The
harbour porpoise is a small, robust odontocete (order Cetacea) rarely more than
2 m in length, males being slightly smaller than females. The species has a
wide coastal and circumpolar distribution in temperate and sub arctic shelf
waters (mainly 11-14°C) of the Northern hemisphere. Harbour porpoises are
usually seen solitarily or in groups of several animals, although relatively
little is known about their social interactions. Displaying little aerial activity
while they briefly roll at the surface, studying this small cetacean faces
special challenges (Evans et al. 2007).
Due to
its cold living environment, small body size relative to its surface area and
energetically demanding reproductive schedule, the species is confined to the
proximity of its prey more than many other cetaceans. The females are
simultaneously pregnant and lactating most of their lives (Lockyer 2003). The
species’ distribution and movements can therefore be expected to respond
strongly to biological and oceanographical features of the environment, as well
as to anthopogenic activity. Habitat preferences of the lactating females are
of special interest in the Welsh coast where selection for certain sites, both
offshore and inshore, has been previously documented (Pierpoint 2001).
Mainly a
coastal species, the harbour porpoise is vulnerable to inshore development,
pollution, entanglement in fishing gear and changes in its prey fish
assemblages. The harbour porpoise is considered vulnerable in the IUCN Red List
of Threatened Species. Thus identifying and modelling suitable porpoise habitat
is vital for the conservation of this species.
Despite
a reliable concentration of porpoises around the strongly tidal Broad Sound,
Skomer and Skokholm Islands, there is little information available on the
fine-scale porpoise habitat in this particular area. This study will throw new
light into the habitat use of the local population. The three-year survey was
initiated in 2007 and is funded by the CCW.
The aim
of this study is to identify any significant patterns in harbour porpoise
occurrence and behaviour in relation to tidal phase, strength and direction
alongside with other temporal and spatial environmental factors in the study
area. A comparison is made between vessel- and land-based survey methods.
The
study area encompasses the waters
from St Anns Head (51º 40.8 N 5º 10.4 W) to the West 5º 20 longitude and to the
North 51º 45 latitude. Both systematic and opportunistic search, during which
data is collected on regular Pembrokeshire Dive Charter passenger trips, are
carried out. Effort minutes spent searching for porpoises are recorded
alongside with prevailing sighting conditions. For each sighting, time,
position, bearing, estimated distance, total number of adults and calves,
behaviour, animal heading direction, any associations with birds and currents
are recorded. Using geographic information system software, the effort minutes
and sighting information is then related to spatial and temporal environmental
data such as bathymetry, substrate type, sea surface temperature and tidal
currents.
Results
CCW
report and summaries of the main results will be made available to the public
by November 2007.
References
Evans P.G.H., Lockyer C.H. &
Read A.J. 2007. The New Handbook
of British Mammals.
Lockyer C. 2003. Harbour porpoises
(Phocoena phocoena) in the North
Atlantic: Biological parameters. In:
Haug T., Desportes G., Víkingsson G. & Witting L. (eds.), NAMMCO
Sci. Pub. 5: 71-90.
Pierpoint
C. 2001 Harbour Porpoise Distribution in the Coastal Waters of SW Wales. A
report to the International Fund for Animal Welfare.