A preliminary investigation of the potential effects of the invasive Mozambique tilapia Oreochromis mossambicus on the native fish assemblages of Lake MacLeod, Western Australia

As one of the major threats to biodiversity in aquatic systems, invasive species can alter the structure and function of a community, often through habitat and resource competition, and/or direct predation. This study aims to determine if invasive tilapia (Oreochromis mossambicus) is likely to have an effect on the native fish communities of Lake MacLeod, a unique and important inland, saline lake system in north-western Western Australia, through competition for key resources, namely habitat and food sources. Seven study sites were selected within the Northern Ponds of Lake MacLeod, from which fish abundances were assessed in three habitats (pneumatophore beds, nearshore and vents) using adapted fyke-nets and unbaited underwater videography. From three of the seven sites, dietary and stable isotope analyses were conducted. The species richness of fish was low, with between 3-4 species caught at each site. Amniataba caudavittata (yellowtail grunters) and Craterocephalus pauciradiatus (hardyheads), the two most prevalent native fish species, were captured in all three habitats. Although observed in every habitat, O. mossambicus was only successfully captured from the vent habitats. The total length of O. mossambicus ranged from 48-385 mm but the cohort at one of the vents, ‘Jack’s vent’, was considerably larger and more abundant than at all other sites. C. pauciradiatus remained consistent in its size structure across sites and habitats, whereas A. caudavittata tended to be slightly smaller in the pneumatophore habitat. Based on stomach contents analysis, O. mossambicus and A. caudavittata consumed many of the same food items, but often in different proportions. Little variation was seen among sites, except for Jack’s, and the diets were more separated by species than by site. A. caudavittata consumed higher percentages of filamentous green algae, amphipods, and seagrasses than O. mossambicus, but O. mossambicus generally consumed a higher percentage of sand and/or sediment, foraminifera, and detritus. A. caudavittata consumed a wider variety of items. From observations on recorded videos, and from the high percentage of sediment in the stomachs of O. mossambicus relative to A. caudavittata, it was concluded that the two species are utilizing different feeding strategies; the former selecting items from the benthos and the latter straight from the water column. A. caudavittata had a higher mean !15N than O. mossambicus, but not high enough to imply they occupy different trophic levels. The mixing models based on stable isotopes that incorporate food assimilated over longer periods than stomach content analysis, suggested a greater similarity in the diets between species within each site. O. mossambicus is occupying some of the same habitats and using some of the same resources as the native fish, particularly A. caudavittata. However, based on behavioural differences, such as feeding strategies, and the observed differences in prey item proportions, along with the observations of high abundances of all species at many sites, it can be hypothesized that the fish species successfully co-exist with the present habitat and food resource availability. It would be prudent to examine other aspects of the ecology of O. mossambicus in this system to determine if they are impacting the system in ways other than its trophic and habitat interactions with other species