Over the bank holiday weekend we were joined by Dave Nairn from the Clyde Marine Mammal Project and his fellow skipper Alistair. Dave and Alistair sailed over to Peel along with their towed hydrophone – an acoustic device which records click trains from Harbour porpoise, to conduct the first ever sea trial in Manx waters using the equipment.
The Clyde Marine Mammal Project's Vessel 'Sy Saorsa'
We went to sea on Tuesday 2nd of May at 05:30 and surveyed for a total of 10 hours. The hydrophone was deployed just as we departed Peel, and was towed 100 metres behind our research vessel Galps. The computer system, with the software PAMguard, was set up in the wheelhouse. The hydrophone picks up every single underwater sound, which shows as black dots on the screen. The hydrophone can pick up the different sound frequencies made by a Harbour porpoise, which appear as red triangles on the screen. The hydrophone can detect sounds up to 1km away and, although we can’t tell exactly where the porpoise is (unless it is accompanied with a visual sighting), we can see if it is in front, behind, or to the side of the hydrophone.
First recording shown as a red triangle on the top right of the screen
The sea state wasn’t very good, with a force 3-4 and a medium swell, so we travelled at around 5 knots. The rough sea makes cetaceans, especially porpoises, almost impossible to spot. However with the hydrophone ‘watching’ for us, we were still able to collect lots of useful data.
Our survey took us 4 miles offshore on the west coast before heading south. We rounded the Calf of Man behind the Chicken Rock, and made our way east towards Langness. We had sporadic porpoise detections en route and found that many detections were collated in particular spots, meaning it is likely there was a pod of porpoise feeding in the area.
With increasingly rough seas, we decided to head inshore back towards the Calf. As we approached Chicken Rock we saw in the distance an unbelievable amount of diving Gannets (an indicator that cetaceans were feeding in the area). As we approached the Gannets, the hydrophone began to pick up loads of porpoise clicks and the screen was covered in red triangles. The sea was so choppy at this point that we didn’t have any visuals, but from the acoustic data, we could tell that there was a fairly large number of porpoise feeding. It was incredibly exciting!
Heightened porpoise activity behind the Calf of Man
Gannet activity above the porpoise detections
We gradually made our way back to Peel around 5 miles offshore and reached the harbour at 16:00. Throughout the survey, we had 2 visual sightings of porpoise along with 33 different detections. A hydrophone is undoubtedly an extremely useful piece of equipment and if we were to acquire our own, it would enable us to survey in higher sea states than normal and still collect vital data. The cost of a hydrophone array is around £11.000, to include the device itself and the associated equipment. There is no doubt that it would be incredibly beneficial in terms of research for us to have our own array, so the next task is to acquire funding to cover the project. If anyone reading this can suggest who we can approach for funding, please do get in touch.
We would like to say a huge thank you to Dave and Alistair for taking time out to travel to the Isle of Man and trialling their hydrophone in Manx waters. You can find out more about the Clyde Marine Mammal Project here: http://www.clydemarinemammalproject.org/