Abstract
The use of active high frequency echo sounders for commercial activities and marine research has been increasing in recent years. Compared to other anthropogenic noise sources, high frequency echo sounders have received little attention in terms of their potential impacts on marine life. However, while these devices typically operate at centre frequencies outside the hearing range of most marine species, recent work has demonstrated that they may produce unintended energy at lower frequencies. These lower frequencies may extend into the audible range for several species of marine mammals and have the potential to affect their behaviour (Deng et al., 2014).
This study measured the full frequency spectrum of the SIMRAD EK60 echo sounder operating at target frequencies of 120 and 200 kHz. This echo sounder is widely used in the marine science and fish stock assessment communities. Results showed that the generation of both signal types produced broadband energy at frequencies below the system’s target frequencies of 120 kHz and 200 kHz, in the range of 70-100 kHz and 90-150 kHz for the 120 and 200 kHz signals, respectively. For harbour porpoises (Phocoena phocoena), the target frequency of the 120 kHz signal and subcomponents of the 200 kHz signal fall within the region of highest hearing sensitivity and are thus potentially detectable. While less sensitive at higher frequencies, measured signal levels indicate that harbour seals (Phoca vitulina) will likely also be able to detect the lower frequencies (70-100 kHz) generated by both signal types. Detection of these signals will be dependent on source power, signal duration, repetition rate, signal directionality and the animal’s proximity to the beam centre. In addition, detection will be dependent on water depth, local ambient noise and seabed and surface scattering, all affecting signal propagation characteristics.
Given the theoretical detectability of these lower frequencies by marine mammals, both signal types have the potential to elicit behavioural responses towards them. This should be considered in environmental impact assessments of activities using these devices and when planning marine mammal monitoring studies alongside ecosystem studies using active acoustic sonar systems.