Timing of spawning and predicted fry emergence by naturalized Chinook salmon (Oncorhynchus tshawytscha) in a Lake Huron tributary

We evaluated how well a naturalized Chinook salmon population matches their seasonal timing of spawning to the period of thermal suitability (i.e., phenological match) in the Sydenham River (Ontario), because this could be a factor in their persistence. In 2010 and 2011, the mean spawning date (= date of nest settlement) in the Sydenham River was in early October, about 7-12 days earlier than that of the source population at around the time of the transplant (Green River/Soos Creek, Washington, USA). Twenty-eight percent (2010) and 72% (2011) of females settled on nests when temperatures were above those considered suitable for spawning (> 12.8 °C). From nest settlement to death, average water temperature was 9.5 °C in 2010 (n = 28) and 14.4 °C in 2011 (n = 22). Median egg retention rate showed a significant seasonal decline in the warmer year (2011), but overall rates (2010: 0.078, n = 20; 2011: 0.264, n = 13) were not significantly higher than rates for native populations of Oncorhynchus spp. (0.10). By applying Belehrádek's model of temperature-dependent development to the Sydenham River population, we predicted fry to emerge from early April to early June at water temperatures of 7.3-17.3 °C, depending on the year and time of spawning. All but the latest emerging fry should experience thermally suitable conditions for growth (< 15.6 °C). We suggest that thermally appropriate timing of juvenile growth may compensate for weak phenological match during the spawning season and may have contributed to the naturalization of Chinook salmon in the Sydenham River.