Invasive species represent a major concern for native flora and fauna in New Zealand waterways. Current surveying methods employed in the detection of these species typically rely on visually observing or catching fish and are often expensive and difficult to implement. Given that freshwater fish release organic materials into the water, and that some of these materials contain volatile elements that are then released into the air above the surface, it was hypothesised that domestic dogs (Canis familiaris) may be able to detect the presence of these species by smelling water samples that have contained them. In this study, four experiments were conducted to determine the validity of this hypothesis. Five pet dogs were trained using a go/no-go procedure to operate an automated apparatus that presented individual water samples through an opening in the front panel. Dogs were presented with samples from aquaria that had or had not contained fish. In Experiment 1, dogs were presented with water from aquaria containing koi carp (Cyprinus carpio) and aquaria containing no fish. In Experiment 2, koi carp samples were systematically diluted until concentrations similar to those found within the natural environment were reached. Experiment 3 sought to determine whether dogs could discriminate koi carp from a distantly-related fish (brown bullhead catfish, Ameiurus nebulosus). Experiment 4 replicated the previous experiment with a closely-related fish (goldfish, Carassius auratus). In all four experiments, dogs were able to correctly identify water that had contained koi carp and largely ignore water samples that had contained either no fish or other species of fish at above 80% accuracy. The overall results of this research indicate that dogs are able to accurately detect the presence of koi carp from water samples at concentrations similar to those found within the natural environment, and to discriminate between at least three species of fish. These findings suggest that dogs may have an important role to play in waterway conservation and management.