Spectrograms from Axial instrument illustrating the 2 primary song sequences observed.
Regional map of the northeast Pacific Ocean showing locations of all instruments used in this study, along with a timeline indicating when they were deployed. At the scale of this map, KENE and KEMF are effectively co-located, but KENE is displayed slightly offset to the north for clarity (AX, Axial; CI, Cascadia Initiative; CZ, COLZA).
Weirathmueller, M.J., K.M. Stafford, W.S.D. Wilcock, R.P. Dziak, and A.M. Tréhu (2017): Spatial and temporal trends in fin whale vocalizations recorded in the NE Pacific Ocean between 2003-2013. PLoS ONE, 12 (10), e0186127, doi:10.1371/journal.pone.0186127.
Fin whales (Balaenoptera physalus) spend most of their lives ranging widely throughout ocean basins and thus can be extremely difficult to study. Nevertheless, understanding the fin whales’ population structure is of particular importance for management and recovery efforts due to their status as endangered under the U.S. Endangered Species Act.
Fin whales produce relatively simple, repeated signals that appear to have not changed significantly over time. The most commonly observed vocalization produced by fin whales—the “20 Hz pulse”—has been recorded throughout the world’s oceans. In order to study the long-term stability of fin whale singing behavior, Weirathmueller et al. (2017) reviewed the frequency and the time interval between individual fin whale vocalizations over a 10-year period (2003–2013) using instruments that measure sound and motion on the seafloor. These instruments were distributed approximately 1000 km across the northeast Pacific Ocean.
The authors found that the peak frequency of the fin whale call had decreased (at a rate of 0.17 Hz/year) and the time interval between their calls increased (at a rate of 0.54 seconds/year) over the 10-year period of observation.
A similar overall decadal decrease in frequency has been documented in numerous blue whale populations. Increasing population size and/or increases in ambient noise have been given as possible explanations for decreasing frequencies. Other marine mammal species have shown evidence of long-term shifts in call frequency that have been attributed to noise resulting from human activity, although these tend to be associated with an increase, rather than a decrease, in frequency. This study illustrates that if song characteristics are going to be used for determining population structure in fin whales, then long-term, decadal-scale data must be considered to avoid potentially identifying populations as acoustically distinct when they are not.