Marianas Cetacean Surveys 2013: Rota Summary (July 4 – 10)

by Marie Hill, Allan Ligon, Adam Ü, and Mark Deakos

This is our third year conducting surveys off of Rota, and we completed six days of surveys aboard two vessels (three surveys aboard each).  One was a 12.2m Ocean Alexander Sport-fisher with twin diesel inboards and flying bridge (Sr. Dung) and the other was a 7.1m Boston Whaler (Nacrina).  Over the six days we surveyed 509km of trackline (Figure 1).

Figure 1- Rota Tracklines

Figure 1: Survey tracklines around Rota (4-10 July, 2013). Both survey vessels departed from Song Song Harbor. Inset: The southern Mariana Islands.

We were hoping that the excitement from our previous two weeks working off of Guam would continue and Rota did not disappoint (see the following link for details of our surveys off of Guam: https://pifscblog.wordpress.com/2013/07/08/marianas-cetacean-surveys-2013-guam-summary-june-22-july-1/?postpost=v2).  On 6 July, just as we were leaving the harbor mouth we had a sighting of false killer whales (Pseudorca crassidens) and bottlenose dolphins (Tursiops truncatus).  We followed the false killer whales for approximately 40km (the northernmost trackline in Figure 1).  There were a total of 16-26 individuals that were in multiple subgroups spread out over several kilometers.  We collected over 2100 photos, 9 biopsy samples and deployed 3 satellite tags (PTT 128903, 128904, 128906).  We also witnessed three feeding events in which some of the animals from the different subgroups converged on, killed, and shared fish (two were tunas and the third was a marlin) (Figure 2).  One of the tagged individuals (128906) (Figure 3) participated in the marlin feeding event along with eleven other individuals.

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Figure 2: False killer whale grabbing a tuna on the surface of the water before taking it under. Photo credit: Marie Hill, collected under NMFS permit 15240.

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Figure 3: False killer whale satellite tagged off of Rota (PTT 128906). Photo credit: Adam Ü, collected under NMFS permit 15240.

As if that day wasn’t exciting enough the very next day we had another encounter with false killer whales!  This was a completely different group of 13-18 individuals.  They were spread over approximately a kilometer but joined together to kill and share a marlin (Figure 4).  See below for a short underwater video of this feeding event: 

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Figure 4: False killer whale with part of a marlin in its mouth. Photo credit: Marie Hill, collected under NMFS permit 15240.

During this encounter with the false killer whales we collected over 1100 photos, 7 biopsy samples and deployed 1 satellite tag (PTT 128908).

All four satellite-tagged false killer whales have traveled long distances since their tags were attached (Figure 5).  The three whales tagged on the first day (6 July) traveled north-northeast together before splitting up.  The tagged false killer whales 128903 and 128904 remained together and traveled west toward the West Mariana Ridge, while 128906 headed back south after going north to Guguan.  The most recently reported satellite transmission from tagged false killer whale 128906 indicates that it is approximately 94km west of Saipan.  Saipan is the location of our next basecamp for our continuing surveys.  Maybe we will see these whales again.  The false killer whale tagged on 7 July (128908) traveled to the northwest approximately 70km before heading due south toward Tracey Seamount and beyond, then turned west.

A comparison of photos from the two false killer whale encounters off Rota with the two distinctive individuals photographed during our first survey day off of Guam revealed no matches.

Figure 5- Rota Pc tag tracks

Figure 5: Tracks of the false killer whales satellite tagged off the northwest side of Rota on 6 and 7 July, 2013 (local time). The last positions shown (stars) were on 11 July @ 9:00 (local time) for tags 128903, 128906, and 128908 and on 10 July @ 5:00 (local time). All whales were tagged under NMFS permit 15240.

In addition to false killer whales, we encountered three other species off Rota: bottlenose dolphins (Tursiops truncatus), spinner dolphins (Stenella longirostris), and pantropical spotted dolphins (Stenella attenuata) (Figure 6).

Figure 6 - Rota cetacean sightings

Figure 6: Locations of cetacean sightings around Rota (5-10 July, 2013). Pc- Pseudorca crassidens (false killer whale), Sa-Stenella attenuata (pantropical spotted dolphin), Sl-Stenella longirostris (spinner dolphin), Tt-Tursiops truncatus (bottlenose dolphin).

During our second encounter of bottlenose dolphins off Rota (9 July), one of the dolphins was an individual that we photographed on 30 June off Guam during the mixed encounter with pilot whales (Globicephala macrorhynchus) (Figure 7).  Seven other individuals within this group of twelve are a part of our photo-identification catalog of bottlenose dolphins and were photographed previously off Rota in 2012.  In addition, during our 2011 surveys two of the dolphins were photographed off Guam and three others were photographed off Saipan.  On 10 July we encountered 8 of the bottlenose dolphins from the previous day’s group, including the individual that was photographed off Guam on 30 June.  The two encounter locations off Rota were approximately 12km apart (Figure 6).

Figure 7 - Bottlenose dolphin resight

Figure 7: Bottlenose dolphin photographed off Guam on 30 June, 2013 and then off Rota on 9 July, 2013.

The two Stenella species continue to be our most frequently encountered species.  This year off Rota we had four encounters with and one resight of pantropical spotted dolphins and two encounters with spinner dolphins (a total of 6 and 7 encounters for the year respectively).  Unlike previous years, we encountered a group of pantropical spotted dolphins off the south side of Rota (Figure 6).  We have not created photo-identification catalogs yet for these dolphins so we do not know whether they are the same individuals that we have photographed before.

We do have a catalog of spinner dolphins that includes 89 individuals from Saipan, Tinian, Aguijan, and Rota.  During our first encounter this year off Rota we photographed 8 of 22 cataloged individuals previously photographed off Rota.  On 9 July, our second encounter with spinners off Rota ended early because bottlenose dolphins showed up on the scene and took priority.  We did however get a photo of one cataloged individual from the previous encounter.  If we had remained with them, it is likely that we would have collected more photos of cataloged spinner dolphins.

We will continue our surveys of the Southern Marianas by working in the waters surrounding Saipan, Tinian, and Aguijan (12-30 July).  We look forward to continued success and possibly some resights of our tagged individuals.

Satellite Tagged Pilot Whale Update:

All three pilot whales that were satellite tagged off Guam continue to have transmitting tags.  They have made some interesting movements since our previous report on 3 July (Figure 8).  At that time they were in three separate locations.  Satellite tagged pilot whale 128884 was off the west side of Tinian, 128885 was south of Rota, and 128886 was on the southwest coast of Guam.  Over the past week tagged pilot whale 128885 returned south on the east side of Guam, met up with 128886 and they spent several days together off of the southwest side of Guam and at Galvez Bank.  These two whales were tagged on separate days and have not been previously photographed together.  During this time tagged pilot whale 128884 visited the west side of Saipan then looped around Tinian and headed back down south along the east side of Rota and Guam, then visited Galvez Bank (on a different day than the others).  On 11 July all three satellite tagged pilot whales were off Guam.  Tagged pilot whale 128885 was off Santa Rosa Bank approximately 55km southwest of Guam.  Tagged pilot whales 128884 and 128886 were both off the west side of Guam both close to the locations where they were each tagged.

Figure 8 - Guam Pilot Whale track update

Figure 8: Tracks of pilot whales satellite tagged off of Guam (30 June and 1 July, 2013). Most recently reported locations were on 11 July. All three pilot whales were tagged under NMFS permit 15240.

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