Definitions for "Year class"
Fish born in a given year. For example, the 1987 year class of cod includes all cod born in 1987. This year class would be age 1 in 1988, age 2 in 1989, and so on.
fish of a given species spawned or hatched in a given year
Most fish species in temperate waters (like those found in the Chesapeake Bay and offshore Virginia) reproduce during a relatively short (one or two month) period each year. That period may be different for each species. Fisheries scientists refer to all of the fish of any species hatched during one annual spawning period as a year class. For mathematical purposes, fishery analysts often treat members of the year class as if all fish were hatched on one day.