Hydroacoustic ambient noise typical for the inner part of Gullmarn

This report presents the analysis of hydrophone recordings from the inner part of the Gullmarn fjord, during the periods September 2013, May 2014 and October 2014. The purpose of the analysis work originated from observations made by Marinbasen and FMV, who recorded the background noise during May 2014. In the recordings a previously unknown type of sound, in the form of short impulses was detected. In addition, low frequency tones of unknown origin were also observed in the recordings. In order to further investigate these two phenomena, as well as to study the noise from the new road ferry between Skår and Finnsbo, a new measurement campaign was carried out by Marinbasen during October 2014, where FOI assisted at the site with personnel. In addition to strong, impulsive sounds, a new type of broad band sound in the range 20-2000 Hz was detected at this campaign, the sound appearing repeatedly and lasting for several seconds. Litterature studies, contacts with marine biologists and analysis of the recordings from October 2014 show unanimously that the broad band, several seconds long lasting sound is a so called roar from the harbor seal. A study of the extent of these roars shows that they mostly appear during the night, and mostly from the direction of the island Ärtern. The time of year for these observations, October, is remarkable, since the harbor seals are reported to roar during mating season only, which in the current regions occurs during May-August. The fin-slap of the harbor seal has also been studied during this work. The conclusion of the analysis is that many of the short (in the order of 1 ms) impulses found in the October 2014 recordings are generated by fin-slaps. The existence of fin-slap sounds during the other measurement campaigns cannot be affirmed with 100% confidence. Unique properties of the sound recorded during May 2014, which was named as klatsch-sound, have not been possible to identify in a feasible manner. Thus, no conclusion can be drawn as to whether this type of sound was recorded during the other two measurement campaigns. However, sequences of a crackling (Swedish: knastrande) sound having similar time series and spectrogram as the klatsch-sound, were found both in the September 2013 and in the October 2014 recordings. A comparison of the low frequency tones, noted during the May 2014 recordings, with the other measurement campaigns shows no obvious resemblance with respect to frequency. The few measurable passages of the new ferry Saturnus are disturbed by background noise due to the bad weather, however the results still clearly show that the noise from Saturnus does not increase the ambient noise levels compared to the old ferries. The results and conclusions of this work lead to the recommendation to perform an extended analysis of the measured data, and to possibly carry out new measurements, in order to answer the remaining vital questions.