Harbour porpoises (Phocoena phocoena) are one of the smallest marine mammals. They are widespread within cooler coastal waters of the North Atlantic, North Pacific and Black Sea. Within the northwest Atlantic, they range from western Greenland down to North Carolina, USA.
The northwest Atlantic harbour porpoise is, on average 1.6 meters long and weighs ca. 50 kg. Within Canada, there are three different populations: Newfoundland & Labrador; the Gulf of St. Lawrence; and, the Bay of Fundy/Gulf of Maine (http://www.dfo-mpo.gc.ca).
The Bay of Fundy (http://en.wikipedia.org) is located on the Atlantic coast of North America between New Brunswick and Nova Scotia, Canada just off the north eastern end of the Gulf of Maine. The Bay of Fundy is famous for having the highest tidal range in the world (ca. 17 m, with a record peak of over 21 m). Harbour porpoises enter the Bay of Fundy during the early summer and then return to the Gulf of Main in the autumn.
Using satellite telemetry (http://en.wikipedia.org), line transect surveys (http://en.wikipedia.org), and remote sensing techniques (http://en.wikipedia.org and http://en.wikipedia.org), researchers found that during summer months, harbour porpoises in the Bay of Fundy have restricted focal regions that coincide with tidal circulation (Johnston et al. 2005). While their overall range is widespread (7,738 – 11,289 km2 in this study), the size of their monthly focal area was quite small (122 – 415 km2). Features such as island and headland wakes that occur during tidal flow aggregate plankton, and therefore augments prey availability for harbour porpoises (e.g. Johnston et al. 2005; Johnston & Read 2007). At the finer scale distribution, relative harbour porpoise density was highest during flood tides around these features (9.59 animals/km2 during flood tide phases, and 1.79 animals/km2 during ebb).
The biggest threat to harbour porpoise in this area is accidental entanglement in commercial bottom-set gill nets (http://en.wikipedia.org).
Using Porpoise Echolocation Detectors (PODs) it was found that chemically enhanced gill nets, which are meant to be more acoustically reflective, did not alter harbour porpoise echolocation occurrence or rate (Cox & Read 2004). This meant that harbour porpoises do not respond to the acoustic nature of the chemically enhanced gill nets. Another study has shown that it may be the stiffness of the net that reduces by-catch (Larsen et al. 2002).
Visual and acoustic T-POD (www.t-pod.co.uk) surveys have established that harbour porpoises both feed and transit through an aquaculture cage site in the Bay of Fundy (Haarr et al. 2009). Harbour porpoises were displaced temporarily from the site during boat traffic or certain work on the cages (e.g. cage cleaning), always returning a short time after the disturbance ceased. General worker activity during the day did not appear to disturb the porpoises, as a similar 24 h cycle in echolocation activity was seen both at the cages and offshore. All study sites showed an increase in echolocation activity during the night. Such diel activities have also been seen at offshore gas installations in the North Sea (Todd et al. 2009).
Acoustic Harassment Devices (AHDs)
Acoustic Harassment Devices (AHDs) are often used at aquaculture sites to deter seals from eating caged fish. Given that harbour porpoises are also known to frequent these areas, there is concern of how they are affected by noise created by AHDs. One study in the Bay of Fundy found that fewer porpoises were sighted when AHDs were active than when they were not (Johnston 2002). Similar results have also been observed on the west coast of Canada in British Columbia (Olesiuk et al. 2002). The long term effects of AHDs on harbour porpoises are still unknown.