Porpoise survey log book 2007

31.7 July summary

Despite the wind and rain, we have had considerable success in finding the porpoise – which, by no means, seem to be bothered by the weather like us terrestrial mammals. In fact, the observed feeding events have often associated with higher sea states due to strong tidal current action. On the 17.7, around 12 individuals were riding waves against the tidal current just two hours after the high tide. The two-metre swell provided a crystal-clear window of observation to their underwater surfing, if somewhat bumpy ride on the rib! Almost all of these recorded feeding occasions have been accompanied by the gannets. Descriptively, the porpoises seem to perform very energetic swim close to the surface into one direction right to the point where the gannets are dining. Sometimes these avian and aquatic rockets attack the prey simultaneously as if behaving one cohesive group.

For long have the lack of recognisable individuals concealed information on harbour porpoise movements, in comparison with, e.g. bottlenose dolphin individuals whose fins can often be positively identified. Therefore we were agog with excitement at the discovery of a pale individual, assumed female, with two notches in the fin and a tiny newborn calf in the North Little Sound on the 17.7. We haven’t seen her since, but anecdotally it would be very interesting to spot her again.

So far, we have recorded 155 effort-related sightings of cetaceans covering an area ranging from Jack Sound all the way to St Anns Head. In addition to vessel-based surveys, timed watches have been carried out on the Wooltack Point in Martin’s Haven, which is a particularly good spot for any member of the public to have a close-up look at porpoise behaviour. Judging by our experience, it might be worth to take the binoculars down when the tidal current is flowing northwards, that is between the two hours before and after high tide.

See the fun sightings list for basking shark, sun fish, common dolphin, minke and pilot whale observations.

2.9 August Summary

Weather finally improving we managed to get to the sea on 16 days yielding 90 more sightings of harbour porpoise! On a few calmer days we also carried out trips all the way to the gannet colony in the Grassholm Island, although no porpoises were seen here.

The female reported earlier, assuming there is only one individual with two notches in the fin, has now been sighted two more times around the Skomer. On the 17.8 the female was associated with a calf and two other juvenile or adult size individuals, whereas on the 27.8 she had joined a larger group of 6 with 3 mother-calf pairs. The latter sighting is especially interesting as we had a chance to watch their behaviour up to half an hour after which they still carried on foraging. The group was particularly tight and facing the same way at the same time against and into the current during the whole observation period. As we had observed many times earlier, the porpoises exhibited energetic surfacing against the current flow as opposed to slow or normal swimming, possibly diving, back to the direction of the current. On most of these occasions also foraging gannets have been present. It is interesting to speculate whether the porpoises are using the speed of the tide to catch their prey. In this case the energetic advantage must counteract the cost of swimming against the tide (and perhaps the cost of sharing the meal with other individuals). So far the average number of individuals in a sighting is 2.8, 29 % and 26% of all sightings including only one and two individuals respectively. However the dispersion of animals in a sighting has varied from tight groups to loose aggregations.

Although the number of very small calves seems to have decreased since July, a newborn was recorded as late as on the 26.8. Many observed calves still form tight pairs with their mothers but are now also seen swimming independently. Almost 25% of sightings have included calves. A similar record (29%) has been reported earlier (Chris Pierpoint 2001: Harbour Porpoise Distribution in the SW Wales) in the local inshore area, and interestingly a considerably lower record (13%) for the offshore, but unfortunately we do not have enough data yet to confirm the latter finding. It remains to see how the season affects the number of individuals and calves further in September.