Decadal-scale frequency shift of migrating bowhead whale calls in the shallow Beaufort Sea.

ABSTRACT

Automated and manual acoustic localizations of bowhead whale calls in the Beaufort Sea were used to estimate the minimum frequency attained by their highly variable FM-modulated call repertoire during seven westerly fall migrations. Analyses of 13 355 manual and 100 009 automated call localizations found that between 2008 and 2014 the proportion of calls that dipped below 75 Hz increased from 27% to 41%, shifting the mean value of the minimum frequency distribution from 94 to 84 Hz. Multivariate regression analyses using both generalized linear models and generalized estimating equations found that this frequency shift persisted even when accounting for ten other factors, including calling depth, call range, call type, noise level, signal-to-noise ratio, local water depth (site), airgun activity, and call spatial density. No single call type was responsible for the observed shift, but so-called "complex" calls experienced larger percentage downward shifts. By contrast, the call source level distribution remained stable over the same period. The observed frequency shift also could not be explained by migration corridor shifts, relative changes in call detectability between different frequency bands, long-term degradation in the automated airgun detector, physiological growth in the population, or behavioral responses to increasing population density (estimated via call density).