Blue and fin whales in the Northern Mariana Islands: Their call characteristics and occurrence.

Many baleen whale vocalizations are species-, or even population-specific and can be used to monitor their occurrence. Although baleen whale occurrence has been well studied in parts of the Pacific Ocean, little is known about the seasonal distribution of blue and fin whales in the Western Pacific. Since 2010, a more concerted visual and acoustic survey effort has occurred around a very remote region of the western Pacific Ocean: the Northern Mariana Islands. Passive acoustic data were collected at two locations in the Northern Mariana Islands, Saipan and Tinian, from 2015-2017 and they were analyzed for call characteristics and occurrence of blue whale and fin whale calls. Low levels of Central North Pacific blue whale tonal calls were detected year-round with peaks in the winter (December) and summer (June). There was a clear seasonal pattern in fin whale calls (20 and 40 Hz calls), with the majority of detections occurring during winter and spring. Moreover, two unknown low-frequency sounds were detected, one tonal and one pulsed. The former was more common at Tinian and the latter at Saipan. By providing their acoustic features and occurrence patterns, we aim to facilitate future identification of their source. Additionally, the observed seasonal patterns in blue and fin whale call occurrences may offer insights into their movement patterns in this remote region.