Feeding ecology and role of two common seagrass (posidonia australis) inhabiting fishes, the Monacanthids Meuschenia freycineti and Meuschenia trachylepis

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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- This thesis is a study of the feeding habits of two omnivorous fish species. Meuschenia freycineti and M trachylepis, temporary residents of Posidonia australis seagrass meadows. These fishes were found to be the most common fish grazers in three estuaries in NSW, Australia (Pittwater, Botany Bay and Port Hacking). The main aims were to determine their food consumption, the extent and spatial and temporal variability of seagrass biomass removal and the contribution to nutrient recycling via faeces egestion. Biomass and abundance of both species showed high spatial and temporal variability over the fourteen months sampling period (e.g. biomass of M. freycineti in Port Hacking was 22 times that in Pittwater and nine times that in Botany Bay in May 04; throughout the sampling period biomass varied up to 37 fold). This variation was probably due to a combination of factors such as patchy larval supply and variable post-settlement growth, movement and mortality. By combining conventional gut content and stable isotope analyses I showed that M. freycineti and M. trachylepis fed almost exclusively on P. australis and its epiphytes and epifauna throughout the year. They obtained the majority of nutrition from epifauna and, to a lesser extent, from epiphytes but were also able to assimilate nutrients from the seagrass itself. The daily ration of both species was estimated on the basis of a logistic gut content evacuation curve, which was established in laboratory experiments, and the daily gut fullness from specimens caught in the field. Mean daily ration of P. australis and attached epibiota (dry weight per wet weight) using winter evacuation estimates was 0.36 % of body weight for M. freycineti and 0.14 % of body weight for M. trachylepis. By combining biomass estimates with those of the daily ration of seagrass and attached epibiota, an estimate of P. australis biomass removed throughout the sampling period was calculated. Seagrass biomass removal was highly variable in space and time, due to the high variability in fish abundance. Although low when averaged across the whole meadow, grazing by the two species was found to result in high localised removal rates (up to 90 mg dry weight per m² in Port Hacking) in certain areas during winter, the low growth season of the seagrass. Mean biomass estimates for P. australis with epiphytes range from 175 to 230 mg dry weight per m². However, a clear preference for epiphyte-covered seagrass blades was established in multiple-choice preference experiments which suggests that both species preferably feed on old P. australis blades since epiphyte cover increases with leaf age. Thus the impact of grazing by the two species on P. australis is likely to be minimal since they apparently avoid young, growing blades of the seagrass. The C:N ratio of faeces of both species indicated that the faeces are a more nutritious food source for detritivores than ungrazed seagrass detritus. Moreover, the atomic C:N:P ratios of 101:17:1 and 103:17:1 measured in the faeces of M. freycineti and M. trachylepis respectively, were close to the C:N:P ratio required for balanced bacterial growth, suggesting that faeces decomposition should be faster and more complete than in ungrazed seagrass detritus. This study examined the impact of fish grazers on Australian seagrass meadows via seagrass biomass removal and nutrient egestion. The results of this study add to our understanding of the basic ecology of seagrass meadows and their inhabitants which is of vital importance for the development of management strategies for these meadows.
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