HOMING AND OLFACTION IN SALMONIDS: A CRITICAL REVIEW WITH SPECIAL REFERENCE TO THE ATLANTIC SALMON

(1) Based on data from the literature, the phenomenon of homing in salmonids is examined with special reference to the Atlantic salmon. Wild, native fish demonstrate an excellent homing ability, judged from percent return to the home river (1–3 %) and straying to non-native watersheds (less than 3 % of returning fish). (2) The homing ability in wild fish is shown to be closely related to the existence of reproductively isolated populations between and within watersheds, as demonstrated by data from salmonid ecology and biochemical genetics. (3) Two main hypotheses have dominated the literature on salmonid homing during recent years: (a) An ‘imprinting’ hypothesis based on a process of learning of stream odours during seaward migration, coupled with sun-orientation for open sea navigation, (b) A ‘pheromone’ hypothesis related to odours from fish and based on inheritance and the seasonal migrating schedules of discrete populations. (4) The olfactory sense has been demonstrated as mandatory for salmonids, both in near range and open sea navigation. According to genetic, sensory and ecological aspects of homing, the pheromone hypothesis is therefore concluded to be the most appropriate. (5) Fish produced from artificially fertilized eggs, released within native systems or transplanted, demonstrate a reduced homing ability. Since hatchery-raised fish demonstrate a survival in sea equivalent to that of wild fish, a genetic disturbance of navigational ability has been suggested, resulting from the production of population hybrids by man. (6) Studies made in the fields of behaviour, electrophysiology and chemistry strongly suggest that population-specific fish odours are involved in home-stream recognition by salmonids. (7) An evaluation of ‘imprinting’ experiments related to artificial organic compounds reveals that: (a) the odorant properties of the applied chemicals must be questioned, (b) imprinting related to olfaction may be based on a weak theoretical foundation, (c) returns obtained in census experiments may be adequately explained through ecological interpretations, and (d) behavioural preferences obtained from exposure to non-natural compounds may be founded on mechanisms not associated with homing. (8) A logical link between the use of olfaction and the role of genetics in salmonid homing is emphasized, together with its practical implications for salmonid management.