As we know, trees add a growth ring each year that can be used to count the age of a tree. Tree rings can be counted on the stump after a tree has fallen or been removed, OR without destroying the tree by using a common forestry tool called an increment borer.
Trout (and all fish for that matter) also put on growth rings each year, but put on groups of growth rings annually- not a single ring like a tree. Growth rings occur on each of their scales as well as every bone in their body. Biologists call these fish growth rings, "annuli".
Groups of annuli are referred to as "curruli" and when fisheries biologists want to age a fish, they are not counting each individual growth ring (as with trees), but instead count the spaces in between the groups of annual rings. Spaces between the rings represent times when the fish slow in their eating, so essentially when biologists age fish by counting these spaces between rings, they are counting the winters that the fish has been alive.
Courtesy of Maureen Stine of Natureology
Fish can have it pretty tough in winter, especially when water levels are low and winters are long. Fish living in our local inland lakes and ponds may not survive the cold due to any number of factors:
Less Water: More likely it will freeze all the way down, trapping fish
Less Water: Less oxygen available
Longer Winter: Less oxygen available as supply is used
More Ice: Water's oxygen supply from air is stopped and fish may suffocate
So next time you're having a rough day, thing about those fish in shallow waters.