The ageing of the low-frequency water disturbances caused by swimming goldfish and its possible relevance to prey detection.

Wakes caused by swimming goldfish (Carassius auratus) were measured with a particle image velocimetry system and analyzed using a cross-correlation technique. Particle velocities in a horizontal plane (size of measuring plane 24 cmx32 cm or 20 cmx27 cm) were determined, and the vorticity in the plane was derived from these data. The wake behind a swimming goldfish can show a clear vortex structure for at least 30 s. Particle velocities significantly higher than background noise could still be detected 3 min after a fish (body length 10 cm) had passed through the measuring plane. Within this time span, the lateral spread of fish-generated wakes could exceed 30 cm for a 10 cm fish and 20 cm for a 6 cm fish. Measurements in a man-made open-air pond showed that water velocities in a quasi-natural still water environment can be as small as 1 mm s(-)(1). Background velocities did not exceed 3 mm s(-)(1) as long as no moving animal was present in the measuring plane. The possible advantage for piscivorous predators of being able to detect and analyze fish-generated wakes is discussed.