The Influence of Tidal Phase on Patterns of Ichthyoplankton Abundance in the Vicinity of an Estuarine Front, Botany Bay, Australia

Abstract

The influence of oceanography on abundance patterns of ichthyoplankton was compared at different phases of the tide in the vicinity of an estuarine front in Botany Bay, Australia. Fronts were observed at high, ebb, low and flood tides, but the most conspicuous colour discontinuities were found on ebb and flood tides. Greatest differences in water density were found across the frontal region on ebb tides. Because rainfall was low, density differences were primarily due to differences in temperature. There was strong advection of drogues into fronts and along frontal margins on ebb and flood tides. Thus, particles are ‘ jetted ’ along fronts. Total ichthyoplankton was always greatest in fronts on low tides (by 1·5–4 times). Rank abundance of ichthyoplankton in the plume, front and ocean, at other phases of the tide (flood, high, ebb), varied among days. Great differences in the composition of ichthyoplankton were found between the plume, front and ocean at all phases of the tide, but differences were most clear on ebb and low tides. Some types of fish were always most abundant in the front (e.g. Exocoetidae), or the plume and front (e.g. Gobiidae, Pleuronectidae, Hemiramphidae, Blenniidae), regardless of the state of the tide. Some pelagic juveniles were only found in tows that contained drift algae, especially in the front. The ‘ jetting ’ of larvae along topographically stable fronts on flood tides may influence spatial patterns of recruitment in bays and estuaries. The intensity of differences in water density across the frontal region, between the plume and ocean, did not explain the relative abundance patterns of fish. The authors argue that a combination of physical advection, responses by ichthyoplankton to the biological attributes of fronts (e.g. abundance of food), lag effects (i.e. response of plankton to aggregation), and differences in larval supply (from spawning) were responsible for patterns of abundance in different water masses at scales of less than 15 km.