Escape behaviour of solitary herring (Clupea harengus) and comparisons with schooling individuals

The escape behaviour of solitary herring (Clupea harengus L.) startled by a sound stimulus was observed by means of high-speed video-filming. The results were compared with data from a previous study on the escape behaviour of schooling herring. Escape responses were divided into ''away responses'' and ''towards responses'' according to the orientation of the C-bend of the body relative to the stimulus. The proportion of away responses was smaller for solitary than for schooling herring. In solitary herring, the subsequent escape trajectories of fish making initial away responses showed a bimodal pattern of distribution, with modes at 130 and 180 degrees from the stimulus. Trajectories following towards responses, however, were mainly within the semicircle directed at the stimulus, and their pattern of distribution differed from that of away responses. This result contrasts with observations on schooling herring, whose trajectories following both initial away and towards responses are directed away from the stimulus. In addition, we measured the response latency, defined as the interval of time between stimulus presentation and the first detectable movement of the fish. Solitary herring showed a higher proportion of short-latency responses (latency <50 ms) than schooling herring. Different behaviours appear to be exhibited by herring depending on whether they are solitary or within a school. We hypothesize that schooling may raise the threshold for initiation of fast escape responses, giving longer latencies and slower responses which are more appropriate in their directionality and reduce the possibility of collisions with neighbours. In addition, we suggest that schooling behaviour enhances the directionality and coordination of the escape response of the whole school, possibly increasing the probability of surviving a predator attack.