During the breeding season, male harbour seals (Phoca vitulina) produce underwater calls used in intra-sexual competition and advertisement. One call type, called a roar, has been documented in every population of this species that has been studied. Because calls vary in structure and temporal patterns among populations, it can be inferred that regional vocal dialects may exist, and that the influence of local environmental and biological conditions may affect the timing of calls. Breeding calls have only been studied in relatively few locations worldwide; however, the effect of ambient noise on the underwater vocal behaviour of harbour seals has not been studied. I investigated the temporal patterns, structure and complexity of harbour seal breeding calls at Hornby Island, British Columbia. Underwater recordings were made near the south end of Hornby Island in the summer breeding season of 2014 using a single omnidirectional hydrophone while concurrent visual observations were conducted at a nearby site on shore.
I investigated the relationships between roars per hour, tide level, ambient noise and time of day. Logistic regression showed that roars were over eight times more likely to be heard during night-time hours than during the day. When roars were heard, roar number was most influenced by time of day and the progression of the breeding season, with more roars heard later into the breeding season. Roar density was inversely related with ambient noise; however, ambient noise and time of day were highly auto-correlated, and therefore the effects of these two variables could not be assessed independently. Whereas harbour seal call timing in other areas has been attributed to tidally-driven haul-out patterns, the tide level at Hornby Island did not statistically correlate with roar density. I hypothesize that ambient noise may be a cause of the strong diel pattern of call density at Hornby Island, and suggest further study to determine the mechanistic link between ambient noise cycles and calling behaviour.
Four breeding call types were identified, one of which was the ‘roar’ call described in every population that has been studied. The structure of the roars at Hornby Island was comparable to those of other populations, but displayed wide variation in several parameters. Each of the three non-roar call types were distinguished from roars qualitatively by aural-visual classification, and quantitatively by trained linear discriminants analysis (LDA). Agreement between these two classification systems was 88%, suggesting that the four call types were distinct. The three non-roar call types contained five call subtypes which were also identified through aural-visual and LDA classification. Agreement was slightly lower at 74%, but more variable, suggesting that some call subtypes were more distinct than others. One non-roar call type, the sweep, was distinct from any call previously described for harbour seals worldwide. I suggest that more than one of these call types are used by each individual, supporting the results of a previous study in California which identified several call types produced by harbour seals. If this is the case, then the vocal breeding repertoire of harbour seals at Hornby Island is more complex than that of most previously studied populations. Further study of this population with the ability to localize calls and identify individuals is required to support or refute the hypothesis that individual harbour seals at Hornby Island produce several call types
