Dall's Porpoise  Phocoenoides dalli

Dall’s porpoises have small, robust bodies and triangular heads.  In addition to a small dorsal fin, they have another small hump located just in front of their tail flukes.  This hump is more pronounced in males.  

Dall’s porpoises eat a variety of prey items including squid, sardines and herring, and even lantern fish.  They do most of their feeding at night. 

They are sexually mature at 3.5 to 8 years of age and have a gestation period of about 10 to 12 months.  Calves typically nurse for less than a year.

Dall’s porpoises travel in groups which vary greatly in size, sometimes ranging to thousands of animals.  They will also sometimes travel with Pacific white-sided dolphins and pilot whales.  These groups structures are not formal and many individuals are only loosely associated with one another.

Dall’s porpoises are presumed numerous in North America despite a lack of reliable population estimates.  This is not the case in Japan however.  Huge numbers of Dall’s porpoises, an estimated 18,000 per year, are lost in gillnet fisheries and to Japanese coastal whaling operations.  Other threats include pollutants which make their way into the blubber layer of these animals and prove toxic.

Books:

Porpoises (WorldLife Library) by Andrew J. Read, 1999
THE PORPOISE WATCHER by Norris S Kenneth, 1980

Links:
http://www.nmfs.noaa.gov/pr/species/mammals/cetaceans/dallsporpoise.htm http://www.acsonline.org/factpack/DallsPorpoise.htm

http://www.nmfs.noaa.gov/pr/species/mammals/cetaceans/dallsporpoise.htm

http://www.acsonline.org/factpack/DallsPorpoise.htm