Rice’s whales in the northwestern Gulf of Mexico: call variation and occurrence beyond the known core habitat

Abstract

The endangered Rice’s whale Balaenoptera ricei, with fewer than 100 individuals remaining, is the only year-round resident baleen whale found in the Gulf of Mexico (GOM) and occurs primarily along the northeastern shelf break near De Soto Canyon. Historical whaling records and predictive density modeling suggest that these whales potentially could occur more broadly throughout the GOM. High levels of anthropogenic activities in the GOM, including oil and gas exploration and extraction, fisheries, shipping, and the unprecedented Deepwater Horizon oil spill, highlight the need to better understand the distribution, ecology, and threats to this small population to improve protection of these endangered whales. We used long-term passive acoustic recordings from the northwestern GOM shelf break to explore the extent of Rice’s whale distribution in the northern GOM and to evaluate whether they exhibit seasonal movements throughout this range. We describe 6 new stereotyped variants of Rice’s whale long-moan calls, found predominantly in the western GOM, that share distinctive features with typical eastern long-moans, including a 150 Hz starting tone, an approximately 100 Hz tail with amplitude modulation, and a long call duration ranging from 10 to 35 s. Western long-moan variants were detected at 3 northwestern sites, occurring sporadically throughout the year on as many as 16% of days at the westernmost site, and infrequently at an eastern core-habitat site. These results indicate that some whales persistently occur over a broader range in the GOM than previously understood, which is important to consider when designating critical habitat and assessing threats to this Critically Endangered species.