Acoustic environmental impact of Swedish Navy active sonars

Anthropogenic sound may affect marine life in different ways. Hearing and internal organs can be directly affected, but sound may also induce behavioural effects that in turn may affect critical life functions such as reproduction and feeding. Several NATO nations and other nations have performed research on the effects of active sonars on marine mammals and developed new results and criteria for safer use of active sonar. However, because the sonars of the Royal Swedish Navy are designed to operate in the shallow brackish water of the Baltic Sea, they use other frequencies and pulses than the previously studied systems. Hence, there is a lack of scientific results that can be applied to estimate the environmental effects of Swedish Navy sonars. The aim of this report is to present an overview of marine mammals in Swedish waters and their sensitivities to acoustic disturbances, to summarise the acoustic properties of the sonar systems used by the Swedish Navy, and to estimate standoff ranges (i.e. ranges from a transmitter within which significant negative effects can occur). The contents of this report can be used to develop an environmental policy for safe use of Swedish Navy sonars. Swedish Navy use sonars to search for and classify objects in on the surface, in the water volume, on the seabed and in the seafloor. A sonar system can typically use a range of different frequencies and pulses. This report focuses on the VDS, HMS and mine hunting sonars used by the Royal Swedish Navy. These systems transmit at frequencies between 20 and 400 kHz. There are four resident species of marine mammals in Swedish waters. There are three seal species; the harbour seal (Phoca vitulina), the grey seal (Halichoerus grypus) and the ringed seal (Pusa hispida botnica), and one whale species, the harbour porpoise (Phocoena phocoena). The distributions of these species overlap. Swedish law prohibits intentionally disturbance or wounding of these animals. All four species have relatively good hearing at Swedish Navy sonar frequencies, but seals are not sensitive to sounds above 100 kHz; porpoise hearing extends up to 200 kHz. This report estimates risks of negative effects on marine mammals by calculating standoff ranges from sonar transmitters. If an animal within the relevant standoff range from a sonar transmitter, it risks injuries or behavioural disturbances. The calculations are based on sound propagation characteristics of the Baltic and Skagerrak seas combined with the most recent scientific data on sound level thresholds for physical and behavioural effects. The results show that the VDS sonars display the greatest standoff ranges among the Swedish Navy's sonar systems. The behaviour of harbour porpoises can be significantly affected at ranges up to 20 km and they may risk hearing damage at ranges up to 6 km. Seals are less sensitive and their standoff ranges are below 2.5 km. The lower salinity in the Baltic Sea implies a lower acoustic absorption, which in turn leads to standoff ranges that are 1.5 to 3 times longer than compared to results in Skagerrak. The results of this report may be seen as a first step in a risk assessment of the active sonar systems of the Swedish Navy. Such an assessment needs to be complemented with information on sonar use and the likelihood of encountering a marine mammal within the standoff range as well as animal behaviour upon exposure to sonar sounds.