HWDT surveys have been running on Silurian with consistent methodology since 2003. We therefore now have a valuable and growing set of trend data to monitor cetacean populations in the Hebrides, and any changes, threats or impacts that are occurring.
We now know that…
- Fewer than 40 bottlenose dolphins live in the waters of the Inner Hebrides, but they range from the Kintyre Peninsula to Skye. Another more localised population of approximately 12-15 animals is found in and around the Sound of Barra in the Outer Hebrides. Photo-identification work on these two populations suggests that they do not associate with each other.
- Patterns of minke whale sightings have been changing, maybe because of the availability of the small schooling fish on which they feed. Over recent years, there have been fewer minke whale sightings in core survey areas. However, populations of basking sharks are following an upward trend.
- The west coast of Scotland is one of the most important harbour porpoise habitats in Europe, but there are certain areas around Mull, the Small Isles and the Sound of Jura which are particular feeding hotspots. Harbour porpoise appear to favour deeper channels or basins close to shore. Consistent small-scale hotspots are currently being identified in collaborative work with the University of St Andrews.
- Through our collaborative research with PhD student Andy Foote, we have discovered there are only around 10 to 12 killer whales in Hebridean waters. It is likely that they don't mix with any other North Atlantic killer whale population (Shetland, Norway, Iceland). Demographically they are interesting because the group consists of 5 adult males and 5 females. There have been no calves for a number of years. This group does not appear to be reproducing and as such, as a population, is in a highly vulnerable state. We will have the full results of this study when Andy completes his PhD in 2009.
- There are two primary local water masses off the west coast of Scotland – relatively warm Atlantic waters, and cooler Coastal Current waters moving north from the Irish Sea. Biodiversity indices show that species diversity is lower in the area which is dominated exclusively by a single water mass from the Irish Sea (in the area south of Mull and around Islay and Jura). Diversity is highest where Atlantic and Irish Sea waters are actively mixing or are extensively mixed. This suggests that oceanographic features are a strong indicator of cetacean diversity on the Scottish west coast.