The average annual natural mortality rate of age-V and older lake trout was estimated to be 25%. Annual fishing rates for these age groups in 1975 ranged from 19% in Statistical District MM5 to 47% in MM7 for a lake-wide mean of 30%. The standing crop of age-V and older lake trout in 1975 was estimated to be 254,000 fish.

No natural reproduction by the planted lake trout was found until 1977. This is believed to be due to the tendency of the hatchery-reared lake trout to home as adults to the inshore area of planting, where spawning occurs over unsuitable substrate and chances of egg survival are low. The hypothesis that reproductive failure is caused by environmental contaminants has been tested and rejected. Harvest of lake trout by the sport fishery is not an important factor in limiting natural reproduction because large aggregations of spawning lake trout have been observed for several years.

Movement by most adult lake trout, as determined from tag returns, appeared to be largely within a 20-mile radius of the tagging locality. The tagged fish showed a strong tendency to home to the tagging site during the spawning season.

Mean lengths of present-day lake trout are much greater than in 1947, ranging from 57% larger at age VII to 150% at age III. Because of reports of decreasing growth rates for Michigan salmon and Wisconsin lake trout, there was concern that a similar trend might exist for lake trout in Michigan waters of Lake Michigan. However, we found no consistent pattern to indicate that lake trout have decreased in average size since 1970. Four- and five-year-old lake trout from southern Lake Michigan were of larger average length than those in the middle and northern sectors of the lake; however, the mid-lake and northern stocks were equal to or greater than the southern population at age VI-IX. Southern and mid-lake stocks were slightly heavier than the northern population at the same length.

The mail creel survey was believed to have overestimated the 1975 sport harvest of lake trout by a factor of 5. Thus the annual catches of lake trout, as determined from the mail survey over the years, may be in serious error. Nevertheless, the surveys were useful in following trends in the sport fishery. The number of sport-caught lake trout increased by nearly 5-fold from 1969 to 1975. The geographical distribution of salmonid catch and angling effort had shifted noticeably from northern to southern Lake Michigan. Since 1970, the percentage of lake trout in the sport catch has progressively increased in those sectors of the lake where the harvest was once dominated by salmon.