These data give length and age for over 3000 walleye (a type of fish) captured in Butternut Lake, Wisconsin, in three periods with different management method in place.
A data frame with 3198 observations on the following 3 variables.
Age of the fish, years
1 = pre 1990, 2 = 1991-1996, 3=1997-2000
Weisberg, S. (2005). Applied Linear Regression, 3rd edition. New York: Wiley, Problem 11.3.