Francisco Marco-Rius, Pablo Caballero, Paloma Morán and Carlos Garcia de Leaniz Can migrants escape from density dependence? Ecology and Evolution 3
Our study tests the hypothesis that organisms can “escape” from density-dependence constrains by migrating, and that this strategy maximizes growth by reducing competition from conspecifics. To test this, we reconstructed individual growth profiles of sea trout (Salmo trutta) before and after migration, and related these to estimates of year class strength over a 13-year period. We found that variation in circuli spacing was generally much greater among individuals than within individuals, and that scale growth was inversely related to year class strength, both during the freshwater and marine growing seasons. However, density-dependence was c. 2.5 times stronger in freshwater (before migration) than at sea (after migration) indicating that although migrants might benefit from enhanced growth by moving into the sea, they do not appear to become free from density-dependence constraints completely. Our findings shed light on the nature of density-dependent regulation in migratory species, and provide a plausible mechanism for the maintenance of partial migrations if, as our study indicates, resident and migratory individuals are subjected to density-dependent forces of varying strength before and after migration.
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