Decadal decline in demersal fish biomass coincident with a prolonged drought and introduction of an exotic starfish

Between 1990 and 2011, Port Phillip Bay in southern Australia, experienced two major ecological disturbances: a prolonged drought from 1997–2010; and the introduction of an invasive starfish, Asterias amurensis. The drought... [ view full abstract ]

Between 1990 and 2011, Port Phillip Bay in southern Australia, experienced two major ecological disturbances: a prolonged drought from 1997–2010; and the introduction of an invasive starfish, Asterias amurensis. The drought reduced nitrogen inputs by 64%, and the biomass of A. amurensis in the deep centre of the bay peaked at 56% of the resident fish biomass in 2000. The impact of these disturbances was examined using a demersal trawl time-series spanning two decades (1990–2011). The timing and spatial extent of changes to fish biomass were analysed using ANCOVA and Bayesian change point analysis .

During the drought, fish biomass declined by 69% in the deep centre of the bay, while increasing slightly around the shallow fringes. This spatial pattern is consistent with hydrodynamic modelling that suggests that, during the drought, a greater proportion of the (lower) nitrogen input was retained near the coastal fringe. Most of the decline in fish biomass was attributed to the cumulative effects of reduced productivity during the 12 year drought. However, declines in three species in the deep region were attributed to competition with A. amurensis. Each of these species exhibited high dietary overlap with A. amurensis and displayed sharp declines in biomass coinciding with the peak abundance of A. amurensis in 2000.