The principal spawning grounds of lake trout (Salvelinus namaycush namaycush) in United States waters of southern Lake Superior are on rocky shoals at depths of less than 20 fathoms. Most spawning occurs in October and early November. Of the mature fish collected on or near the spawning grounds, 60 to 69 percent were males. Among mature fish the average length of females was greater than that of males; few males less than 24 inches or females less than 26 inches in total length were caught. Recoveries of lake trout tagged on the spawning grounds showed that some males remained in the immediate area for a period of several weeks during the spawning season. Marked fish showed a tendency to return during later years to spawning grounds on which they had been tagged, even though many of them ranged long distances between spawning seasons.
The percentage of the total weight composed by gonads of maturing females collected in 1950 was 5.0 percent on August 11–30 and 12.3 percent on October 6–9, but this increase was not accompanied by an increase in the coefficient of condition.
One volumetric and two gravimetric methods for estimating numbers of eggs in ovaries were evaluated. Some advantage in accuracy of estimate resulted from the drying of eggs in an oven before weighing on an analytical balance. Production of eggs by 70 lake trout ranging from 25.1 to 38.0 inches in total length and from 5.8 to 19.6 pounds in weight ranged from 2,476 to 17,119. Average production was 646 per pound of fish; this number was greater among fish 31.0 inches long or larger than for fish of smaller size.
Thirteen fish identified by commercial fishermen as siscowets (Salvelinus namaycush siscowet) also showed extensive individual variation in numbers of eggs, although egg production tended to increase with increasing length and weight of the fish. A mature female siscowet as small as 16.5 inches long and a 15.7-inch mature male were collected about 45 miles north of Grand Marais, Michigan. Combined data from all collections showed that spawning of lake trout or siscowets may occur during at least 6 months of the year (June to November).