Converging results in different scientific fields (behavioural ecology, fisheries
biology, acoustic tagging, fisheries acoustics, behavioural modelling) suggest the
existence of “micro-groups” inside fish schools. These would comprise a few (5–10) fish
maintaining contact during a period long enough to allow individuals to recognise each
other. It is hypothesised that they would prefer to share the space with familiar rather
than anonymous conspecifics. To evaluate whether acoustic methods could be used to
recognise “micro-structures” inside fish schools and help test the “micro-group” hypothesis
we analysed acoustic data from anchovy schools off Peru, and gadoids in the North Sea.
Data collection used a multibeam sonar (Reson SeaBat 6012). In the Peruvian case study,
the sonar was mounted set horizontally on a drifting research vessel and the internal
structure of the schools of anchovies was analysed, although individual fish could not be
discriminated. In the North Sea case study, the sonar was orientated vertically above a
demersal trawl to allow observation of individual fish entering the trawl. Geostatistical
analyses were used to evaluate the existence of small spatial structures in anchovy
schools. In these schools, “micro-structures” with a scale as small as 0.5 m were observed
acoustically. For the gadoids nearest neighbour distance (NDD) measurements were carried
out, suggesting that the fish aggregated in small groups (2 to 25 individuals, with an
average of 3.7 fish per group) in the trawl catches. The perspectives and limitations of
these results are discussed.