Cetacean Surveys of the Southern Mariana Islands: Rota (June 16-20, 2014)

by Marie Hill, Andrea Bendlin, Allan Ligon, Adam Ü, and Erin Oleson

During 5 days of surveys off Rota, we covered 488 km of trackline and encountered 14 groups of cetacean species including  Blainville’s beaked whale (Mesoplodon densirostris), bottlenose dolphin (Tursiops truncatus), pantropical spotted dolphin (Stenella attenuata), short-finned pilot whale (Globicephala macrorhynchus), spinner dolphin (Stenella longirostris), and unidentified beaked whales (Figure 1).

Figure 1: Survey tracklines (grey lines) and cetacean sightings around Rota (16-21 June, 2014). Gm- Globicephala macrorhynchus (short-finned pilot whale), Md- Mesoplodon densirostris (Blainville’s  beaked whale),  Sa- Stenella attenuata (pantropical spotted dolphin), Sl- Stenella longirostris (spinner dolphin), Tt- Tursiops truncatus (bottlenose dolphin), uZ- unidentified Ziphiid whale (unidentified beaked whale).

Figure 1: Survey tracklines (grey lines) and cetacean sightings around Rota (16-21 June, 2014). Gm- Globicephala macrorhynchus (short-finned pilot whale), Md- Mesoplodon densirostris (Blainville’s beaked whale), Sa- Stenella attenuata (pantropical spotted dolphin), Sl- Stenella longirostris (spinner dolphin), Tt- Tursiops truncatus (bottlenose dolphin), uZ- unidentified Ziphiid whale (unidentified beaked whale).

We had incredibly calm conditions while we were on Rota which allowed us to survey further offshore than we have in previous years.

During our first day on the water, while off the easternmost tip of the island (Figure 1), we encountered the largest group of short-finned pilot whales that we have seen to date in the Marianas (approximately 44 individuals).  We recognized some individuals from our photo-identification catalog.  We deployed 3 location-only satellite tags on individuals from different subgroups (tag ID#s 128899, 137726, and 137727) and collected 9 biopsy samples.

The following day (17 June) we encountered some of the same short-finned whales off the south side of the island (Figure 1), including two of the three satellite-tagged individuals (tag ID#s 128899 and 137726).  They were traveling with the group of short-finned pilot whales that we had encountered off of Guam on 19 May, including the two satellite-tagged individuals (tag ID#s 128889 and 128920).  We deployed another satellite tag on an adult male from the Guam group (tag ID# 137728).

These five individuals (tag ID#s 128889, 128920, 128899, 137726, 137728) continued to travel together during the day on 17 June (Figure 2).  At approximately 17:00 they were less than 3 km from the location where we encountered the group of pilot whales just the day before (Figures 1 and 2).  During our encounter with the short-finned pilot whales on 17 June, we did not see the individual with tag ID# 137727. The satellite transmission locations from this tag were off the north side of Rota (Figure 2).

Figure 2: Satellite tag locations of short-finned pilot whales between 15:00 and 16:00 on 17 June.

Figure 2: Satellite tag locations of short-finned pilot whales between 15:00 and 16:00 on 17 June.

Approximately 24 hours later some of the tagged individuals were in different locations from each other (Figure 3).  Individuals with tag ID#s 128899 and 137726 were off the northwest side of Rota.  Individuals with tag ID#s 128920 and 137728 were off the north side of Guam, while the individual with tag ID# 133727 was approximately 12 km south of Rota.  The individual with tag ID# 128889 is not pictured in Figure 3 because no transmission was received at that time.

Figure 3: Satellite tag locations for short-finned pilot whales between 16:00 and 17:00 on 18 June.

Figure 3: Satellite tag locations for short-finned pilot whales between 16:00 and 17:00 on 18 June.

Another 48 hours later, between 16:00 and 17:00 on 20 June, individuals with tag ID#s 128899 and 137726 were off the southeast side of Tinian.  Individuals with tag ID#s 128889, 128920, 137727, and 137728 were off the south side of Rota (Figure 4).

Figure 4: Satellite tag locations for short-finned pilot whales between 16:00 and 17:00 on 21 June.

Figure 4: Satellite tag locations for short-finned pilot whales between 16:00 and 17:00 on 21 June.

The most recent locations for the satellite transmissions from the tagged short-finned pilot whales are shown in Figure 5. It will be interesting to see how they continue to move around the islands and if they join up with the other two Guam-tagged short-finned pilot whales (tag ID#s 128910 and 128914) whose tags were still transmitting on 22 June and were off the west side of Guam.

Figure 5: Satellite tag locations for short-finned pilot whales on 22 June.

Figure 5: Satellite tag locations for short-finned pilot whales on 22 June.

June 18 was an exciting day for us surveying around Rota.  Not only did we have 5 encounters with 5 different species, but we had our first encounter with a Mesoplodont beaked whale in which we were able to confirm the species.   All of our previous encounters were in rough conditions or distant.  This time we were able to get great photos of the head of a solitary adult male Blainville’s beaked whale (Mesoplodon densirostris) showing the high bottom jaw line and the erupted teeth (Figure 6).  We have known from acoustic recordings that they occur in the Marianas, but it is good to finally have visual confirmation.

Figure 6:  Adult male Blainville’s beaked whale photographed off Rota on 18 June (photo credit: Adam Ü)

Figure 6: Adult male Blainville’s beaked whale photographed off Rota on 18 June (photo credit: Adam Ü)

During our Rota surveys, pantropical spotted dolphins won out over spinner dolphins as the most encountered species.  They were our first and last encounter around the Island.  On our last day (20 June) we encountered a group of nearly 150 individuals.  It was another beautifully calm day on the ocean and a great way to wrap up our surveys (Figure 7).   We are already looking forward to next year.

Figure 7: Spotted dolphin leaping in front of our boat as we returned to the harbor off of Rota (photo credit: Adam Ü).

Figure 7: Spotted dolphin leaping in front of our boat as we returned to the harbor off of Rota (photo credit: Adam Ü).

All survey operations including satellite tagging, photo-id, and biopsy sampling were conducted under NMFS permit 15240 and CNMI Fish and Game License 14-02868.  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 figures above.

 

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