Surviving the 4th ObSERVE offshore acoustic survey on board Celtic Voyager27th May 2016
Fom Wednesday the 18th to Wednesday the 26th of May 2016 I had the privilege of being part of the visual observation team on board R.V. Celtic Voyager as she carried out Passive Acoustic Monitoring (or PAM) as part of the ObSERVE acoustic monitoring program, a government funded project assessing the importance of the Irish continental shelf edge habitats for whales & dolphins. The project is led by GMIT along with its partners The Marine Institute, Irish Whale and Dolphin Group and Sea Mammal Research Unit Consulting. The plan, to tow hydrophones some 300 m behind the Voyager and listen for cetacean activity whilst adding visual sightings of behaviour to this acoustic data which offers a clearer picture of the importance of the shelf edge as vital habitat for a myriad of cetacean species.
Figure 1: Leaving lovely Falmouth
Joining the ship in Falmouth in beautiful Cornwall, we left port at 9 o’clock in the evening on the 18th of May and sailed west around Lands’ End. Thus began a 40 hour steam to the beginning of our survey. Our planned track lines were to take us on a zig zagging course from just south of the Goban Spur, off the south west coast of Ireland, to west of Achill Island of the west coast of Co. Mayo.
Our first couple of days at sea were marred by high winds and heavy swells, however this did not stop our intrepid team of acoustic observers (Johnathan Gordon, Chloe Malinka and Arthur Lee) from carrying out test to check that their equipment was running correctly. Nor did it stop our uber-fantastic visual team (Simon Berrow, Joanne O’Brien, Rossa Meade and I, Mick Marrinan) from collecting our first auxiliary sightings (sightings off track). These consisted of one unidentified dolphin, one unidentified cetacean species which looked very like a minke whale, but was sighted well offshore and not in expected minke whale habitat. Some wiser head suggested that it may have been a beaked whale as they proport to be minke whales at times. Along with three sightings of one of my favourite species, the common dolphin, the trip had gotten off to a good start.
Figure 3: Good old common dolphin (Delphinus delphis)
Friday afternoon saw the official start of the survey track. With the hydrophone in the water, the ship set off at a slow steady pace of six knots. This speed was decided upon as the universal speed all of these surveys should be carried out at, in the interest of consistency. Given that there are several different platforms (or boats) that these surveys are conducted from, e.g. The Song of the Whale, R.V. Celtic Voyager two completely different types of vessel, consistency is required for more robust data.
For the next five days, the ship alternated tracks between northwest and northeast along the western slope of the shelf edge. Visual surveys were carried out from the monkey bridge (above the wheel house) where possible and from the wheel house wings when the weather was a bit too rough for the monkey bridge, and finally from the wheel house itself in particularly inclement weather. Sightings were recorded using Logger software, which keeps track of all sightings, the position, species, behaviour, age (adult, juvenile and calf) and number of animals. It also keeps track of environmental conditions as the survey progresses all of which is vital in reporting the species, and behaviour, that live in these habitats.
Figure 2: Pilot whales (Globicephala melas) along the shelf edge
Throughout the four days on dedicated survey track lines, there were a total of 21 sightings, one of which was an unidentified shark species. Given the choppy conditions and the glare of the sun, identification of most of the sightings was impossible, however many of the usual suspects were noted such as common dolphins, large groups of pilot whales (one with at least 30 individual animals including adults and calves), sperm and fin whales. I myself am convinced that I saw my first Sei whale, others are not so sure, but I am putting it down on my own personal list with a little asterix!
The highlights of the trip, however, occurred on Monday the 23rd of May. Whilst surveying from the bridge, Simon spotted the breach of what turned out to be one of a pair of Sowerby’s beaked whale. The animals breached up to ten times in succession, affording us some wonderful views on a beautiful day, of this deep diving species about which very little is known. But wait! It got better; several hours later Joanne was to have the sighting of the trip as a group of at least 5 or 6 beaked whales, which included at least two mother and calf pairs, cruised by the starboard side of the boat about 150m out. These animals also proved to be Sowerby’s beaked whale, but the chance to get a good picture of the adults and tiny calves was one that will not be forgotten soon (especially since I have the photos! Bwah-ha-ha-ha).
Figure 4: Group of Sowerby's Beaked Whales, can you see how many are in the group?
So as a member of the visual team, I have written a lot about the sightings and not a lot about the acoustic side of the survey so here goes. They heard stuff!........................................... only kidding - I will know hand you to Chloe Malinka from the acoustics team:
Figure 5: PAMGuard running at the acoustic station
The three of us took turns listening, through the day and night, to what was being recorded by the towed hydrophone array, consisting of a chain of four hydrophones (underwater microphones) in an oil-filled tube. These were recording at very high frequencies (500 kHz) in order to be able to pick up the wide vocal range of marine mammals, particularly the high-frequency echolocation clicks of toothed whales. While listening to what was often the white-noise hypnosis through the headphones, we were also watching real-time click and whistle detectors in PAMGuard – the industry-standard open-source free software used by academia and industry alike for passive acoustic monitoring (PAM). We set up the click detector to automatically detect echolocation clicks many of the species we would expect to encounter, such as beaked whales, sperm whales and dolphins. Odontocetes use these sounds to forage and sense their environment, and arguably for social purposes as well. Beaked whales hunt at deep depths (Cuvier’s beaked whales have been recorded going down to nearly 3000 m!), and as such, when you see them at the surface, you’re unlikely to get acoustic recordings, and vice versa. Fortunately, this survey gifted us with both visual sightings and acoustic recordings of the rarely-encountered Sowerby’s beaked whale. A whistle detector was also running, and highlit instances of when pilot whales or dolphins were recorded producing these sounds. Acoustic indications of animal presence often accompanied visual sightings, but the same cannot be said of the converse, due to visual observation being much more difficult in higher sea states.
Figure 6: Computers for processing incoming multi-channel recordings from the hydrophones.
We can use the differences in time it took for echolocation clicks to arrive at different hydrophones to triangulate the origin of the sound. This localisation process is not possible from one sound as collected on a 2D hydrophone array, due to ambiguities in whether a sound comes from the port or starboard side of the array, but since our hydrophones are being towed, the relative bearings calculated to the potential sound origins will overlap on the side in which the sound is coming from – allowing us to estimate the whereabouts of a vocalising whale. If an animal, or group of animals, were clicking successively, we could use a mapping feature in PAMGuard to help us discern, in real-time, the bearing and distance that the given species was in relation to our vessel. It was encouraging to hear animals, and be reminded that you weren’t in a vast ocean emptiness, but instead, in the habitat of the deep-diving and infrequently-surfacing animals of the continental shelf.
Figure7: Beaked whale click train as it appears on the PAMGaurd display.
We heard many species, including pilot whales, dolphins, beaked whales, and the metronome of the sperm whale click. Arthur Lee is doing the acoustic analysis for all 6 of the ObSERVE surveys – watch this space.
Thank you Chloe! So now all that is left for me to do as we sail into Rossaveal, Connemara on a lovely May afternoon, is to thank the crew of the Voyager for all their help and patience, the whole team for just being totes awesome and the IWDG for giving me the opportunity to partake in this survey.
Hope to speak to you again!
And remember, keep your eye on the sea because you never know what you will see !
Mick Marrinan (Marine Mammal Observer) and Chloe Malinka (Acoustic Scientist)
All images by Mick Marrinan
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