by Marie Hill, Allan Ligon, Adam Ü, Mark Deakos, and Erin Oleson
We are wrapping up this year’s project here in the Marianas and have completed our small boat surveys for cetaceans in the waters surrounding Saipan, Tinian, and Aguijan. Over a period of 14 days we surveyed more than 1600 km of trackline using two different vessels (Figure 1). The first was a 12.2m Sport-fisher with twin diesel inboards and flying bridge, Sea Hunter (6 days). The second vessel was a 7.9 m Regulator with twin outboard engines, Regulator (8 days).
This continues to be a year for firsts as we have added another species to our list of encounters since the overall project began in 2010. We were incredibly thrilled to encounter rough-toothed dolphins (Steno bredanensis) off Aguijan (Goat Island). They are one of the most unusual looking species of dolphin with their sloping heads and mottled body coloration (Figure 2). They also have great dorsal fins for photo-identification. This was a small group of six individuals. We collected photo-Ids from all, a biopsy sample from one and deployed a satellite tag on another.
In the days since the tag (128896) was deployed (15 July), the Steno has roamed back and forth (north and south) along the west sides of Saipan, Tinian, and Aguijan (Figure 3). The tag has transmitted for a total of 12 days. We resighted the group on 20 July off of Saipan after getting a satellite transmission update (smart phones can be a helpful technology in the field if there is cellular telephone service). Only four of the six individuals were present. We collected photo-Ids and another biopsy sample.
During the first encounter with the rough-toothed dolphins (on 15 July) we also experienced another first when we had three different species bow riding at the same time! First, individuals from a group of bottlenose dolphins (Tursiops truncatus) were swimming right alongside the rough-toothed dolphins then a couple of spinner dolphins (Stenella longirostris) from a nearby group came along to follow the other two species. What an incredible sight!
Two of the bottlenose dolphins are part of our existing photo-identification catalog. Both were photographed last year; one was at Rota Bank (between Guam and Rota) and the other was near Orote Pt. off the west side of Guam.
During the 15 July encounter we deployed a satellite tag on a bottlenose dolphin (128897). Although the tagged bottlenose dolphin moved up and down the west sides of Saipan, Tinian, and Aguijan in the days immediately following the tagging events (Figure 5), it did not travel with the tagged Steno. On 18 July the tagged bottlenose dolphin’s track apparently began matching that of the tagged false killer whale 128904 (see Figure 2 from our update on July 30th). On the day that we tagged this false killer whale off of Rota there were bottlenose dolphins present. We were only able to get a photo of one of the bottlenose dolphins and a preliminary it does not match any that we have photographed before or since. We do not know if the satellite tagged bottlenose dolphin 128897 was present on that day. The satellite tagged bottlenose dolphin and false killer whale apparently traveled along the same route through 20 July. After that the bottlenose dolphin (128897) returned south and apparently met up again with the tagged Steno 128986 along the north side of Aguijan on 25 July. They were separated by the following day and the most recently reported position (27 July) of the bottlenose dolphin (128897) was off the east side of Tinian (Figure 4). The satellite tag has transmitted for a total of 12 days.
Only two days after deploying a satellite tag on our first bottlenose dolphin we had another encounter with bottlenose dolphins off the west side of Saipan during which we deployed another satellite tag (128898). This was a completely different group of individuals. One of the dolphins was already in our catalog. We had photographed and collected a biopsy sample from it during an encounter off of Rota in 2012. The satellite tagged bottlenose dolphin (128898) has primarily moved up and down along the west sides of Saipan, Tinian, and Aguijan with a trip out to Coke Reef on 22 July (Figure 6). The most recent transmission from tag 128898 was on 26 July at 9:26 when the bottlenose dolphin was approximately 23 km west of Aguijan. It is possible that the tag is no longer transmitting because we have received messages from our other tagged individuals since that time but not from 128898. The tag has transmitted for a total of 8 days.
Just when we thought that things couldn’t possibly get more interesting, we had an encounter with sperm whales (Physeter macrocephalus) off the northwest side of Saipan (Figure 7). What is even more interesting is that they were within a kilometer of the only other sperm whale sighting that we have had in this area, which was in 2010. We also had one sighting off Guam in 2010, but no others until now.
For sperm whale photo-identification we use tail flukes. We spent several hours following the whales moving from subgroup to subgroup trying to collect fluke photos and biopsy samples, but the whales were not cooperating fully. So we moved to plan B, which was to investigate the “footprints” of the whales to collect skin samples from the surface of the water using a net. Footprints are areas of disturbance on the surface of the water caused by the motion of the whale’s flukes underwater as it swims. They actually appear as smooth circular areas on the surface of the water. Sperm whales have a tendency to slough skin at high rates and as they swim and move their flukes vigorously they leave trails of skin behind. This proved to be an effective method of getting skin samples, which can also be used for genetic analysis. We managed to collect one biopsy sample and 8 skin samples.
Continuing to be our most encountered species and rounding out our surveys (the first and the last encounters of our surveys off Saipan, Tinian, and Aguijan) were the spinner dolphins (Figure 8). They confound us! Their choices of location defy all logic and go against what we know of them from other places like the Hawaii. There they hang out in calm, serene locations. Here we repeatedly find some of them in areas unprotected from the wind, where the current is strong and the backwash from the cliffs make the conditions reminiscent of the opening scene in “Victory at Sea”. Needless to say, it makes photo-identification difficult!
We have learned from our existing photo-identification catalogs that individual spinner dolphins are moving throughout these Islands, up to Marpi Reef and down to Rota. We are currently working on the photo-identification catalog from Guam and will soon know if they are moving there also.
HARP Deployment and Recovery
Another part of our project here in the Marianas is the collection of passive acoustic recordings using HARPs (High-frequency Acoustic Recording Packages) (Figure 10). The devices are deployed on the seafloor for a period of one year and are then recovered using an acoustic release. We successfully recovered and redeployed two HARPs. One is located off the west side of Saipan near 300 Reef and the other is located off the east side of Tinian. Both are at approximately 1000 m depth. The devices that we recovered have been recording almost continuously (5 out of every 7 minutes) for the past year. The collection of acoustic data is useful in answering questions about the year-round and seasonal occurrence of certain cetacean species within these areas. It is particularly useful since we are unable to conduct small boat surveys throughout the year because both cost and weather are prohibitive.
This has been an excellent year for our small boat surveys and we can’t wait to see what next year brings!!
All survey operations including satellite tagging, photo-id, and biopsy sampling were conducted under NMFS permit 15240 and CNMI Fish and Game License 02694. 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 figures above.