. The Canadian field-naturalist. 10 The Canadian Field-Naturalist Vol. 106. Figure 4. Distribution records of Ichthyomyzon fossor in Canada. maturity around mid-May. The post-spawning period probably lasts only a few days, after which all spawners die. Since degeneration of the alimentary canal occurs at the beginning of transformation, there is a period of eight or nine months during which no food is taken (Churchill 1947). Three physical factors in streams are essential for successful spawning: first, for nest building, a suit- able substrate of gravel is required that includes at least a sm
. The Canadian field-naturalist. 10 The Canadian Field-Naturalist Vol. 106. Figure 4. Distribution records of Ichthyomyzon fossor in Canada. maturity around mid-May. The post-spawning period probably lasts only a few days, after which all spawners die. Since degeneration of the alimentary canal occurs at the beginning of transformation, there is a period of eight or nine months during which no food is taken (Churchill 1947). Three physical factors in streams are essential for successful spawning: first, for nest building, a suit- able substrate of gravel is required that includes at least a small amount of silt-free sand or other fine material to which the eggs can adhere, thereby increasing the probability of their retention in the nest. Second, a current must be flowing uni-direc- tionally over the nest. Third, the water temperatures must be suitable. In Manitoba, fourteen mature individuals were cap- tured in the Birch River in mid-May 1977 (Jyrkkanen and Wright 1977). Of these, 10 were males and four were females. They were assumed to be spawning at the time of collection. No details on the reproductive behavior were reported. Two more sexually mature individuals, a male and a female whose eggs were free in the body cavity, were captured in the Whitemouth River (into which the Birch River empties) in mid- July 1977 (Lanteigne 1981). For two nests in which sex ratio was determined in a Michigan river, 11 males and two females were in one and three males and one female in the other (Morman 1979). In a trib- utary of southern Lake Superior, Purvis (1970) noted that 97% of the metamorphosed specimens collected in August were males and in June, 75% of the spawn- ers were also males. In a Michigan river, Ichthyomyzon fossor was observed in seven nests on 13 June when water tem- peratures ranged from ° to °C (daily mean 18°C) [Morman 1979]. Spawning occurred in a shal- low, pool-riffle, high-gradient stretch of the stream. Nests were inconspicuously located
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