by Marie Hill
We continue our surveys for cetaceans in the waters surrounding the southern islands of the Mariana Archipelago (Figure 1).
Our first stop was Guam where we completed 11 days of surveys aboard small boats. During those 11 days we covered 1,045 km (564 nmi) of trackline and encountered 11 groups of cetaceans (Figure 2). When we come across a group we approach them to take photos for individual identification, we collect biopsy samples from some species for genetic studies, and we deploy satellite tags on certain species to investigate their movements.
This is our fifth year conducting small boat surveys around the southern Mariana Islands and we currently have individual photo-identification catalogs for short-finned pilot whales (Globicephala macrorhynchus), bottlenose dolphins (Tursiops truncatus), and spinner dolphins (Stenella longirostris). Although we are seeing some of the same individuals, we are still photographing new individuals.
We encountered two different groups of short-finned pilot whales, south of Orote Point on the southwest side of the island. On 19 May, we encountered a group of 23 individuals. After a preliminary scan of the photos we found 7 individuals in our catalog that were previously photographed off Guam in August 2011 but were not photographed during any of our other pilot whale encounters over the years. We deployed two satellite tags (ID#s: 128889 and 128920) on adult males that were new to us; ones that we had not photographed previously. We also collected biopsy samples from these two individuals that will be used for genetic analysis to determine how they are related to other short-finned pilot whales within the Marianas and across the Pacific Ocean.
On 25 May, we encountered the second group of short-finned pilot whales. The group size was similar; approximately 20 individuals, but they were different individuals from the previous sighting. After a preliminary scan of photos we found 11 individuals within our catalog; 10 were from an encounter we had off Guam in July 2013. One of these 10 was an individual that we satellite tagged in 2013. That was our longest satellite tag deployment, which lasted 235 days. The 11th individual is a large male that had been photographed off Tinian in September 2011 and off Guam in March 2012. We deployed two satellite tags (ID#s: 128910 and 128914); one on an adult male (Figure 3) and another on a smaller individual.
As of 28 May all four satellite tags on the short-finned pilot whales were still transmitting. The individuals tagged together appeared to be traveling together (Figure 4). The two individuals tagged on 19 May traveled north to Rota and then spent the following days north of Rota Bank between the two islands. The other two traveled south of Guam (Figure 4).
Another interesting day was 21 May when we encountered false killer whales (Pseudorca crassidens) approximately 11 km off the west side of Guam (Figures 2 and 5). We found multiple small subgroups of 2-3 individuals that merged to hunt and share mahi-mahi, dispersed then merged again into a final group of 9 individuals. We deployed satellite tags on 2 (ID# 128887 and 128902) and collected biopsy samples from 9. Although we have not yet developed false killer whale photo-identification catalog we did recognize one individual from a group that we encountered on 7 July 2013 off Rota.
During the false killer whale encounter on 21 May, a group of 4 bottlenose dolphins (2 adults and 2 juveniles) showed up on our bow. They were primarily interested in surfing the waves. It is not unusual for bottlenose dolphins to show up during an encounter with another species. We have seen them on multiple occasions during encounters with short-finned pilot whales, rough-toothed dolphins, spinner dolphins, and false killer whales.
Since tag deployment, the 2 false killer whales circled out to the southwest of Guam then moved north before circling out to the west of the islands and back south toward Guam (Figure 6).
On 23 May we encountered the largest group of pantropical spotted dolphins that we have seen out here; more than 100 individuals. They were in a location where we expected to see them but in greater numbers and much more active than in past encounters. They were spread of several kilometers and small subgroups were porpoising together up-swell into the waves (Figure 7).
One final encounter of note was a quick glimpse of a Mesoplodont beaked whale at Tracey Seamount approximately 30 km west of Guam. We saw it just before it dove and didn’t see it again. Beaked whales are of particular interest because of their deep diving behavior and the known potential impacts of sonar.
All survey operations including satellite tagging, photo-id, and biopsy sampling were conducted under NMFS permit 15240. 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.