Through the Gauntlet Again: Demographic Restructuring of American Shad by Migration

Abstract

Long-distance migration in fishes is one of the more spectacular examples of ontogenetic niche switching, yet details are often obscured because of the difficulty in making direct observations on individual migrants. Juvenile emigration and ultimate recruitment to the spawning population were studied retrospectively in American shad (Alosa sapidissima), to look for patterns of size-biased survivorship as a consequence of migration. Adults were collected on or near the spawning grounds in the Hudson River, New York, USA, in spring 1995. Their early life histories were reconstructed by measuring strontium and calcium concentrations in the inner portions of their otoliths, which were deposited when the fish were young-of-the-year (YOY). Sudden elevations of the Sr:Ca ratio were interpreted as seaward migration events, and the physical location of these elevations on the otolith could be related to the age and size at which the fish first emigrated from fresh water. Recruited adults showed distributions of first emigration that were bimodal with respect to size and age, and these patterns were repeated in three of four year classes analyzed. In 1989 and 1990, years when YOY data were available, size distributions of apparently emigrating YOY do not agree with the distributions displayed in the otoliths of returning adults from the same year classes, suggesting that differential mortality had occurred, which was dependent on the timing of emigration. Detailed analysis was possible for the 1990 year class, which showed strong evidence in both sexes of at least two major recruitment bottlenecks—one affecting larvae hatched early, and the second affecting emigrating juveniles hatched from late cohorts. Potential causes of differential mortality on emigrating juveniles include the presence of large predators outside of the Hudson, as well as the presence of hypoxic waters into which the fish may have entered. This study indicates that long-distance migration of juveniles can be a strong factor that restructures the demography of a year class of fish.