Seasonal, diel, tidal, and geographical variation in male harbour seal (Phoca vitulina) vocalizations in southern Scandinavia

Male harbour seals make underwater calls in the mating season but remarkable little is known about the circumstances under which the calls are made and the role of the calls in mating.
This study reports on the first recordings of harbour seal calls from Danish waters and investigates the feasibility of using passive acoustic monitoring to determine seasonal and possible diel and tidal variation in calling behavior as well as site preference.
Two autonomous data-loggers (Loggerheadinstruments, Florida) were deployed approximately 2 meters above the sea bed in shallow waters off a haul-out site (Blinderøn) in Limfjorden, Denmark, from June 9th to July 7th 2011. Same method was used at a haul-out site in the Wadden Sea (north of the island Rømø), Denmark, from July 7th to August 1st, 2010.
More than 2000 calls were recorded in Limfjorden and more than 5000 calls from Rømø. Individual calls varied in structure but overall resembled signals recorded by others. Peak energy was around 120 Hz and little energy above 2 kHz.
The recordings from Limfjorden showed that the calling begins by the end of June, and ends in early August. A strong diel pattern in the vocalization rate with a peak around midnight was found. Recordings of calls on both sides of the haul-out site (a narrow sand bar) showed no indication of preference of one side over the other. Recordings from Rømø showed peak in call-rate at high tide.
The study has demonstrated passive acoustic monitoring as a feasible method for studying underwater calling behavior of harbour seals. This gives prospects of a tool, which can aid in identifying essential underwater areas used in mating behavior by the seals, and can allow assessment of the need for adequate protection of these areas as required by the Habitats Directive.