Echograms of data collected with a side-looking DIDSON often show temporal patterns within the echo traces that are generated by fish swimming through the beam. For upstream migrating salmon, these patterns resemble the shape of caterpillars with legs extending to one side.
Frame by frame comparison of the echogram and the corresponding images shows that these “caterpillar patterns” are created by periodic changes in the range extent covered by the fish image. Each “caterpillar leg” coincides with a consistent position of the tail within the tail beat cycle. The time interval between successive peaks is thus related to the tail beat frequency. Tail beat frequency can potentially be useful for species identification and bioenergetics studies.
For more details see:
Mueller, A. M., D. L. Burwen, K. M. Boswell, and T. Mulligan. 2010. Tail beat patterns in DIDSON echograms and their potential use for species identification and bioenergetics studies. Transactions of the American Fisheries Society 139:900-910 Download pdf (379KB)
Here are some of the examples discussed in the paper:
Caterpillar patterns in the echogram of upstream migrating salmon.
Data provided by Debby Burwen, Alaska Department of Fish and Game.
DIDSON movie clip of a sockeye-sized salmon (at 16 m range) and a Chinook salmon (at 15 m range).
Data provided by Debby Burwen, Alaska Department of Fish and Game.
Zoom view of caterpillar pattern of upstream migrating salmon.
Data provided by Debby Burwen, Alaska Department of Fish and Game.
Echogram patterns of three adult eels with different geometries between
eel body, its trajectory and the transducer.
See DIDSON movie clips below.
DIDSON movie clip of eel 1.
DIDSON movie clip of eel 2.
DIDSON movie clip of eel 3.