Year Class

A year class (or cohort) consists of all fish born in the same year. Because fish recruitment is highly variable (driven by environmental conditions during the larval and juvenile period), year classes vary enormously in strength. A strong year class — produced by exceptionally favourable environmental conditions — can dominate the commercial catch for the entire lifetime of those fish (sometimes 10–20 years in long-lived species). Conversely, a series of weak year classes can cause stock collapse even without overfishing. Year class strength is monitored by fisheries scientists using research surveys and commercial catch-at-age data.

The 1970 North Sea herring year class was exceptionally strong; it was still detectable in the stock 10 years later and contributed disproportionately to spawning stock biomass through the late 1970s.