Synchronicity between zooplankton biomass and larval fish concentrations along a highly flushed Tasmanian estuary: assessment using net and acoustic methods

We examined the spatio-temporal synchronicity between zooplankton biomass and larval fish concentrations within a highly flushed system in northern Tasmania, Australia, combining the data from nets and acoustic methods obtained between October 2001 and November 2002. Zooplankton and larval fish data from nets were analysed in terms of water temperature, salinity and freshwater flow, while backscatter strength from an Acoustic Doppler Current Profiler (ADCP) was employed to complement zooplankton-net data and identify the likely areas of high secondary productivity. Zooplankton and fishes varied significantly across months, peaking simultaneously during late spring (November) at an average temperature of similar to 15 degrees C. Maximum zooplankton (20.5 mgC/m(3)) and fishes (874 larvae/100 m(3)) were recorded within mesohaline (5-17) and polyhaline (18-29) zones, respectively, also in spring. Peaks in zooplankton and larval fish occurred a month after peak freshwater flow, with temperature explaining variability better than did flow or salinity. The coupling of spring peaks in zooplankton biomass and larval fish implies that estuary-spawning fishes may have a fixed spawning period timed to increasing temperatures to ensure a match with abundant microplankton food supply. Backscatter strength complemented zooplankton biomass from nets, and could arguably be used as a proxy for zooplankton abundance even within noisy estuarine systems.