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Investigating harbour porpoise distribution and drivers

Investigating harbour porpoise distribution and drivers
ORCA stock image - harbour porpoise - Andy Gilbert

In the 2018 State of European Cetaceans report, we featured research by Kimberly Nielsen, who was completing a Masters at Plymouth University, using ORCA data investigate harbour porpoise distribution and drivers. This research has now been published in a new paper! 

Below Kimberly has written a blog for ORCA about her research. 

Harbour porpoises (Phocoena phocoena) are ubiquitous in UK seas, but despite their prevalence, information on space-use is patchy. Given the level of bycatch in some areas, understanding how porpoise abundance and distribution changes over time and across space is necessary for conservation management.

The paper “Spatio-temporal patterns in harbour porpoise density: citizen science and conservation in UK seas” has just been published in the journal Marine Ecology Progress Series (https://doi.org/10.3354/meps13828). We used ORCA’s 2006-2017 data to assess patterns in porpoise density along ferry routes in the North Sea, English Channel, and from Penzance to the Isles of Scilly in the Celtic Sea. As small, inconspicuous cetaceans, porpoises can be difficult to spot at a distance and in choppy seas. Distance sampling methods allowed us to estimate how many porpoises were missed on the surveys, providing a better idea of abundance along routes in each region. This analysis showed that average porpoise abundance was highest on the Scilly route, followed by the North Sea, and English Channel.

Density surface models were used to fit the relationship between harbour porpoise abundance and environmental variables that may drive their distribution. We tested depth, slope of the seafloor, distance from the coast, sea surface temperature, the concentration of chlorophyll-a in surface waters, depth of the mixed layer, and the difference in temperature between the surface and seabed. These are all variables that had either been correlated with porpoise distributions in other studies, or are known to have some effect on productivity or the distribution of prey species.  

Location (longitude, latitude), distance to coast, sea surface temperature, chlorophyll-a, as well as the surface to seabed temperature difference impacted porpoise distribution and density, although the exact relationship was region-dependent. In the English Channel porpoises preferred well-mixed areas, and in the North Sea there was a significant preference for highly-stratified waters.

Over the course of the study, densities have increased substantially along routes in the North Sea, while other regions have remained stable. As a whole, the greatest average densities were found on the Scilly route, although relatively high density areas were also identified in northeast Scotland near Aberdeen and Shetland, and in the southern North Sea near the Netherlands.

Other studies and previous ORCA surveys have documented few porpoises in the English Channel. While abundance is still low there, data are starting to show a shift in space-use into the Channel from the southwest. Interestingly, porpoises that strand in the Channel can be genetically differentiated from those farther north in the Atlantic as a result of genetically mixing with a larger subspecies found on the Iberian coast (Phocoena phocoena meridionalis). The difference in preferred habitat found in our study suggests that porpoises in the Channel and North Sea are exploiting distinct prey species, and could indicate that the range of the southwestern porpoise is expanding. More research needs to be done in order to understand the differences between harbour porpoise populations in the northeast Atlantic, including the density and distribution of their prey.

The prevalence of harbour porpoises in coastal waters makes them particularly vulnerable to a suite of anthropogenic stressors, including noise pollution, habitat degradation, and fisheries bycatch. Although they are a common species in the UK, threats such as bycatch could be having a more substantial impact on subpopulations in localized areas. There is a clear need for the work ORCA does to monitor cetaceans in specific areas over time. When considered with larger studies of absolute abundance, such surveys help provide the necessary resolution to monitor vulnerability to anthropogenic threats, detect change, and focus conservation efforts.

   Nielsen PorpoiseManuscript DensityMap                          Nielsen PorpoiseManuscript ChangeInDensity

                                  Harbour porpoise density map                                                                                          Figure showing the change in density