School level structural and dynamic adjustments to risk promote information transfer and collective evasion in herring

Abstract

1 transfer and collective evasion in herring 2 3Many large-scale animal groups have the ability to react in a rapid and coordinated manner to 4 environmental perturbations or predators. Information transfer among organisms during such 5 events is thought to confer important antipredatory advantages. However, it remains unknown 6 whether individuals in large aggregations can change the structural properties of their 7 collective in response to higher predation risk, and if so whether such adjustments promote 8 responsiveness and information transfer. We examined the role of risk perception on the 9 schooling dynamics and collective evasions of a large herring school (~ 60 000 fish) during 10 simulated-predator encounters in a sea-cage. Using acoustics we quantified swimming 11 dynamics, collective reactions and the speed of the propagating waves of evasion induced by 12 a mobile predator model. In the higher risk condition, fish swam faster, exhibited stronger 13 circular swimming pattern, and we found an increased correlation strength indicating that the 14 school had a greater ability to collectively respond to a perturbation. When exposed to a 15 simulated threat, collective evasions were stronger and behavioural change (evasion 16 manoeuvres) propagated more quickly within the school under environmental conditions 17 perceived as being more risky. Our results demonstrate that large schools make structural and 18 behavioural adjustments in response to perceived risk in a way that improves collective 19 information transfer, and thus responsiveness, during predator attacks. 20 22Keywords: Acoustics, collective evasions, information transfer, large aggregations, predation 23 risk, schooling behaviour. 24 25

Prey in oceanic sound scattering layers organize to get a little help from their friends

“…The variability of within group inter‐individual spacing was also affected by the presence of a dolphin within the school (Fig. ) with a marked decrease in variability when dolphins were present, even when controlling for taxa, similar to the increase in organization observed in individual schools in experiments exposing the group to a predator, a process shown to improve collective responsiveness to risk (Rieucau et al ). Note, however, that unlike observations on fish schools which respond more intensely at high densities (Rieucau et al ), we observed very similar responses by squid over a range of packing densities (here measured as inter‐individual distance), a measurement that was positively correlated with the target strength (e.g., length) of individual squid that made up each group in the absence of observed predation pressure.…”

Using unmanned aerial vehicle (UAV) surveys and image analysis in the study of large surface‐associated marine species: a case study on reef sharksCarcharhinus melanopterusshoaling behaviour

“…Further efforts should be directed at implementing automated video post‐processing to track and quantify swimming dynamics. Recent advances in automated tracking techniques such as the use of particle image velocimetry (PIV) allow researchers to measure swimming dynamics and collective reactions of pelagic fish with great precision, even in very dense shoals (Rieucau et al , a,b). To overcome the important challenge of controlling for the drone movement, our tracking algorithm could be implemented with optic‐flow based techniques.…”

Use of Drones in Fishery Science

“…Automated tracking techniques, such as the use of particle image velocimetry, have been successfully employed by researchers to measure collective reactions of pelagic fish (Rieucau et al. , ). A current challenge for the further development of UAS video analysis resides in the capability to estimate and subtract the UAS's movement.…”

“…employed by researchers to measure collective reactions of pelagic fish (Rieucau et al 2016a(Rieucau et al , 2016b. A current challenge for the further development of UAS video analysis resides in the capability to estimate and subtract the UAS's movement.…”