ECOLOG is the acoustic instrumentation system developed at the Bedford Institute of Oceanography in Dartmouth, Nova Scotia, for research on and survey of fish populations. A dual-beam, vertical echo sounder is towed from a ship, transmitting sound and receiving echoes at a frequency of 50 or 120 kHz (echoes from swim bladders of fish are strongest in the 30-120 kHz range). Echoes are received simultaneously on narrow- and wide-beam acoustic receivers. Echo signals are digitized and recorded on magnetic tape. Computer programs on board ship, or later in the laboratory, give position, depth, number and size of fish encountered, allowing estimates of local abundance.