Experimental evidence of homing to site of incubation by mature sockeye salmon, Oncorhynchus nerka

Fish of the family Salmonidae (salmon, trout and charr) are famous for their ability to learn (‘imprint’) olfactory information as juveniles and to use those odour memories as adults to home to their natal site for reproduction years later. However, the spatial scale at which salmonids home has not been determined, and this is important not only for an understanding of the imprinting and homing processes but also because homing determines the spatial scale at which salmon populations are structured. To test the hypothesis that salmon home to specific habitat units within a single small creek, we induced banding patterns on the otoliths (ear bones) of pre-hatch sockeye salmon embryos by altering the temperatures at which they were incubated. Thermally marked embryos were buried in a small pond associated with Hansen Creek, in southwestern Alaska, and allowed to complete development and migrate. In the year when the salmon were expected to return to the creek, examination of otoliths from carcasses revealed that 12 of 324 salmon in the pond were marked, whereas all 138 salmon examined in the creek were unmarked. This distribution of marked fish indicates that more of the experimental salmon returned to the pond than would have occurred by chance, although we cannot be certain that all surviving salmon did so. These results contribute to the growing literature indicating the capacity for very fine-scale populations structure in salmon.