Age-at-Maturity

Age at 50% maturity (A₅₀) is the age at which half the fish in a cohort become reproductively active. It is one of the most important life-history parameters for calculating spawning stock biomass and MSY reference points. Heavy fishing pressure on large fish can cause evolutionary changes in maturation — selecting for earlier, smaller maturation (fisheries-induced evolution). This has been documented in northeast Arctic cod, eastern Baltic cod, and Norwegian spring-spawning herring. The phenomenon can permanently reduce stock productivity even after fishing pressure is relaxed.

Atlantic cod in the early 20th century matured at 4–5 years; by the 1990s, heavily fished populations matured at 2–3 years — earlier and at smaller size due to fishing-induced evolutionary change.