The behavioural homing response of adult chum salmon Oncorhynchus keta to amino-acid profiles

Relationships between maturational status and migration behavior of homing chum salmon Oncorhynchus keta in inner bays of the Sanriku coast

The correlations among gonad maturity and various homing behaviors of chum salmon, Oncorhynchus keta, were evaluated using acoustic tracking of tagged fish in Otsuchi Bay, Japan. There was a negative correlation between the time duration from release of tagged fish until river entry and the plasma 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) levels, an indicator of final maturation. Females with high DHP entered the rivers soon after the release, whereas females with low DHP (<10 ng/ml) took a few days to more than one week until river entry. Similar correlation was also found in males. A pattern of river entry correlated with maturational conditions was also observed in fish entering the rivers of neighboring bays. DHP concentrations of fish caught in the rivers were consistently higher. On the other hand, more than half of released salmon departed from the bay regardless of their plasma DHP level, suggesting that maturational status does not force homing adults to enter the most available nearest rivers. Fish entering the rivers experienced ambient temperatures less than 8 °C, which is approximately 5 °C lower than that of the bay. These results indicate that homing salmon hold their position in the bay until just before spawning, which may be attributable to low temperature avoidance. This characteristic type of river entry may be suitable to geographical features and thermal regimes of this region.

Enhanced osmoregulatory ability marks the smoltification period in developing chum salmon (Oncorhynchus keta)

The freshwater (FW) life of chum salmon is short, as they migrate to the ocean soon after emergence from the substrate gravel of natal waters. The alevins achieve seawater (SW) acclimating ability at an early developmental stage and the details of smoltification are not clear. We examined the stage-dependent SW acclimating ability in chum salmon alevins and found a sharp increase in SW tolerance during development that resembles the physiological parr-smolt transformation seen in other salmonids. Perturbation of plasma Na⁺ after SW exposure was prominent from the hatched embryo stage to emerged alevins, but the plasma Na⁺ became highly stable and more resistant to perturbation soon after complete absorption of yolk. Marker gene expression for SW-ionocytes including Na/K-ATPase (NKA α1b), Na-K-Cl cotransporter 1a (NKCC1a), Na/H exchanger 3a (NHE3a), cystic fibrosis transmembrane conductance regulators (CFTR I and CFTR II) were all upregulated profoundly at the same stage when the alevins were challenged by SW, suggesting that the stability of plasma Na⁺ concentration was partly a result of elevated osmoregulatory capability. FW-ionocyte markers including NKA α1a and NHE3b were consistently downregulated independent of stage by SW exposure, suggesting that embryos at all stages respond to salinity challenge, but the increase in SW osmoregulatory capability is restricted to the developmental stage after emergence. We propose that the "smoltification period" is condensed and integrated into the early development of chum salmon, and our results can be extrapolated to the future studies on hormonal controls and developmental triggers for smoltification in salmonids.

Sensory mechanisms of natal stream imprinting and homing in Oncorhynchus spp

Juvenile Oncorhynchus spp. can memorise their natal stream during downstream migration; juveniles migrate to feed during their growth phase and then they migrate long distances from their feeding habitat to their natal stream to reproduce as adults. Two different sensory mechanisms, olfaction and navigation, are involved in the imprinting and homing processes during short-distance migration within the natal stream and long-distance migration in open water, respectively. Here, olfactory functions are reviewed from both neurophysiological studies on the olfactory discrimination ability of natal stream odours and neuroendocrinological studies on the hormonal controlling mechanisms of olfactory memory formation and retrieval in the brain. These studies revealed that the long-term stability of dissolved free amino-acid composition in the natal stream is crucial for olfactory imprinting and homing. Additionally, the brain-pituitary-thyroid and brain-pituitary-gonadal hormones play important roles in olfactory memory formation and retrieval, respectively. Navigation functions were reviewed from physiological biotelemetry techniques with sensory interference experiments during the homing migration of anadromous and lacustrine Oncorhynchus spp. The experiments demonstrated that Oncorhynchus spp. used compass navigation mechanisms in the open water. These findings are discussed in relation to the sensory mechanisms involved in natal stream imprinting and homing in Oncorhynchus spp.