Here is the reply I received from WDFW:
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Dear Mr. Carey:

Thank you for your e-mail correspondence to the Washington Department of Fish and Wildlife (WDFW) Fish Program.

Both grass and common carp reside within Green Lake. Common carp were probably first observed in Green Lake somewhere between the late 1940's to the early 1960's and have been present (and very prevalent) since. Grass carp, a recent introduction, were stocked into the lake in 2001 to control severe infestations of Eurasian water milfoil.

Presently, anglers may fish for common carp in Green Lake. There is no bag or size limits for common carp and at present, no licenses are required either. In Washington, it is illegal to possess a grass carp. If an angler happens to catch a grass carp it must be released back into the lake immediately.

Common and grass carp can be differentiated from each other by several physical characteristics. These include the following:

Common Carp: stocky and laterally compressed, suction-type mouth with barbels (whiskers), very large diamond-shaped scales (however, some individuals may have scattered scales [mirror carp] or have no scales [leather carp]), coloration is brownish-orange (in general).

Grass Carp: longer and more slender (but still robust) than common carp, their mouth is closer to a trout's (no whiskers though), they have very rigid jaws used to grasp and tear aquatic vegetation, they are grayish-white in appearance, and have large diamond-shaped scales (grass carp do not have a mirror or leather form), all grass carp stocked into Washington waters (as well as most other states) are sterile.

I hope this answers you and your reader's questions. If you have any other questions feel free to contact me either by e-mail or phone.

Sincerely,

Chad Jackson
Area Inland Fish Biologist
425-775-1311, ext. 113