Waves of agitation inside anchovy schools observed with multibeam sonar: a way to transmit information in response to predation

Abstract

M. 2006. Waves of agitation inside anchovy schools observed with multibeam sonar: a way to transmit information in response to predation. e ICES Journal of Marine Science, 63: 1405e1417.Most pelagic fish live in schools. To allow fast reactions, for instance to predator attacks, these collective structures require behavioural mechanisms authorizing fast, coordinated movements. Considering the large number of individuals constituting a school of small pelagic fish, a crucial premise to coordinated movements and school reorganization is an ability to transfer quickly and efficiently information across the whole collective structure. We observed anchovy school movements and reactions to sea-lion attacks while the ship was drifting in Peruvian waters. The main process of information transfer we could observe was that of waves of agitation crossing large anchovy schools. The average speed of these waves (7.45 m s À1 ) was much greater than the average 0.3 m s À1 school speeds measured during this experiment. The internal organization of each school modified dramatically after the waves of agitation had crossed them. Changes in school external morphology and internal structure were described and measured using geostatistics. Our results show that information transfer is a crucial process for the cohesion and plasticity of schools. As such, it allows efficient reactions of schools of pelagic fish to variations in their immediate environment in general, and to predation in particular.

When good neighbours become good friends: observing small scale structures in fish aggregations using multibeam sonar

“…The geostatistical results on the test series (Table 1) as well as on the whole images of the studied school (Table 2) showed that the anchovy schools examined in this study had broadly the same characteristics as those reported by Gerlotto et al (2006). For the purposes of the present study, we concentrated on structures that were less than 10 m in approximate diameter.…”

“…For this data set we followed the analytical approach described by Gerlotto et al (2006), which used a geostatistical analysis (Rivoirard et al 2000) of the acoustic data recorded from a horizontally orientated multi-beam sonar mounted on a drifting vessel. Small scale (∼40 cm diameter) spatial structure was observed within this school Anchovy are a relatively small schooling species, the other data set we examined was for larger gadoid fish in the North Sea.…”

“…Figure 1 shows one image recorded from the school (Gerlotto and Paramo 2003). In order to avoid bias in the data structure, such as those described by Gerlotto et al (2006) at short ranges, for each beam of a given image, we calculated the variogram of the raw polar acoustic data in one direction parallel to the beam axis. Finally, we computed a mean unidirectional variogram for the 60 beams.…”

Fast and accurate decisions through collective vigilance in fish shoals

“…Despite the fact that numerous studies have provided evidence in support of this functional benefit of larger group sizes, little is known about the decision-making mechanisms that underlie this key benefit of sociality. If we assume that members independently gather information and scan different parts of the environment, then the first individual to detect a given cue could initiate a response that proliferates rapidly throughout the group (14)(15)(16)(17). Under this scenario, improvements in both speed and accuracy of collective decision making with group size can result because a) the probability that the group contains individuals that are better at scanning increases with group size and/or b) individuals do not need to scan as much of their environment when in groups, effectively focusing their information gathering to a smaller area of space.…”

Overview of the Applications of Hydroacoustic Methods in South Korea and Fish Abundance Estimation Methods

“…Research also showed that the TS of anchovy from the side aspect was 2 dB higher than that from the dorsal research purposes are available in South Korea. Sonar systems are relatively new instruments, compared with echosounders, but have wide coverage for high-resolution data sets; thus, they have been used increasingly to understand the behavioral and geometric characteristics of marine organisms, in particular epi-pelagic fish schools (Gerlotto et al, 2006;Paramo et al, 2007) and precise topographic sea bottom measurements (Cutter et al, 2010). Therefore, trial research using existing sonar systems is expected to be performed in the near future.…”