Disturbance cues in freshwater prey fishes: do juvenile convict cichlids and rainbow trout respond to ammonium as an ‘early warning’ signal?

Abstract

Several freshwater invertebrate and vertebrate prey species rely on chemosensory cues, including non-injury released disturbance cues, to assess and avoid local predation threats. The prevailing hypothesis is that a pulse of ammonia released by disturbed or stressed prey functions as the disturbance cue. Here, we test this hypothesis in two phylogenetically distant freshwater prey fishes, convict cichlids and rainbow trout. In our first experiment, we measured NH4+ concentration in tanks containing shoals of cichlid or trout before and after exposure to a realistic model predator (or left undisturbed). We failed to find an increase in ambient NH4+ concentration for either cichlids or trout. In our second experiment, we exposed cichlids or trout to NH4+ at 0.1 or 0.5 mg L−1 (or a distilled water control) and measured the change in antipredator behaviour (time moving, foraging rate and area use). We found no consistent increase in antipredator behaviour in response to NH4+. In our third study, we exposed cichlids and trout to the disturbance cues of cichlids or trout (versus the odour of undisturbed donors). We found significant increases in antipredator behaviour, regardless of donor species, for both cichlids and trout. Thus, the results of our first two experiments do not support the hypothesis that ammonium functions as a disturbance cue in prey fishes. However, the results of our final experiment do confirm the use of disturbance cues in convict cichlids and rainbow trout and support that hypothesis that the disturbance cue is indeed some generalized metabolic byproduct.