Trophic flexibility and wetland dominance: the diet and population structure of an invasive fish species, Geophagus brasiliensis (Quoy and Gaimard, 1824)

Sam J Attwater

Invasive species are both benefiting from and fuelling a decline in freshwater ecosystems. The Pearl Cichlid (Geophagus brasiliensis) is a euryhaline fish native to South America that has shown remarkable adaptability to novel freshwater and brackish environments in several continents, including Australia, Asia and North America. Within Australia, established feral populations are found in Queensland, New South Wales and Western Australia. The species was first reported in Western Australia in 2006 from Bennett Brook, a tributary of the Swan River near Perth. Since that time, the species has invaded numerous freshwater environments on the Swan Coastal Plain (SCP), which is within south-western Australia, a global biodiversity hotspot. Since its initial detection, the species has substantially expanded its range across the SCP and has been detected in both lotic and lentic systems. However, the population status and feeding ecology in all but one of the lotic systems (Bennett Brook) were previously undescribed. This study aimed to determine the population viability and feeding ecology of G. brasiliensis in lentic systems to provide insight into their potential ecological impact in wetlands of the SCP. The study spanned five different wetlands and two seasons, and combined fyke netting and electrofishing to capture fish, littoral sweep sampling to capture available macroinvertebrates, and gut content analysis to characterise dietary and prey assemblage composition. Spatial, seasonal, and ontogenetic patterns were quantified using ANOSIM, PERMANOVA, and SIMPER, while reproductive traits were assessed using maturity staging and gonadosomatic indices (GSI). It was found that G. brasiliensis exhibits all the traits characteristic of a successful invasive species, such as evidence of early maturation and prolonged recruitment, a broad diet, habitat generalism, and a lack of predators. All wetland sites supported self-maintaining populations, demonstrating strong summer recruitment, particularly in response to rising temperature cues. All populations had a generalist diet that varied with prey availability, namely microcrustacea (Copepoda, Cladocera, Ostracoda), while larger fish exhibited selective predation on more nutritious prey items such as Chironomidae, Gastropoda, and Atyidae. This dietary composition and prey selection closely matched that of the primarily carnivorous native fish species. Furthermore, Pearl Cichlids exhibited size-based dominance over the co-occurring native freshwater fishes. Their ability to track seasonal prey availability, exploit benthic habitats, and reproduce vigorously positions them as a particularly challenging invader capable of reshaping wetland food webs and dispersing through drains and estuarine corridors. In addition to this species’ ecological traits, the detection of the Asian Fish Tapeworm (Schyzocotyle acheilognathi) in these fish posed an additional ecological risk. This discovery implicates the potential spread of this parasite with the rapid colonisation of G. brasiliensis into native species in neighbouring freshwater and brackish systems. The magnitude of the detected impacts of this highly invasive species highlights the need for prompt, targeted research into distribution mapping, dispersal mechanisms, and ecological disruption, and for the urgent development of potential control strategies.

Trophic flexibility and wetland dominance: the diet and population structure of an invasive fish species, Geophagus brasiliensis (Quoy and Gaimard, 1824)

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