The longjaw (Leucichthys alpenae) was found at all of the 109 stations fished in the open lake during 1930–1932 and at 29 of the 32 stations in Green Bay in 1930 and 1932. Koelz (1929) found the longjaw at 35 localities, 33 of which were different from those fished in 1930–1932. The species was most abundant in water with a depth of less than 70 fathoms, but was found as deep as 97 fathoms. The longjaw was estimated to be about 4 1/3 times as abundant along the east shore of southern Lake Michigan as along the west shore in 1930 and 1931. The abundance along the east shore in 1930–1931 was estimated to be approximately equal to that in northern Lake Michigan in 1932. The best explanation for the relative scarcity of L. alpenae along the west shore is that a more intensive fishery, with smaller sizes of mesh, was operating on a separate population.

The length distribution of 6,954 specimens taken in 1930–1932 revealed little difference in frequencies between the sexes, the samples taken in each of the years 1930 and 1931, or the fish taken on the two shores of the southern region. The individuals caught in 1932 from northern Lake Michigan averaged 12.5 inches, total length, and were from 1.0 to 1.3 inches longer than fish taken by the same sizes of mesh from the southern area in 1930–1931.

Of the 378 longjaws whose ages were determined, 320 were taken during the summer of 1923 in northeastern Lake Michigan, and 58 were secured off Grand Haven, Michigan, in November 1928. The age groups represented ranged from II to IX. Age-group IV dominated in the 1923 samples making up 53.8 percent of the total, and age-groups III and V with 20.9 and 10.6 percent, respectively, were the next best represented groups. The III group dominated the 1928 collection with 55.2 percent of the total, and age-groups IV and II, that were represented by 31.0 and 12.1 percent, respectively, were the only other well represented groups.

The sexes grew at approximately the same rate. A total length of 11.1 inches and a weight of 6.4 ounces were reached at the end of 4 years of growth by the longjaws in northeastern Lake Michigan. The fish from Grand Haven averaged 11.0 inches, total length, and 6.1 ounces at the end of 4 years. Growth in length was most rapid during the first year and decreased continuously thereafter through the fourth year (fifth year in 1928). The increments in length of the 1923 fish during the fourth through the eighth years were approximately the same. Growth in weight of the fish taken in 1923 was computed to be most rapid following the fifth year of life. Growth compensation occurred among the longjaws of Lake Michigan. The major part of the annual growth in length had taken place before June 15, 1923.

Although the empirical data on the length-weight relationship of 5,314 fish failed to fall along a simple curve, it is believed that the following equation is the one best suited to computing the weight of the longjaw: W = 0.96288 × 10⁻⁵ L^3.06060.

The coefficient of condition (K) of all longjaws increased as the standard length increased to 194 millimeters. The values then remained high up to 225 millimeters but decreased progressively until the fish reached a length of 265 millimeters. The coefficient changed little at fish lengths of 265 to 284 but began to increase again at 285 millimeters. Condition was best in 1931 and poorest in 1932. The individuals of both sexes were in the poorest condition during some month previous to August each year. Although the females were somewhat heavier in relation to their length than the males in practically all months, the weighted averages of K for all males and all females were almost identical (1.30 for the males and 1.31 for the females).

The sex ratio was determined from 7,457 individuals taken in 1930–1932 and 373 of the 1923 and 1928 specimens whose ages were determined. The percentage of females was relatively low in May 1931 and relatively high in October, but in the other months it remained more or less constant. In 1932 the females tended to become relatively less abundant in each month from April through September. There was little difference between the relative abundance of the sexes in 1930 and 1931 (72.4 percent females in 1930 and 67.5 percent in 1931). The consistently slightly higher percentage of females each month in 1932 (average of 80.6) probably represents a difference between populations. The relative abundance of the males decreased as the age increased. A natural differential mortality is suggested as the cause for the more rapid disappearance of the males.

Females may contain ripe eggs as early as July 9, and some females may spawn as early as October 16 despite the fact that the spawning season is in November.