Utility of direct observational methods for assessing competitive interactions between non-native and native freshwater fishes

Biological invasions are a major threat to global freshwater biodiversity. Competition is a frequent negative interaction between non-native and native species, and this process is commonly quantified using indirect methods (e.g. dietary overlap, comparison of habitat use/selection). However, direct observation can provide crucial information on these biotic interactions. This paper reviews studies that used direct observational methods to quantify interactions between non-native and native freshwater fishes. Although laboratory and field studies using direct observations are not common, both have been used to demonstrate impacts by non-native species on native species. These effects include: (1) altered habitat selection and foraging behaviour and (2) disruption of reproductive behaviour. Direct observational techniques have great potential for quantifying the impacts of non-natives on native freshwater fishes and can detect negative behavioural impacts that would be missed using indirect methods.