Baseline survey of the fish and invertebrate fauna of Hill Inlet, the northernmost estuary in south-western Australia

Abstract

Microtidal estuaries in Mediterranean climates are particularly vulnerable to the effects of anthropogenic degradation. This study provides the first data on the fish and benthic macroinvertebrate fauna of Hill Inlet, the northernmost estuary in south-western Australia. Sampling was conducted in June 2019 (Austral winter), when water levels were very high due to recent heavy rainfall and the bar at the mouth of the estuary was intact. Surface salinities were oligohaline and declined along the longitudinal axis, ranging from 12 to 3. A marked halocline was present at most sites, resulting in pronounced hypoxia. High water levels precluded the use of a seine net at some sites to sample the nearshore fish fauna, however, two species were recorded (Pseudogobius olorum and Acanthopagrus butcheri), both of which complete their life-cycle within the estuary. Deeper, offshore waters, sampled using gill nets, yielded only four species (Mugil cephalus, A. butcheri, Adrichetta forsteri and Pomatomus saltatrix), due to the bar at the mouth of the estuary being closed prior to sampling thus limiting recruitment from marine species. Ten benthic macroinvertebrates species were collected, representing mainly polychaetes, molluscs and crustaceans. The low number of species was likely caused by the hypoxia present throughout most of the bottom waters. Although these data represent a benchmark against which future changes can be detected, it is recommended that additional sampling is conducted when water levels are lower and the bar has been open to provide a more holistic assessment of the fauna of Hill Inlet.