Echograms of data collected with side-looking imaging sonars (e.g., DIDSON, ARIS) often show temporal patterns in 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 with its corresponding images shows that these “caterpillar patterns” are created by periodic changes in the fish image's range extent. 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. An interesting aspect of this is that this could potentially be useful for species identification and bioenergetics studies.
Here are some of the examples discussed in the paper:
Caterpillar patterns in the echogram of upstream migrating salmon (left). Zoom view showing the caterpillar pattern at the sample resolution of the underlying data (top right). DIDSON movie clip of a Chinook salmon between 15 and 17 m range, followed by three sockeye-sized salmon between 17 and 19 m range (bottom right). Data provided by Alaska Department of Fish and Game
Echogram patterns (top) and movie clips (bottom) of three adult eels with different geometries between eel body, trajectory
and the transducer.
A terrestrial analogy not discussed in the paper: tail-beat patterns in echograms are like animal tracks. Both record differences in gait and body shape over time.
The tracks of three river otters moving along with their characteristic hop-hop-slide, superimposed on a track left behind by a lynx ambling across.