Intra-patch clustering in mysid swarms revealed through multifrequency acoustics

Simultaneous multifrequency acoustic echosounders (70, 120, 200, 333, and 710 kHz), net tows, and video sampling were conducted on dense mysid aggregations in coastal waters off OR, USA, to study intra-patch dynamics in mysid swarms. Multifrequency differencing techniques demonstrated submetre, intra-patch clustering of individual mysids of different frequency response characteristics in all 33 samples collected. For samples that contained multiple sizes of mysids, frequency response curves showed a consistent peak 200 kHz. Samples dominated by only a single size class of small (5 mm long mysids) had a peak in scattering at 710 kHz. Intra-patch clustering of heterogeneous individuals shown here demonstrates patchiness at the scales of interactions among individual mysids; however, this patchiness is likely averaged out at the scale of large predator-mysid swarm interactions or the scale typical of most scientific surveys.