Investigation of fin whales using ocean-bottom recordings

Instruments used for seismic monitoring (OBS) have been recording baleen whales along with the target data. These long-term datasets provide valuable information for the study of large cetaceans that would otherwise be difficult to obtain due to economic and logistic reasons. Fin whales are classified as ‘Endangered’ species and therefore knowledge about population size and spatial and temporal distribution patterns is essential for good management strategies. In Portugal, sightings of fin whales off mainland waters are rare and are insufficient to assess any kind of trend. In this context, seismic datasets may become one of the primary sources of information on a year-round basis. The general aim of this thesis was to demonstrate the use of acoustic datasets, collected for non-biological focussed studies, such as seismic data, to study fin whales. Between September 2007 and August 2008 a network of 24 OBS was deployed off southwest Portugal with the main aim to obtain data to study the microseismicity originating from potential tsunami sources. During this time fin whale sounds were also recorded. The main acoustic signal produced by fin whales used in this thesis was the 20-Hz call. A detection process to detect the 20-Hz call was developed (Chapter 2) and the resulting dataset of automatic detections was used to: 1) estimate a relative location, i.e. a range from the recording instrument, of the automatic detections with the single-station method and develop a classification scheme that help deciding if a detection was from a 20-Hz call and if the range estimates were inside the critical area of the single station (Chapter 3); 2) characterize the 20-Hz call and assess the existence of different acoustic groups (Chapter 4); 3) obtain an average fin whale density and abundance (Chapter 5); and 4) assess the impact of acoustic interference on the 20-Hz call and infer about the depth of the calling fin whale (Chapter 6). This thesis showed how OBS data can be used in different types of studies about fin whales and provide valuable information for cetacean conservation.