Hydrological and Ecological Survey of the Port Davey/Bathurst Channel Estuary 1988-1989

Abstract

Five field trips were conducted to the Bathurst Harbour/Port Davey area between October 1988 and July 1989. During each trip the hydrology, benthic fauna, plankton and fish assemblages in the Bathurst Harbour estuary were surveyed. During the winter period of peak freshwater outflow, a brackish surface layer (=15 %0) was present to a depth of =4 m throughout the estuary. The halocline broke down during summer with surface salinities of =29 %0 prevailing. Bottom waters were close to fully marine (=33%0) throughout the year. Surface waters in Bathurst Harbour were found to be extremely depleted in nitrates (<0.1 uM). Oxygen levels were also very low (=35% saturation) during February but were near ? saturation during the other sampling periods. Dinoflagellates and copepods were both extremely abundant in Bathurst Harbour. Diatoms were also abundant during the summer field trips, but were much less common than dinoflagellates on other occasions. Diatoms did, however, occur in higher densities than dinoflagellates in the waters of Port Davey. The plankton community (>20 um mesh size) in Bathurst Harbour was dominated by few taxa, particularly during the winter months. The dinoflagellate Dinophysis acuminata provided >99% of phytoplankton numbers in October 1988 and July 1989, while the copepods GJadioferens inermis and Oithona austraJis and the appendicularian OikopJeura sp. provided almost all of the zooplankton numbers. The diversity of benthic plants and animals decreased rapidly up the Bathurst Harbour estuary. Discrete communities of benthic organisms, both sessile and motile, were found at the western entrance to Bathurst Channel and in the rest of the estuary. The overlap zone between the different communities was a relatively small area between Sarah Island and Schooner Cove. Mobile invertebrates were most abundant at the Bathurst Channel and Port Davey sites in February, and at the Bathurst Harbour sites in December. They showed high fidelity to location, with little indication in this study of a migration up or down the estuary in response to changes in salinity. Very few mobile invertebrates were found in Bathurst Harbour below 4 m depth. Fishes in the Bathurst Harbour estuary also showed little response to seasonal changes in salinity. A number of fishes were, however, possibly prevented from entering the estuary during February because of low oxygen concentrations then prevailing. The assemblage of Bathurst Harbour fishes collected by gillnet was unusual in being dominated by sharks and skates, with all of the common species also being distributed in water depths >50m along the continentalshelf. The assemblage thus has many characteristics typical of a deep water assemblage. Included amongst the Bathurst Harbour fishes was a previously unknown species of skate which may be restricted to the estuary. By far the most common of the larger fishes was the white-spotted dogfish Squalus acanthias; this species comprised 86% of the total gillnet catch. Despite these large catches, a tagging study in which a very high proportion (26 %) of tagged dogfish were recaptured indicated that the total population in the estuary was not huge, being in the order of only 2,500 animals. Tagged animals roamed widely through the estuary. The major recommendations of the study are that gillnets be banned from the Bathurst Harbour estuary, that the low nutrient status of the estuary be maintained, and that steps be taken to ensure that the unique Caulerpa algal beds in Kelly Basin be preserved.