Introduction & Objectives

Since 2007, visual and acoustic cetacean monitoring efforts in the Mariana Islands have documented the occurrence of humpback whales (Megaptera novaeangliae) during winter months. However, these efforts have focused mainly around the southern portion of the archipelago, in particular Saipan and Tinian. This project analyzed passive acoustic data for humpback whale song near the islands of Maug and Pagan in the northern portion of the archipelago. Data were recorded using Ecological Acoustic Recorders (EARs) from April 2009 through February 2010 (Pagan) or October 2010 (Maug). EARs sampled for 30 seconds at 15-minute intervals with an effective recording bandwidth of 0-20 kHz. Analysts visually browsed spectrograms from the entire Pagan dataset and from the winter months (Nov-May) at Maug. 

Technical Approach

Acoustic data were recorded using bottom-mounted Ecological Acoustic Recorders (EARs) deployed at depths between 10-20 m from 2009-2010 off the islands of Maug and Pagan. The EARs sampled at 40 kHz for an effective bandwidth of 20 kHz, and recorded data for 30 s “on” at 15 m intervals (3.3% duty cycle).

The Maug EAR recorded from 04/29/2009 - 10/17/2010, and the Pagan EAR recorded from 04/23/2009 - 02/23/2010.  Data were searched manually by analysts who visually scanned spectrograms of each individual 30-s recording using the Matlab program Triton. The search effort focused on winter months (January to March) when humpback whales were most likely to be detected, as well as adjacent months in late autumn and early spring when data were available. Two recording periods were analyzed from the Maug EAR: late April 2009 - May 2009, and November 2009 - May 2010.

The entire Pagan dataset was analyzed, i.e. from late April 2009 through late February 2010. Potential humpback whale detections were logged by the analyst and reviewed by two additional experienced researchers.

Progress & Results

No evidence of humpback whale singing was detected at Maug in the months searched (late April-May 2009 and November 2009 through May 2010).  At Pagan, humpback whale song was detected with high confidence on 8 days in February 2010 (35% of days with recording effort that month), but not in any other months.  For the entire analysis period, a total of 123 recordings with humpback song were documented. Humpback song was detected during all hours of the day, with slightly greater numbers of detections during evening and pre-dawn hours.

Future PAM for large cetaceans in the northern portion of the CNMI would be optimized by siting instruments for good acoustic propagation, e.g. at greater depths and on outer slopes of islands with more exposure to the open ocean, by deploying archival moorings close to the start of whale season (e.g. in autumn) to maximize battery life and hence data collection throughout an entire season, and by increasing the recording duration to at least a few minutes of continuous sampling to increase the probability of picking up song fragments or distant song. Placement of multiple recorders at several islands may provide more detailed information on the movements, habitat use, and relative abundance of humpback whales within the CNMI during the winter season.