Wednesday, December 23, 2009

Gray Whales (Eschrichtius robustus)




The gray whale is the most common large whale seen from shore along the west coast of North America. Gray whales are found off the Oregon coast all year. They feed in shallow water near shore during the summer and fall, migrate south for breeding and calving during the winter, and migrate north in the spring. The gray whale gets its name from its blotchy color pattern. Some of this pattern is present at birth, but most of it is caused by barnacles growing in the skin or by depigmented areas where barnacles have been.

Gray whales reach 45 feet in length and weigh 35 tons. For comparison, a cross-country bus is 40 feet long. Adult females on average are larger than males. Whales are mammals. They are warm blooded, breathe air, have hair (single hairs around the front of the head that are visible on calves), and give birth to live young that suckle on milk from their mothers. Mid spring to mid fall is the gray whales feeding season. Most of the population spends this time in the Bering and Chukehi Seas off Alaska, although every summer some whales are observed feeding from British Columbia to Mexico. The summer population off the Oregon coast is about 200 to 400 animals, with many of the same individuals returning year after year. Summer feeding is better at higher latitudes because the long days produce lots of phytoplankton (small marine plants), which are eaten by zooplankton (small marine animals).

These are the basic food for all ocean life, stimulating the growth of the marine food web, including bottom-dwelling amphipods, the primary prey of gray whales. There are two basic types of whales: toothed and baleen. The gray whale is a baleen whale. Instead of true teeth, a row of 138-180 baleen plates grows along each side of the upper gum line. The baleen is made of material like a human fingernail. These are quite stiff and solid at its outer edge, each piece of baleen is “fringed” inside the mouth and tapers from 3 inches wide at the gum line to nearly a point at its bottom. These plates are separated by approximately ¼ inch inside the mouth, where their fringes overlap to form an effective screen.

Gray whales feed primarily on benthic (bottom-dwelling) amphipods (shrimp like animals). They go to the seafloor and suck up an area of the bottom about the size of a desktop and a foot deep. Sometimes this makes conspicuous pits on the bottom. The amphipods are trapped on the baleen filter inside the mouth, while mud, sand, and water pass between the baleen plates. This is the way the whale washes the amphipods clear of sand and mud. It then uses its tongue to suck the amphipods off the inside of the baleen fringe. Since gray whales filter animals from mud and water, their baleen is stiffer and has coarser fringes than that of other baleen whales.

 
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