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WhaleNet 's Satellite Tagging Program - Contains information on satellite tags and tagging , data from current and archived tagging projects on whales , porpoises , seals , and turtles , and questions and activities to help use the data in the classroom . The first , when I struck , came up , porpoised , and looked like a whale . If you find yourself porpoising , then you simply landed too " hot , " meaning too fast . The S-38 seemingly has no bad habits in the water and makes consistently smooth landings without a noticeable tendency to porpoise . By examining the stomach contents of stranded porpoises from the east coast of Scotland collected each spring between 1992 and 2001 , researchers from the University of Aberdeen found that at this time of year , harbour porpoises rely heavily on sandeels for food . Porpoise habitat in the Yangtze has been degraded by water development , including the Gezhouba and Three Gorges dams and about 1300 smaller dams in tributaries . As we head offshore to the domain of the great whales you will be greeted by grey seals hauled out on ledges , harbor porpoise scurrying along in small groups , and extensive rafts of sea birds . These large dolphins are often seen surfacing slowly , although they can be energetic , sometimes breaching or porpoising , and occasionally bowriding . Excessive porpoising can cause a prop strike on some models , such as Mooney . Albirostris ) and the harbour porpoise ( Phocoena phocoena ) . Usually this will arrest the porpoising , and you can mush in , but if not , you're ready to add more power and go around . There are also a large number of harbour porpoises inhabiting these waters due to high concentrations of their prey species , herring and capelin . They travel mainly by porpoising , whipping low over the surface , and like to bowride boats up to speeds of 35kph . Instead of a dorsal fin , the finless porpoise has an area of small bumps or tubercles on its back , running from just forward of mid-back to the tail stock . Some toothed Cetaceans are called whales , some are dolphins , and some are porpoises . " Harbour porpoises are found in the temperate waters of the northern hemisphere in a nearly circumpolar distribution .