Smolt development in distinct populations of coho salmon (oncorhynchus kiutch): The influence of water temperature and water temperature preference

Smolting is an important time in the life of anadromous salmon when juveniles undergo changes that prepare them for life in the ocean. Widely distributed salmon populations have evolved based on the selective pressures of their local environments. I examined population differences in the development, duration (smolt window) and loss of seawater tolerance, how temperature influenced development among populations, and temperature preference throughout smolting in coho salmon (Oncorhynchus kisutch). For my first objective, I compared smolt development among three populations in British Columbia, Canada that each had different migration distances to the ocean. In year one, fish appeared to undergo an incomplete smoltification based on the biochemical marker Na+/K+-ATPase (NKA). Additionally, molecular markers (mRNA expression of gill NKA α1a, NKA α1b, as well as growth hormone (GHR), prolactin (PrlR) and glucocorticoid (GR1) receptors) suggest that fish also did fully develop physiological changes associated with smolting. In year two, the smolt window (approximately 300 accumulated thermal units – ATU) did not differ by population as evident by elevated NKA activity. Molecular markers (gill NKA α1a, NKA α1b, GHR, and PrlR) for smolting also did not show a population difference – indicating that distinct populations do not differ in their development, duration, or loss of seawater tolerance. For my second objective, I examined temperature preference in short- and long-distance migrating populations of coho salmon juveniles. Mean temperature preference did not differ between the two populations (15.9 °C and 16.1 °C) and did not change throughout smolting. Based on the observed temperature preferences, smolts do not prefer temperatures that are advantageous for prolonging the smolt window. Additionally, the disparity between temperature preference and what is likely available in the environment at the time of smolting demonstrates that temperature is not a selective pressure for juveniles to behaviourally regulate the development of seawater tolerance.