By Marie Hill
We have returned to the Marianas to conduct small boat surveys and to look for cetaceans in the waters surrounding Saipan, Tinian, Aguijan, and Guam (Figure 1). We timed these surveys with humpback whales in mind. We know from sightings by locals that they occur here seasonally during the “winter” months. In Hawaii, humpback whales are encountered between November and May. Here in the Marianas most of the reports of humpbacks have been in January-April so we are hoping to find some late season individuals. The occurrence of all cetacean species during this time of year is of interest to us since most of our surveys have been during the summer time (June-August).
Our first stop was on Saipan where we have spent the past 9 days surveying the waters surrounding Saipan, Tinian, and Aguijan. We have covered approximately 865 km of trackline and have had 8 encounters with 4 different species of cetaceans (Figure 2).
Those who have followed our efforts in previous years will not be surprised to learn that spinner dolphins (Stenella longirostris) were our most frequently encountered species (6 encounters) (Figure 3). Each of the spinner dolphin encounters were within 200m of encounters that we had in previous years.
We had an interesting mixed-species encounter off of Aguijan (Goat Island). Bottlenose (Tursiops truncatus) and rough-toothed (Steno bredanensis) dolphins were closely associated. Some individuals of each species swam right next to one another (Figure 4). This was a particularly interesting encounter because we had a similar encounter last summer of rough-toothed and bottlenose dolphins off Aguijan. Three of four rough-toothed dolphins photographed last year were present during this encounter. After some preliminary matching, we did not find the same individual bottlenose dolphins that were photographed during last year’s mixed encounter. There were some individuals that are in our catalog of bottlenose dolphins that have been photographed off Saipan, Tinian, and Rota in previous years.
Our last day on the water proved to be our most exciting because we encountered a large group of over 300 melon-headed whales (Figure 5). This was the first time that we have encountered them out here.
We successfully deployed 2 satellite tags (PTT IDs 128916 and 128918) and collected 10 biopsy samples and just under 3500 photos. The satellite tracks from the tagged individuals show that they have traveled 65 km offshore to the west of Saipan during the first 24 hours after being tagged (Figure 6).
We head to Guam next where we will spend the next week. We look forward to more exciting encounters!
All survey operations including satellite tagging, photo-id, and biopsy sampling were conducted under NMFS permit 15240 and CNMI Fish and Game License 02868. Funding was provided by the NOAA Fisheries and the U.S. Pacific Fleet. The satellite tag tracks shown are based on raw transmission data and have not been quality checked. The final products may vary from those shown in the figure above.