Hydrodynamics and zooplankton ecology in the Tamar Estuary, northern Tasmania, with special emphasis on larval fishes

This thesis describes results of a comprehensive study of the Tamar Estuary, a highly flushed system in northern Tasmania, conducted to investigate the link between hydrodynamic processes and zooplankton biomass with particular emphasis on larval fish dynamics. It comprised extensive plankton sampling together with measurements of physical parameters, between October 2001 and November 2002, including three 24-hour sampling sessions in the lower estuary to ascertain the effect of tidal currents in the transport of fishes. The Tamar was classified as a partially mixed estuary (Type 2a), based on salinity distribution, current velocities and freshwater flow. The weak vertical temperature and salinity stratification, together with strong mean current velocities (2 m/s), indicated a lack of two-layered circulation, and hence absence of a net upstream flow along the bottom. The study yielded 80,837 larval fishes representing 44 families, with catches dominated by gobiids, blenniids, clinids and engraulids. Temperature was found to be the most important variable driving temporal changes in zooplankton biomass and larval concentrations. Peaks in zooplankton biomass and larval concentrations occurred simultaneously in November, both in 2001 and 2002, at temperatures approx. 15 C, and lasted approx.2 months. Results suggest that the commencement and intensity of spawning in the Tamar may be triggered by changes in abiotic factors, such as increasing temperature and moderate freshwater flow, whereas spawning duration may be linked to biotic factors such as the presence of potential predators and food availability. No evident pattern in the spatial distribution, both of zooplankton biomass and larval fishes was observed during the peak abundance period. The spatial structuring of the larval assemblage was driven by salinity, with one assemblage representing mostly estuarine-spawned larvae (mouth to 35 km upstream), and the other representing larvae of freshwater taxa (>35 km). The geographical extent of each assemblage could be associated with the strong diffusive forces of tidal currents, which also affected zooplankton biomass. An estuarine transport model used to investigate fish movements along the lower estuary was able to accurately predict concentrations of virtual larvae early in the season, as they behaved similar to passive particles. However, it was unable to accurately predict concentrations later in the season, likely because most larvae were displaying a behaviour different to passive particles due to increased swimming ability. Strategies likely helping retention and survival of larvae in the Tamar Estuary include a short (approx.months), defined occurrence period that is well-timed with zooplankton production, and location where these larvae are spawned within the estuary.