Timing of spawning and assessment of a degree-day model to predict the in situ embryonic developmental rate of white sucker, Catostomus commersoni
We sampled eggs and larvae of white sucker, Catostomus commersoni, on six spawning grounds to (i) determine the influence of water temperature and discharge on spawning and (ii) assess the reliability of four degree-day equations developed to predict the attainment of organogenesis, eyed egg, hatching, and swim-up phases. We estimated a threshold temperature for spawning of 13°C at four sites and 10°C at two colder sites. A delay of 2 or 3 days was observed between the threshold and the onset of spawning. On average, the degree-day equations predicted the attainment of the developmental phases with an accuracy of 1.6 days, except at the two colder sites, where developmental rates were faster than predicted. The lower spawning temperatures and faster developmental rates observed at the colder sites were interpreted as adaptations to the reduced growing season. Finally, at one site the drift of swim-up larvae occurred over a 6-day period even though spawning lasted 24 days; it was not possible to determine the exact duration of drift at the other sites. This synchronization could be an adaptation lowering the risk of predation on individual larvae, as all the larvae enter the lake within a few days.