As the title suggests, we now hear from WDCS's Sarah Dolman on what's being done around the UK to protect harbour porpoises. Sarah also wants to thank Jo Clark for text taken from the WDCS UK MPA report. Over to you Sarah (and Jo) ...!
Harbour porpoises are the most commonly sighted and stranded cetacean in UK waters. The smallest and most numerous and widely distributed of UK cetaceans, they can live for up to 20 years. Calves are seen between February and September in UK waters, with a peak in June. In the UK you can find the highest densities of porpoises in the Hebrides, north-east Scotland and coastal Wales.
There has been considerable concern for the porpoise status in some regions in recent decades due to apparent declines in numbers of sightings and strandings, and impacts which include ongoing high levels of incidental catches in fishing nets. Pile driving is likely to be a conservation issue for porpoises in the future as this intense and impulsive sound will dominate in coastal waters in coming decades as we erect wind farms around the coastline.
In many areas porpoises are present throughout the year but there do seem to be seasonal changes in distribution and sightings rates, most likely linked to prey availability and the location of suitable breeding and calving habitat. For example, a pattern of peak harbour porpoise numbers off the shelf in May and June, followed by a peak in numbers on the shelf two months later, is thought to relate to calving. These aggregations, occurring in August-September, have been noted for several coastal locations around the UK and coincide with the peak final months of the mating season for harbour porpoises, with social and sexual behaviour frequently observed.
Due to their small size and the lack of markings on their fins that could otherwise allow recognition of individual animals, understanding site fidelity and whether porpoises show regular movement patterns between areas is challenging. However, resightings of highly marked individuals do occur and this indicates a degree of site fidelity. Female porpoises disperse less than males and this may indicate that females stay in the places that they calve and nurse their young.
Important foraging habitat for harbour porpoises includes areas of strong tidal currents, usually near islands or headlands, where the currents combine with the seafloor topography and seem to create conditions where prey become aggregated. For this reason, development of a wide-scale tidal industry, likely to be utilizing these same tidal currents, could have a negative impact on porpoises but no research has unvestigated this yet.
Although there is some variation in prey depending on area, season and age of the porpoise, dietary studies have found that sandeels are the most important prey species for north east Atlantic harbour porpoises during the spring and summer, and whiting in the autumn and winter months. These will be supplemented with other fish such as herring, mackerel and gobies throughout the year.
Anatomical and genetic studies on harbour porpoises to understand population structure have indicated that several populations and subpopulations exist in the North Atlantic. In UK waters, genetically differentiated subpopulations exist in the Irish Sea/Wales and the North Sea, with some possible further divisions of the North Sea population.
Understanding population structure is important for conservation efforts. It is likely that the effects of regional threats such as bycatch, naval sonar or marine renewable energy will be underestimated if animals are thought to be part of a single, wider population but actually form discrete subpopulations.
The harbour porpoise is protected under UK and European law (on paper), yet no marine Special Areas of Conservation for harbour porpoise have been established in UK waters and all the threats they face, including bycatch, continue.