1. In a stock, a group of fish generated during the same spawning season and born during the same time period; 2. In cold and temperate areas, where fish are long-lived, a cohort corresponds usually to fish born during the same year (a year class). For instance, the 1987 cohort would refer to fish that are age 0 in 1987, age 1 in 1988, and so on. In the tropics, where fish tend to be short-lived, cohorts may refer to shorter time intervals (e.g. spring cohort, autumn cohort, monthly cohorts).