Here you go, Bob. Anytime you catch one of your famous little fish, you can just say "it's the season, they shrink." Or if it is the middle of summer you can say, "Must be a winter holdover."
From Midcurrent if you want to read the whole story: http://www.midcurrent.com/news/2010/11/scientists-brown-trout-shrink.html
If you were looking for an excuse not to go in search of a "trophy" trout in the middle of winter, scientists in Norway and Finland may have provided it. A team lead by the Finnish Game and Fisheries Research Institute in Paltamo discovered that "juvenile brown trout reduced in length by as much as 1cm -- a shrinkage of approximately 10%. They say this could help the young fish to conserve energy when food is in short supply." From the BBC News.