Abstract
A common assumption in fish studies is that iteroparous fishes, once mature, normally reproduce in all consecutive seasons. An analysis of scales from Norwegian spring-spawning herring collected between 1935 and 1973 revealed a strong under-representation (47% of expected) of second-time spawners on the spawning grounds. This reduction is not satisfactorily explained by errors in scale-reading, suggesting that the second reproductive season is frequently skipped. Skipping a season may relate to trade-offs between growth, current and future reproduction, and survival, which are likely to be particularly strong for young adult herring.