, and greater pool volume. Local habitat associations therefore varied among kokopu species and size classes. Our study demonstrates the importance of considering the ontogenetic shift in species' habitat use and provides an effective modelling approach for quantifying size-specific local habitat use of stream-dwelling fish.

Population sinks resulting from degraded habitats of an obligate life-history pathway.

Many species traverse multiple habitats across ecosystems to complete their life histories. Degradation of critical, life stage-specific habitats can therefore lead to population bottlenecks and demographic deficits in sub-populations. The riparian zone of waterways is one of the most impacted areas of the coastal zone because of urbanisation, deforestation, farming and livestock grazing. We hypothesised that sink populations can result from alterations of habitats critical to the early life stages of diadromous fish that use this zone, and tested this with field-based sampling and experiments. We found that for Galaxias maculatus, one of the most widely distributed fishes of the southern hemisphere, obligate riparian spawning habitat was very limited and highly vulnerable to disturbance across 14 rivers in New Zealand. Eggs were laid only during spring tides, in the highest tidally influenced vegetation of waterways. Egg survival increased to >90% when laid in three riparian plant species and where stem densities were great enough to prevent desiccation, compared to no survival where vegetation was comprised of other species or was less dense. Experimental exclusion of livestock, one of the major sources of riparian degradation in rural waterways, resulted in quick regeneration, a tenfold increase in egg laying by fish and a threefold increase in survival, compared to adjacent controls. Overall, there was an inverse relationship between river size and egg production. Some of the largest rivers had little or no spawning habitat and very little egg production, effectively becoming sink populations despite supporting large adult populations, whereas some of the smallest pristine streams produced millions of eggs. We demonstrate that even a wide-ranging species with many robust adult populations can be compromised if a stage-specific habitat required to complete a life history is degraded by localised or more diffuse impacts.

Synergistic interactions within disturbed habitats between temperature, relative humidity and UVB radiation on egg survival in a diadromous fish.

Anthropogenic impacts, including urbanization, deforestation, farming, and livestock grazing have altered riparian margins worldwide. One effect of changes to riparian vegetation is that the ground-level light, temperature, and humidity environment has also been altered. Galaxias maculatus, one of the most widely distributed fishes of the southern hemisphere, lays eggs almost exclusively beneath riparian vegetation in tidally influenced reaches of rivers. We hypothesized that the survival of these eggs is greatly affected by the micro-environment afforded by vegetation, particularly relating to temperature, humidity and UVB radiation. We experimentally reduced riparian vegetation height and altered shading characteristics, tracked egg survival, and used small ground-level temperature, humidity and UVB sensors to relate survival to ground-level effects around egg masses. The ground-level physical environment was markedly different from the surrounding ambient conditions. Tall dense riparian vegetation modified ambient conditions to produce a buffered temperature regime with constant high relative humidity, generally above 90%, and negligible UVB radiation at ground-level. Where vegetation height was reduced, frequent high temperatures, low humidity, and high UVB irradiances reduced egg survival by up to 95%. Temperature effects on egg survival were probably indirect, through reduced humidity, because developing eggs are known to survive in a wide range of temperatures. In this study, it was remarkable how such small variations in relatively small sites could have such a large effect on egg survival. It appears that modifications to riparian vegetation and the associated changes in the physical conditions of egg laying sites are major mechanisms affecting egg survival. The impacts associated with vegetational changes through human-induced disturbances are complex yet potentially devastating. These effects are particularly important because they affect a very small portion of habitat that is required to complete the life history of a species, despite the wide distribution of adults and juveniles across aquatic and marine environments.