Drift dispersal of mayfly nymphs in the presence of chemical and visual cues from diurnal drift‐ and nocturnal benthic‐foraging fishes

Abstract

1. The drift of Baetis thermicus nymphs in the presence of chemical, visual and hydrodynamic cues, considered individually and in combination, produced by different predatory fishes was examined experimentally in laboratory streams. Masu salmon (Oncorhynchus masou) and freshwater sculpin (Cottus nozawae) are typical drift‐ and benthic‐foraging fishes, respectively.2. Observations of fish swimming in the streams revealed differing diel periodicity between the species; sculpin were nocturnal foragers and salmon diurnal.3. The drift rate of Baetis by night increased in the presence of chemical cues from sculpin, with other cues having no interactive effects. In contrast, the drift rate increased primarily in the presence of both chemical and, particularly, visual cues from salmon, although no additional effects were found for any non‐visual cues. Visual cues could enable Baetis to assess precisely the predation risk from foraging salmon by day, whereas Baetis could not use visual cues to detect sculpin either at night, because of the low light intensity, or during the day, because of the low activity of sculpin at that time.4. In natural streams, which are often inhabited by several predatory fish employing different modes of foraging, invertebrates may be able to precisely assess the risk and effectively to avoid predators by using cues unique to each.