Herring (Clupea harengus) migrations tend to remain constant over periods of several years or even decades, despite environmental variation. When a migration pattern is changed, apparently in response to an environmental stimulus, the change in migratory behavior sometimes lasts longer than the environmental stimulus itself. This paper reviews evidence that the response of herring to environmental change is restrained by conservatism. Herring apparently develop certain migratory habits during the early stage of adult life and tend to adhere to these habits for life, even when the environment changes. Two elements of conservatism are distinguished: the formation of habits within one generation, and the transfer of habits between generations. A number of case studies on North Sea herring and Norwegian spring spawning herring are reviewed in order to find evidence for habit formation and tradition in the migrations of these populations. By eliminating the possibility that homing to spawning, wintering and feeding grounds is due to innate behavior or environmental constraints, it is shown that the return of these fish to the same areas each year is most likely a form of learned behavior. In many cases, new year-classes adopt the same migration pattern as their predecessors, which is explained by the social transfer of habits from old herring to young ones. A change of migration pattern is usually initiated by a recruiting year-class that lacks the ``guidance'' of older herring at the time it has to start its first migration. This may be the effect of a special environmental condition that affects the distribution of the recruiting age group more than that of the adult stock component, and thereby results in a separation of the two stock components. After the recruiting year-class has adopted the new migration route, it tends to repeat it in subsequent years. The new migration thus becomes habitual. Subsequent generations may copy the migration and continue this behavior even after the original environmental cause has disappeared.
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