Spotted seal Phoca largha underwater vocalisations in relation to ambient noise

Abstract

Animals use different strategies to adjust their vocalisations to compensate for ambient noise interference. This is true for some marine mammals, especially cetaceans, but relatively little is known about this for pinnipeds. We recorded 4 major call types (drum, growl, knock, and sweep) of spotted seals Phoca largha in Liaodong Bay, China, to investigate if seals adjusted their vocalisation parameters in relation to broadband (50-4000 Hz) ambient noise recorded immediately preceding each seal vocalisation. Regression analyses showed that the received level of growls, in both broadband and 1/3-octave bands, centred at 200 and 400 Hz, significantly increased with increasing ambient noise levels. These relationships were not observed in the other 3 call types. Further, regardless of call type, the duration, centroid frequency, and root mean squared bandwidth parameters showed no statistical relationship with noise levels. The noise measured in this study had relatively low broadband levels of 116-132 dB re 1 µPa, and no masking was predicted for any of the 4 call types at 200 and 400 Hz when applying a standard critical ratio approach. It is therefore possible that the ambient noise levels in the study area were not sufficiently loud to induce vocal compensation to avoid masking, but loud enough for the seals to adjust their growl vocalisations. To our knowledge, this study is the first to investigate potential vocal adjustment of spotted seals in relation to ambient noise and is important in light of increasing anthropogenic noise in the marine environment.