AbstractPhotographic and genetic studies have revealed the complex population structure of migratory large whales. Most research and management are geographically based, focusing on either wintering or summering areas, which in most cases fails to capture this complexity. We use examples from humpback whales (Megaptera novaeangliae) and gray whales (Eschrichtius robustus) to illustrate the concept of migratory herds, defined here as conspecific whales that migrate between the same wintering and feeding ground. In cases where individuals show strong fidelity to both feeding and wintering grounds, and therefore to a herd, dispersal between herds can be low enough to render them as demographically independent populations, where population dynamics are determined more by net internal recruitment (births minus deaths) than by immigration from outside sources. In these cases, the migratory whale herd is the appropriate unit to conserve under the ecological paradigm that focuses on groups united by demographic forces. We suggest that when addressing questions focused on ecological timescales for whale species with strong fidelity to migratory destinations, the migratory whale herd is a better initial working hypothesis rather than one based on geographically defined strata.