Mechanisms of Fish Migration in Rivers

Abstract

The unidirectional flow of rivers provides a strong orientational cue and the confined channel provides fixed reference points for migrating fishes. Within a dendritic river system there may be great spatial and temporal variation in physical and chemical characteristics of the flow which though perhaps confounding might provide a means for branch recognition by migrants.

In rivers migration must ultimately involve both a downstream and an upstream component. The former is generally, but not always, a feature of early life-history stages and vice versa. Migration of a given life-history stage, however, may involve both downstream and upstream components. Downstream migration may be passive or active, but upstream migration must be active. Downstream migration of juveniles and adults is mainly nocturnal, but sometimes diurnal, especially in turbid rivers. Characteristically, maturing adults of riverine and anadromous species move upstream to spawn, but some species or populations may move downstream to spawn. Upstream migration may be nocturnal or diurnal.

Timing of migration in rivers depends on the physiological state of the fish, as influenced for example by thyroid and corticosteroid hormones, and external triggering factors. An environmental stimulus, such as water current, temperature or light, may alter fish orientation and act as a “director” of migration, or the stimulus may trigger movement or alter the intensity of movement and act as a “regulator” of migration. Celestial and magnetic cues seem to be involved in some migrations in river-lake systems, but use of landmarks has not been well documented. Orientation to water current is undoubtedly the most important “director” of upstream migration. Together with the co-role played by detection of unique stream odors and odors from conspecifics, it forms the most well-studied, and perhaps most important, guidance system for upstream migration, especially of anadromous salmonids. The precise nature of the odors, of the imprinting process (to both natural and synthetic substances), and of the orientation mechanism itself remain to be learned.

Population-specific, genetically determined factors may influence the responses to an environmental stimulus, e.g. the response to water current in fry of lake inlet and outlet spawning populations. The control mechanisms for riverine migrations are likely organized in sequences or hierarchies appropriate to the ontongeny and evolutionary history of a population. The adaptive significance of migration is enigmatic, but probably lies in the fact that optimal habitats for different functions (survival, growth, reproduction) are spatially, seasonally and onotogenetically separated.