Hydrodynamic patterns from fast-starts in teleost fish and their possible relevance to predator-prey interactions.

Fast-starts are distributed over a wide phylogenetic range of fish and are used for different purposes such as striking at prey or escaping from predators. Here we investigated 42 fast-starts of rainbow trouts (Oncorhynchus mykiss) elicited by a startle stimulus. We investigated the patterns of water movements left behind by the escaping fish and their possible value as a source of information to piscivorous predators that rely on hydrodynamic sensory systems. Particle image velocimetry (PIV) measurements revealed a temporal extension of up to 25.5 min and a spatial extension of up to 1.53 m (extrapolated) for a certain flow structure called jet 1, that is the flow produced by the tail fin. Duration and spatial extension of jet 2, the flow produced by the body, were on average lower, and both jets differed in size. The fish escaped in a mean direction approximately parallel to jet 1, and antiparallel to jet 2, with a range well above 200°. This study quantified the flow patterns generated by escaping fish and, as piscivorous predators would greatly benefit from being able to analyse these flow patterns, provides cues for the behavioural and physiological investigation of hydrodynamic sensory systems.