Egg Development Timing for Riverine Spawning Pygmy Whitefish (Prosopium coulterii)

Despite the large numbers of individuals present during riverine broadcast spawning, little is known about the spawning behavior or egg development timing of pygmy whitefish (Prosopium coulterii). We captured pygmy whitefish from spawning schools in the upper Cedar River, Washington State, and live-spawned to collect milt and eggs. Once fertilized, eggs were placed in Whitlock-Vibert incubation boxes in natural river conditions until hatching. Egg development was monitored weekly by counting eggs and alevin present in boxes, examining a previously undisturbed box each week. Pygmy whitefish hatched over a range of 324 to 370 accumulated temperature units (ATUs). The range in hatch times in this population may be a result of multiple selection pressures (e. g., high flow events frequency, predation, food availability) that confer differing advantages to early and late-hatching individuals. Consequently, even though all broadcast spawning occurs within two weeks, hatching and emergence is spread over a broader temporal period so that not all individuals in the cohort are subjected to the same environmental conditions. These results give better understanding of the timing of hatch and emergence in a pygmy whitefish population and contribute to better management of the species in the face of environmental uncertainty resulting from global climate change.