Dynamics in frequency of skipped reproduction in Norwegian spring-spawning herring
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A common presumption in fisheries science is that adult iteroparous fish, once matured, typically spawn in all consecutive years. Recent evidence suggests, however, that skipping of reproduction occurs more commonly than is usually believed. Adult Norwegian spring-spawning herring (Clupea harengus) undertake long annual migrations between feeding, overwintering, and spawning areas. Analysis of extensive historical data on scales suggested that, on average, almost one in two herring may skip their second spawning migration. Moreover, the frequency of skipping may vary considerably from year to year. Based on annual variations in skipped reproduction, relationships are examined here between the mean weight and condition of spawning herring in a given year, and the fraction of fish skipping the second reproductive season in the following year. Environmental influences on skipping are examined based on indices of annual temperature and zooplankton abundance in the Norwegian Sea. The results corroborate with the hypothesis that skipped reproduction results from trade-offs between current and future reproduction, growth and survival: participation in distant, energetically costly and risky spawning migrations may only pay off in terms of fitness if individuals are sufficiently large and in sufficient condition to both successfully migrate and spawn.
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