Fish use of Tasmanian saltmarsh wetlands

<p dir="ltr">During inundation, coastal saltmarshes provide important feeding and refuge habitat for fishes. Saltmarshes vary in their structure and function as fish habitat, however information on this variation and its drivers is not uniformly available around the world. Although fish use of mainland Australian subtropical to temperate saltmarshes is well documented, little is known of fish use of the higher latitude saltmarshes of Tasmania. To determine the factors influencing fish use of Tasmanian coastal saltmarshes, this thesis investigates the suitability of methods required to undertake fish surveys in saltmarsh environments, the spatial, temporal, and environmental variation in fish use of Tasmanian coastal saltmarsh, the effect of varying access to saltmarsh on the diet of the most numerous species, and the availability of saltmarsh as a habitat for fish. A systematic review of methods found medium-scale, portable methods that provide fish density data to be most appropriate, specifically drop samplers in graminoid marshes and buoyant pop nets in succulent marshes. To investigate the effect of location, season, and environmental conditions on abundance and diversity of fish in Tasmanian saltmarshes, fishes were sampled in the field over one year at three southeast Tasmanian saltmarshes using pop nets. To investigate variation in fish abundance, diversity, and size in introduced <i>Spartina anglica</i> marsh compared to adjacent Tasmanian native succulent saltmarsh vegetation, fishes were sampled in paired patches of native and introduced vegetation in northwest Tasmania, again using pop nets. To explore whether fishes gain functional benefits, specifically foraging benefits, from occupying inundated saltmarsh, variation in dietary composition between fishes with and without direct access to the vegetated saltmarsh flat was investigated using gut content analyses. To understand variation in the availability of saltmarsh for fish in Tasmania, water depth loggers were installed across the low-mid marsh. Seventeen fish species were recorded and abundances of 23-397 individuals per 100 m² were observed. There was significant variation in abundance and diversity between locations and variation in abundance between seasons. Water depth was also positively associated with fish diversity. Fewer individuals, fish species, and small individuals of the most common saltmarsh fish (<i>Atherinosoma microstoma</i>) were found in S. anglica compared to native succulent saltmarsh. Two common marsh species, <i>A. microstoma</i> and <i>Aldrichetta forsteri</i>, were found to consume different quantities of some prey types, and different richness of prey types, when sampled with and without direct access to the vegetated marsh flat. The data indicated some foraging benefits. Saltmarshes in Tasmania are inundated 1 – 14 % of the time, 0.1 – 1.5 times per day, accessible to fish 0.5 – 11.4 % of the time, for 44 – 216 minutes each inundation occasion, similar figures to those recorded in mainland microtidal saltmarshes. The high densities of fish recorded in Tasmanian compared to mainland saltmarshes are unlikely to be attributable to different inundation regimes and more likely to be related to differences in species and habitat context. Future research using comparable techniques in southern Victoria may help separate latitudinal species trends from seascape context drivers. The notably high densities are likely to have important benefits to coastal productivity including fisheries. The findings of this thesis support the continuation of <i>S. anglica</i> removal programs, particularly in zones where swards are well established. Hydrological restoration of saltmarshes for fish habitat is also likely to have fisheries benefits where seascape context is appropriate. More broadly, ongoing monitoring and modelling of changes in inundation regimes should be carried out to better understand effects of climate change and to plan for management of marshes under future sea level rise scenarios.</p>