The Jackson Hole News & Guide has an excellent report about biologists using detective work to determine when lake trout (Salvelinus namaycush) were first introduced into Yellowstone Lake, the largest body of water in Yellowstone National Park. Lake trout are native to the Great Lakes region, but have been artificially introduced throughout the rest of the U.S. Ironically, while they are considered invasive in some bodies of water, lake trout were nearly extirpated from the Upper Great Lakes in the mid-20th century.
By examining the composition of the otolith, a structure found in the inner ear of the trout, the researchers can estimate how long a fish has been in a particular body of water. The otolith, also called an "ear stone," grows in successive layers as the fish ages, so that outmost layers are formed when the fish is oldest. Its chemical composition includes the elements calcium and strontium, and the ratio of these two elements can be calculated and compared over several parts of the otolith. A significant change in the strontium:calcium ratio indicates the fish was displaced (or purposely introduced) to a new body of water.
The researchers identified two years, 1986 and 1994, when the Yellowstone Lake lake trout were introduced. They also used the strontium:calcium ratio to determine that the fish originated from nearby Lewis Lake, which doesn't sound quite so insidious until you consider that the two lakes are separated by the Continental Divide. :-) Lake trout have been in Lewis Lake since the 1800s.
For more details about this research project, and about efforts to conserve the native Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri), check out the spring edition (v.14, #12, .pdf) of the journal Yellowstone Science.