Inter- and intra-annual variation in the frequency of blue whale songs in Aotearoa New Zealand

Blue whales produce stereotyped songs that are declining in fundamental frequency worldwide. We examine inter- and intra-annual frequency variation in the New Zealand population, which can be monitored year-round in the South Taranaki Bight. We document a decline of 0.13 Hz/yr between 1964 and 2025, and 0.08 Hz/yr between 2016 and 2025. Furthermore, we demonstrate higher fundamental frequencies from February to June, and a significant positive relationship between fundamental frequency and the song intensity index. These results indicate that blue whales sing at higher fundamental frequencies during the putative breeding season. Future work should aim to resolve relationships between song and morphological, demographic, behavioral, and cultural factors.