A common assumption in fish studies is that iteroparous fish, once mature, normally reproduce in all consecutive seasons. Analysis of scales from Norwegian spring-spawning herring collected between 1935-1973 revealed strong under-representation (47% of expected) of second-time spawners on the spawning grounds. This reduction is not explained satisfactorily by possible 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
