Evidence for reproductive senescence in a broadly distributed harvested marine fish

Senescence is the physiological deterioration of adult organisms leading to an age-specific decline in fitness principally associated with an increase in mortality rate (actuarial senescence) and decline in fecundity (reproductive senescence). Senescence is common in natural populations of many taxa; however, there are few examples among fishes, even though they are the most speciose vertebrates and comprise a disproportionate number of long-lived animal species. A notable example in fish is Nor wegian spring-spawning Atlantic herring Clupea har engus, which experiences actuarial senescence. Given expected associations between actuarial and reproductive senescence, Atlantic herring likely also experience reproductive senescence. We examined biological data obtained over 4 decades for 15 Canadian Atlantic herring stocks for evidence of reproductive senescence. Age- and length-specific increases in the relative frequency of morphologically non-reproductive herring and age-related decreases in reproductive investment were consistent with reproductive senescence in both sexes, combined with a post-reproductive increase in growth rate. Available evidence was not consistent with 2 alternative hypotheses: delayed maturation combined with higher survival for late-maturing fish, or an increasing frequency of skipped spawning with age. Although the incidence of post-reproductive herring was generally low, this was likely because very few individuals survive to older ages due to fishing. The present study presents the first strong example of reproductive senescence in a marine fish, notably one that is both ecologically important and a target of large fisheries throughout its range.