Saturday, August 8, 2009

A day in the work of tagging a beaked whale

The equipment and training that the teams on the Alliance have been doing are all dedicated to the difficult task of tagging beaked whales. The species we are working on in the Mediterranean Sea is Cuvier's beaked whale, Ziphius cavirostris. Today pretty much everyone on the ship was focused on this job all day.

The ideal weather for tagging a beaked whale is flat calm. Calm weather makes it easier to sight these hard-to-see animals when they surface for a series of blows lasting about 8 minutes. After a deep foraging dive that will typically last about an hour, they do a series of shallow dives each of which lasts for about 15 minutes or so. This means that the observers have to wait about an hour for their opportunity to sight whales when there is a deep foraging dive, but the duration of shallow dives is short enough that this provides the beset opportunity to tag. This means that it is critical for the research vessel to get near enough to the whales during the deep foraging dive that it can direct the tag boat near the whales during the first surfacings after shallow dives.

This morning there were light swells and light whitecaps from wind, which makes sighting beaked whales more difficult, but still possible with the excellent height of eye on this ship and with the powerful bigeye binoculars used by trained observers who know the cues that indicate a beaked whale sighting. The acoustic monitors have no problem hearing beaked whale clicks in these conditions. During the morning we detected several groups of beaked whales but they had small calves, which made them not good candidates for tagging efforts.

We continued the search throughout the day, with the teams rotating through a watch schedule to maintain high levels of attention for a difficult task. Looking for beaked whales, you need to scan the water systematically, looking for a cryptic surfacing. Unlike baleen or sperm whales, the blow is not very visible, and the beaked whale usually surfaces quickly and goes down again, meaning you have to be very attentive to see the surfacing and be able to recognize the species. Similarly for acoustics, when you are far from a beaked whale, all you will hear is a scan of several clicks as the whale’s sonar beam passes the hydrophone. This means that if the monitors take their gaze away from the computer screen for more than 10 seconds, they may miss the cue.

We continued to survey, focusing all of our efforts on detecting the beaked whales. At 5 in the afternoon, the visual team detected a group of 3-4 Ziphius at the surface with no young calf. These initial sightings are marked with a yellow triangle to the right of the figure, and are marked H and I. We left the survey line and headed towards this group. Within about 10 minutes after the whales dove, the acoustic monitors started to pick up beaked whale clicks in the direction of the group. A few minutes later as we approached closer, the acoustic monitors started hearing relatively constant series of clicks with the 0.4 sec interval between clicks that is typical of beaked whales. This meant that we were close enough to hear the clicks even when the whale was not pointing at the hydrophone. The two straight yellow lines on the plot here give acoustic bearings to the sounds of these whales at two different times. Where they intersect indicates an estimate for the location of the whales.

The ship was maneuvered to stay close enough to continue to pick up these clicks until they petered out about 20 minutes later. At this point, the ship was maneuvered to the west so that the visual observers could look away from the glare of the sun, and then was repositioned so the observers, who must orient towards the bow of the ship, had a clear view of where they expected the whales to come up. There was a strong current to the east, which caused the ship to move east. It typically takes about 20 minutes for whales to ascend to the surface after they stop clicking. The visual observers did not sight the whales for about 45 minutes after the stop of clicking, so they probably missed the first surfacing. Over this interval, the ship moved farther east than was ideal for observing the area where we expected the whales to come up. The next sighting is the one marked J1-J3 in the center of the plot, just to the left of the orange line indicating our survey track. As soon as that sighting was made, the ship could move closer and reposition.

This is the critical point to get the information required to place the tag boat near where the whales will next come up on one of their shorter dives. While one surfacing tells us where the whales are, we need a second surfacing to get an idea of their direction of travel. We prepared the tag boat for deployment, waited until the group was next sighted 19 minutes later, and deployed the tag boat 6 minutes later. For the next two hours, the visual team radioed the tag boat to position it near their estimates of where the whales would surface, then to direct it towards the whales as soon as they surfaced. Even though the tag boat was closer to the whales, the elevated platform and excellent optics of the visual team usually allowed them to sight the whales first. The whales usually only spend around 8 minutes at the surface, so the tag boat does not have long to approach the whales. We use a small quiet vessel for the tag boat and move slowly so as not to disturb the whales, so the tag boat needs to be within a few hundred meters of the whales when they surface in order to have a chance to tag.

One complication with our tagging attempt today, was that the whales, which had been moving consistently WSW the entire time until the sightings marked J13-J14, suddenly switched direction, surfacing to the E in surfacings J16 and J17, then off to the NE for the final two surfacings. While the tag boat could see the whales, they did not get close enough for any opportunities to tag before sunset and we recovered the tag boat a little after 9 PM.

The equipment and teams are intensely focused on tagging and are operating at a high level. Even so, beaked whales are so difficult, that our previous experience suggests that it will take days of these efforts to succeed in tagging a whale. We just need to keep doing what I have described here.