Resumen
The spatial extent of a dual-frequency echosounder and its potential impact on cetacean species were examined. Sound emissions of output frequencies of 83 kHz and 200 kHz were tested at a maximum distance of 400 m. This is the minimum vessel approach distance for killer whales (Orcinus orca) in southern British Columbia, which was introduced as a measure for limiting disturbance of southern resident killer whales (SRKWs). The experiment was conducted in shallow (34 m) and deep (220?235 m) water. Recordings were made at depths of 5 and 30 m for both locations, as well as at 100 and 200 m in the deeper water to examine the effect of the echosounder through the water column and at SRKW diving depths. The recordings showed that 200 kHz echosounder emissions were contained within a 250 m radius of the source, with most of the acoustic energy focused within 100 m in shallow waters. For the 83 kHz signal and for deeper water testing of the 200 kHz signal, the echosounder transmissions exceeded the 400 m threshold, intimating that whales could experience noise additions of 30 dB or more above the ambient level at the minimum vessel approach distance. Evaluating the noise additions to the ambient level from the echosounder in frequencies used by SRKWs for echolocation (15?100 kHz) further showed the potential impact on whales in close proximity to vessels (=400 m) when using echosounders or fish-finders.