Abundance and Distribution of the Non-indigenous Acentrogobius pflaumii and Native Gobiids in a Temperate Australian Estuary

Abstract

Non-indigenous species are recognised as a significant threat to estuaries as they can negatively impact native fauna and ecosystem functioning. This study compared the abundance and distribution of the introduced gobiid, Acentrogobius pflaumii, with native gobiids in the shallow, nearshore and deeper, offshore waters of the Swan-Canning Estuary over 31 consecutive months and determined the influence of salinity and water temperature on the gobiid assemblage. Eight gobiid species were recorded (six native and two non-indigenous). Each species was present in offshore waters, with five also occurring in nearshore waters. The contribution of A. pflaumii to the gobiid fauna varied markedly between depths, not being recorded in the 1240 samples from nearshore waters, but representing almost half of all gobiids caught in offshore waters. The percentage contribution of this species to the total gobiid abundance declined with distance from the estuary mouth, from 96% in the most downstream region to only 0.4% in the uppermost. The overwhelming contributions of A. pflaumii in the lower estuary, which historically contained few gobiids, indicate that it has successfully exploited an unoccupied niche. Gobiid composition, in both depths, differed mainly among regions, with species partitioning themselves along the longitudinal axis of the estuary. In offshore waters, Favonigobius lateralis and A. pflaumii dominated the lower regions and Arenigobius bifrenatus, Favonigobius punctatus and Pseudogobius olorum the regions further upstream. The distributions of these species are influenced by salinity and, to a lesser extent, temperature. While there is spatial overlap between A. pflaumii and native gobiid species, its preference for deeper, more saline waters may limit the impact of this non-indigenous species.