Interaction between non-native predatory fishes and native galaxiids (Pisces: Galaxiidae) shapes food web structure in Tasmanian lakes

Abstract

Non-native fish invasions threaten native fauna andecosystem functioning, not least in isolated island lakes. In Tasmania, wherethe native fish are mostly galaxiids, 9 non-native freshwater fish species havebeen introduced over the past 150 years, with uncertain ecological outcomes. Weevaluated the effects of non-native predatory fishes (NNPF) and variousenvironmental and biological variables on the trophic niche of native fish(galaxiids) and potential cascading effects. We analysed Layman´s food web metricsbased on both stable isotope (δ15N and δ13C) values and fish stomach contentsin 14 shallow Tasmanian lakes along a NNPF abundance gradient. The food webmetrics calculated were (1) range of δ13C (CR) and δ15N (NR) centroid distance (CD)and (2) standard ellipse area. Our results showed that NNPF relative abundancein the fish catch per unit effort was negatively related to the galaxiids trophic niche metrics (e.g., CRG, NRG, and CDG), trophic position, and thepelagic contribution to the diet. Moreover, the proportion of galaxiids in thediet of NNPF was higher in turbid lakes. The zooplankton standard ellipse area wasnegatively correlated with the pelagic contribution to the NNPF diet, and NNPFrelative abundance was positively correlated with the maximum body size ofcalanoid copepods. While our results suggest a negative effect of NNPF on thetrophic niche of galaxiids, the cascading effect on phytoplankton biomass wasweak. Non-native predatory fish affect native fish prey, and the outcome of theseinteractions should be considered for conservation purposes, particularly forisland lakes, such as those in Tasmania.