Predation of lake trout and lake whitefish embryos by crayfish: Implications of shifts in crayfish dominance in Lake Simcoe

We compared predation rates of two Lake Simcoe crayfish, Orconectes virilis and Orconectes propinquus, on lake trout and lake whitefish eggs and alevins in laboratory experiments and assessed the potential impact of predation on egg survival in the lake. Experiments were conducted at 4 and 8 °C on gravel and one or three layers of cobble substrate. Predation rates on lake trout eggs at 8 °C were 2.3 and 3.6 eggs/crayfish/day on gravel, 1.3 and 1.4 on one layer of rock, and 1.2 and 0.2 on three layers of rock for O. propinquus and O. virilis, respectively. Similar rates were observed for lake whitefish eggs but the effect of substrate depth was greatly reduced. Predation rates were significantly lower at 4 °C. Overall O. propinquus was a more effective predator of eggs than the larger O. virilis, even after adjustment to a common body size. Lake trout alevins were eaten by both species, but at about one third the rate of lake trout eggs. Vulnerability of alevins declined in deeper substrate, especially for older alevins. In the lake O. propinquus was abundant on rocky substrates to the near exclusion of O. virilis. Based on observed densities of O. propinquus in Lake Simcoe, estimated consumption of lake trout eggs ranged from 5.6 to 16.6 eggs/m2/day depending on temperature and substrate depth. Shifting crayfish dominance from O. virilis to O. propinquus, or vice versa, has the potential to significantly influence egg survival and recruitment success of lake trout and to a lesser degree lake whitefish in Lake Simcoe.