OSC Marine Mammal Observers out in the field are being kept busy with sightings of harbour porpoise (Phocoena phocoena).
Harbour porpoise are some of the smallest cetaceans, with females reaching ca. 1.6 m in size, and males 1.4 m. Distributed in coastal and offshore Northern temperate and subarctic waters, they are the most abundant cetacean in the North Sea. Diet varies with location but small fish such as herring, sprat and sandeel form a large part of the diet in some areas; squid is also common prey.
Visually, harbour porpoise are difficult to observe in the wild. Travelling alone or in small pods of 2 – 4 animals, their small size and quiet surface behaviour means they are easy to miss, even in calm seas. Typically, only the back and small triangular dorsal fin are visible when they surface to breathe.
Unlike the majority of odontocetes (toothed whales and dolphins) harbour porpoise do not whistle, but instead communicate, navigate, explore and hunt using echolocation. High frequency echolocation clicks are produced almost contnuously, making acoustic monitoring an ideal method of observation. Passive Acoustic Monitoring (www.passiveacousticmonitoring.co.uk) methods used commonly are C-PODs (www.cpodclickdetector.co.uk) and T-PODs (www.t-pod.co.uk).
C-POD research conducted by OSC has found that harbour porpoise occur frequently around offshore platforms, which act as artificial reefs. Analysis of echolocation data showed that harbour porpoise are potentially using these areas as feeding grounds.