I recently spent week at an otter conference in southern Ireland. About 50 people from 13 European countries gathered at Kinsale to talk about otter ecology and conservation. Some of us were lucky enough to visit Roaring Water Bay afterwards to take part in a project whereby otter droppings (spraint) are collected and DNA extracted to follow the movements of individuals.
One talk at the conference was particularly interesting in the light of the question of whether otters might be scared of lights at the CUBC boathouse. It dealt with the subject of how to discourage otter predation at fish ponds. Lauren Harrington of Oxford University described efforts to discourage otters from entering the water by playing sounds beneath the surface. Early trials in the field were only moderately successful and they concluded that it would be more practical to work with captive animals where you don’t have to rely on unpredictable visits by otters to your study site.
It seemed that some discordant sounds were disliked by otters though Lauren hadn’t yet worked out best ones to use. Whether any system will work in the wild and whether or not the fish it is supposed to protect will be affected remains to be seen. Most mammals get used to predictable disturbance, so there are no guarantees. On the other hand, sound travels extremely well in water so a single source might protect a substantial sized pond – if it works.
Meanwhile, it seems that at one site in Northern Ireland otters have been successfully discouraged from eating rather different type of prey. The Copeland Islands just south of the entrance to Belfast Lough have a colony of 5,000 pairs of Manx shearwaters which were being preyed on by what was believed to be a single otter. Although the number of birds being killed was believed to be no more than 2% of the breeding population, this did represent an increase in mortality of about 30%.
There are three islands, Copeland, the largest is a mile (1.6km) offshore and Mew Island and Lighthouse Island (where the shearwater breed) a further three quarters of a mile out. Their combined coastlines come to less than 10km which is large enough to support a small number of breeding otters but not a viable population. It isn’t clear whether predation was caused by a resident otter or whether one or more otters visit the island during the bird breeding season when vulnerable avian prey is available.
Kerry Leonard investigated the losses and, having found clear evidence that an otter was responsible, tried to prevent further losses with deterrents. He used a combination of two methods: four high intensity red flashing LED lights which automatically turned on at dusk and off in daylight and ten ultrasonic animal deterrents which generated a noise which swept through the frequencies from 20 – 45 kHz. The ultrasonic deterrents have a sensor similar to the one used in some household security lights so that the noise was only generated when an ‘intruder’ approached. Unlike the Oxford project, this work wasn’t done on an experimental basis so it is not possible to know which method (if either) worked. However, after they were deployed, predation dropped dramatically. In the 50 days before the lights and sound were used, 67 shearwaters were killed but from mid-August to mid-October, only three were taken.
Three thoughts come to mind. First the evidence that the technique worked is good, even though it had only been tried once at the time of the report. Shearwater chicks take a long time to develop and are particularly vulnerable during September – right in the middle of the period when deterrents were used. Second, it would be interesting to know whether it would work with only one deterrent. In general, one would expect unpredictable disturbance to be more effective, which would suggest that the continuously flashing light (at night) would be less likely to work as well as, possibly, less unpleasant. However the sound was only produced when the otter approached the area and given Lauren’s preliminary results, looks likely to be the important effect.
Third, I am intrigued that the light should be red. I wonder whether this reflects our human association of red with danger. Most carnivores are unable to discriminate between red and green light so the otter won’t know whether it was safe to cross!
The presence of otters on offshore islands is also interesting. I have seen otter spraints at the Shiant Islands off the east coast of Lewis in the Outer Hebrides. The coast line of these is a little larger than the Copelands but they are further off shore (4miles; 6.5km). I suppose it is possible that one or a few otters permanently live on the Shiants but it I suspect that they come out for the breeding birds. There are tens of thousands during the summer, an impressive sight. Auks are very common: guillemots (ca 8,000), razorbills (ca 3,500) and puffins (>75,000) – plenty of food for an otter.