Current controversies in the study of salmon homing

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Abstract
Salmonid fishes are famous for their tendency to home from feeding areas to spawn in their natal stream. In spite of considerable research effort, many aspects of homing are still poorly understood. This paper reviews four topics in the study of homing mechanisms which are presently unresolved or controversial. Pacific salmon (Oncorhynchus spp.) are emphasized but the issues apply widely within the family. The first topic is the migration from open ocean to coastal waters; tagging studies and correlations between ocean temperatures and migratory timing are in conflict regarding the possibility of true navigation by salmon. The second controversy involves homing through coastal waters and estuaries. These regions have physical characteristics which differ from both ocean and river environments and it is not clear how salmon make the transition from open water orientation to upstream migration. The third controversy concerns the process by which young salmon learn the chemical characteristics of their natal stream and the way in which adults use this information to return home. Specifically, do salmon imprint on the odors of their river only once, just prior to seaward migration, or sequentially during freshwater residence and migration? The fourth controversy concerns the role of species-specific and population- specific odors («pheromones») in homing. Salmon can distinguish between populations within their species and between full-sibling families within their population but the relevance of these olfactory responses to homing is not clear.