The Alaska Department of Fish and Game confirmed in July that the salmon was the first documented Atlantic salmon caught in Cook Inlet, but the fish’s origin remained unknown until it was further tested in a lab using its otoliths — small, rounded bone-like structures found in the inner ears of fish.

The otoliths grow unevenly depending on the fish’s growth rate, forming rings like those found in the trunk of a tree.

The Atlantic salmon found in Cook Inlet had very even growth rings, indicating it had received regular feedings, as do salmon raised in hatcheries and farms, said Bob Piorkowski, invasive species program coordinator for Fish and Game.

Once Fish and Game had inspected the salmon’s otolith rings, it searched the otoliths for additional distinguishing marks and with the help of the Washington Department Fish and Wildlife determined the fish had likely escaped from a private hatchery near Scatter Creek in Rochester, Wash.

While inspecting the salmon’s otoliths, Fish and Game discovered what is called a thermal mark.

Washington requires Atlantic salmon raised in the state’s waters to be thermally marked to aid identification should the fish escape. A thermally marked fish has been briefly exposed to higher than normal temperatures, which leaves a distinguishing mark on its otolith.

Although the distinguishing mark found on the otolith used to trace the salmon back to the hatchery could have been caused by something other than the thermal exposure at the hatchery, it is unlikely, Piorkowski said.

According to the production manager of the hatchery near Scatter Creek, the hatchery’s last major escape occurred in the spring of 2005, releasing 4,500 one-pound fish, Piorkowski said.

However, the production manager said the salmon discovered in Cook Inlet likely escaped in May when fish from the hatchery were loaded onto a transfer barge, Piorkowski said.

Atlantic salmon can jump much higher than Pacific salmon, and the salmon discovered in Cook Inlet is suspected of jumping insufficient barriers during the transfer.

Alaska prohibits fish farmers from raising Atlantic salmon, but nearly 600 specimen of this exotic species have been documented in state waters thanks to fish farms in neighboring waters.

According to Piorkowski, approximately one out of every 100 Atlantic salmon raised on fish farms in British Colombia and Washington escape.

Many worry Atlantic salmon that escape into Pacific waters could spoil native salmon stocks through colonization, interbreeding, predation, habitat destruction and competition.

Among some of the key identifying characteristics of an Atlantic salmon are an anal fin with eight to 11 rays (but they are occasionally found with 12), large scales and a mouth that does not extend past the far edge of the fish’s eye.

Pacific salmon have an anal fin with 13 or more rays and large mouths that reach past the eye.

Also, Atlantic salmon can be distinguished by the uniquely “x” and “y” shaped, large, black spots found on their scales and cheek plates. Their dark tails, however, are unspotted.

To receive an Atlantic salmon identification card or to report a fish suspected of being an Atlantic salmon, fishermen should call 1-877-INVASIV.

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