Comparative trophic impacts of two globally invasive cyprinid fishes reveal species-specific invasion consequences for a threatened native fish.

Abstract

1. Predicting the ecological consequences of invasions by non-native species is a fundamental aspect of their risk-based management. As impacts can include the negative consequences of resource sharing with native species, the application of in situ cohabitation field experiments can test hypotheses relating to invasion ecology via competitive interactions and processes. As fish are adaptable and tractable experimental organisms, they are strong model species for use in studies on competitive interactions. 2. The trophic consequences of invasion by two globally invasive freshwater fish, common carp Cyprinus carpio and goldfish Carassius auratus, were tested on the threatened native fish crucian carp Carassius carassius. Cohabitation experiments, completed in pond enclosures, used all species in allopatric and sympatric treatments using a substitutive design where the number of fish oper treatment was kept constant. Stable isotope analysis (δ13C and δ15N) assessed alterations in the trophic ecology of each species across treatments, with growth rates used to assess any consequent impacts on the fish. 3. When in sympatry with C. auratus and C. carassius, the C. carpio isotopic niche was at a significantly lower trophic position compared to allopatry. This resulted in niche overlap with C. auratus, whilst for C. carassius, their isotopic niche shifted to a higher trophic position compared with allopatry. The growth rate of C. carpio was always significantly higher in sympatry than in allopatry, whereas growth rates for C. carassius and C. auratus were significantly depressed in C. carpio presence. In contrast, the isotopic niche sizes and positions and growth rates of the Carassius fishes were not significantly different between allopatry and when they co-habited. 4. Plasticity in the isotopic niche of C. carpio resulted in significant alterations in their trophic position between allopatry and sympatry and, when coupled with their depressed growth in allopatry, suggests the competitive processes driving this were intra-specific rather than inter-specific. This then resulted in detrimental impacts in cohabiting Carassius fishes. These results emphasise that ecological consequences of C. carpio in invaded freshwaters include impacts on the trophic ecology of native fishes.