did it have a big head and small body....? If so the length and the low weight could mean that either the fish had trouble finding food or there may be a forage problem in the lake. Not that there is no forage for the trout to eat, there just may be an imbalance in the numbers of trout to Forage. Or the fish could be just real old and was not going to live that much longer.
I Know that in some of our streams here in PA that do have wild trout in them alot of the bigger older fish are the big heads and small bodies fish. one stream in particular has a very large population of trout in it and the older trout have to spend more energy then they are getting from they food that they are able to catch, thus causeing the fish to suffer from malnutrition of sorts. they have to spend more energy because the have to compete with the younger quicker trout for the food.
Just my 2 cents,
JiggerMan