Harbour porpoises (Phocoena phocoena) are small marine mammals that resemble dolphins, but are actually not very closely related. They belong to their own separate family called the Phocoenidae. Porpoises do fall into the wider category known as cetaceans (whales, dolphins, and porpoises) and belong to the Odontoceti suborder, commonly known as the toothed whales (http://en.wikipedia.org/wiki/Odontoceti).
Porpoises are small and stocky, growing to a maximum length of 1.9 m, with small triangular dorsal fins. They are found in cool coastal waters of the North Atlantic, North Pacific, and the Black Sea, with an estimated population in the North Sea of 250,000 individuals (SCANS 2008). For more information on harbour porpoises, including the use of porpoise acoustic click detectors, please see www.porpoisedetectors.co.uk
Seismic surveys introduce high intensity, low frequency (most energy <1 kHz) noise into the marine environment. For a review of seismic noise and cetaceans, see www.marinemammalseismic.co.uk. The effects of seismic airgun noise on cetaceans in general, is difficult to quantify; effects are likely species-specific and vary between seasons, locations, and oceanographic and bathymetric conditions (for information on seismic surveys and other species, see www.towedarray.org and www.staticacousticmonitoring.com). In many Northern Hemisphere shelf seas (areas that are currently, or have the potential to be exposed to offshore energy developments), harbour porpoises are the most common cetacean species, and thus the species most likely to be exposed to seismic survey noise.
HARBOUR PORPOISES DISPLAY ONLY SHORT-TERM DISPLACEMENT DURING 2D SEISMIC SURVEYS
Thompson et al. (2013) undertook a study to assess the impacts of a two-dimensional (2D) seismic survey on harbour porpoises in the North Sea. The seismic survey was conducted in two areas of the central Moray Firth between 1st and 11th September 2011. Harbour porpoise displacement was measured using porpoise click detectors (C-PODs) (www.c-podclickdetector.com) and aerial visual surveys. For an explanation on how C-PODs work see www.dolphindetectors.com
PASSIVE ACOUSTIC MONITORING METHODOLOGY & DATA QUANTITY
C-PODs were deployed throughout August, September, and October of 2010 and 2011 in control (no seismic activity) and impact (seismic activity) areas up to 70 km from the sound source. In 2010, baseline data were collected by C-PODs at 70 sites, although only 60 provided useful data. In 2011, C-PODs were deployed at the same 70 sites in July, and useful data was retrieved from 49 devices four months later. Only click trains with high or medium levels of confidence were analysed, and used to compare spatial and temporal variation of porpoises.
AERIAL SURVEY METHODOLOGY
In 2010, in order to estimate harbour porpoise density in the survey area, visual aerial surveys were conducted using line-transect methods. In 2011, digital aerial surveys were flown on three days before the seismic survey started and four days during it. Methods were based on video techniques used for bird surveys (www.seabirdsurveys.co.uk). Flights were conducted along transects that covered the survey area and were only performed on days with suitable weather for observing (Beaufort sea state <4, swell <1.5m, cloud base >300m). Data from aerial surveys allowed relative density of porpoises to be estimated in 5 x 5 km blocks.
As soon as the seismic surveys commenced, time between porpoise detections (waiting time) increased when compared to baseline data collected the week before. The change in waiting time was also linked to distance from the seismic source, with waiting time decreasing as distance increased. When acoustic detection data were compared with digital aerial survey data, results showed that the relative density of porpoises decreased within 10 km of the seismic vessel, but increased at distances greater than 10 km. Once the seismic vessel passed a C-POD site, waiting time for porpoises increased, but detections always resumed within 19 hours (average 3.05 hours). This was, however, significantly longer than matched random waiting times recorded during baseline assessment, which had an average of 57 minutes. A gradual decline in waiting time was observed throughout the survey, suggesting that increased exposure reduced the response of porpoises to seismic sound.
Thompson et al. (2013) showed that a two-dimensional survey can cause avoidance behaviour in harbour porpoises, but effects appear to be short lived.