48 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 23. — Calculated lengths {sums of mean increments of groivth in inches) of unmarked lake trout caught by 4V2-inch-mesh 



gill nets in 1947 in Lake Michigan 



[Year classes 1939-43 combined] 



seventh aiimilus. Differences at the eighth annu- 

 lus were due to tiie small number of measurements 

 (6 for area 7 and only 1 for area 8). As explained 

 earlier, the calculated length at the first annulus 

 for lake trout caught in area 7 (as published), was 

 not based on the same criteria as the data on the 

 O-mark and the first annulus treated in this paper. 

 For this reason, the calculated lengths of the 

 fish from areas 7 and 8 were combined only from 

 the second to the eighth annulus. The mean 

 calculated length at the first annulus of the fish 

 from area 8 was 6.1 inches and the average annual 

 increase in length of the combined groups was 3.0 

 inches (table 23). Comparison cannot be made 

 of these figures with like figures from lake trout of 

 year classes 1944-46 from the southern areas of the 

 lake as none of those fish were caught in large-mesh 

 nets. The calculated lengths for the early year 

 classes, however, were very much larger than those 

 for the fish of year classes 1944-46 caught in small- 

 mesh nets (table 22). 



Selective destruction of the more rapidly grow- 

 ing individuals by sea lampreys and by nets of the 

 commercial fishery leads to a decrease of gi'owth 

 rate with increase of age which would not exist 

 within a stock not subject to such selective mor- 

 tality. It is a natural consequence of continued 

 selective destruction of large fish, that each older 

 age group should be composed of slower-growing 

 fish than the younger age groups. 



Because the combined eft'ects of biased samplitig 

 and selective destruction of the marked lake trout 

 by lampreys cannot be measured, it must be 

 recognized that tiie "normal" growtli of lake trout 

 in Lake Michigan probably was not determined 

 precisely. However, tlie use of summations of 

 the mean increments of growth in length to 

 describe general growth tends to lessen the effects 

 of selective mortalitv and thus to vield curves 



more representative of the true rate of growth 

 than otherwise could be obtained from these data. 

 A third cause for discrepancies in estimates of 

 growth of lake trout in Lake Michigan, not, 

 however, affecting area estimates, is geographic 

 differences in size and growth. Lake trout in- 

 habiting the northern part of the lake were larger 

 at each year of life than those in the southern 

 part of the lake. Tliis difference in size is ap- 

 parent in comparisons of fish in the same year 

 classes caught in nets of the same mesh size. 

 Examples: the early year classes (1939-43) caught 

 in 4}^inch-mesh nets (table 23, fig. 25), and the 

 later year classes (1944-46) caught in 2)^inch- 

 mesh nets (table 22, fig. 23). For tliese and 

 other groups of lake trout from the two parts of 

 the lake, the differences appear to stem principally 

 from a slower growth of the southern fish during 

 their first summer to formation of the first annulus. 

 The southern fish of the early year classes caught 

 in the large-mesh nets, at formation of the first 

 annulus, were 0.8 inch shorter than a similar 

 group of the more northern fish, but the average 

 annual increases in length in later years were 

 identical. Those of the more recent year classes 

 caught in 2K-inch-mesh nets, at formation of the 

 first annulus were 0.6 inch shorter and the average 

 annual increases in length were 0.4 inch less than 

 the annual gains of the unmarked northern fish 

 of the same year classes. The consistency of the 

 discrepancies between the calculated lengths of 

 lake trout from southern and northern Lake 

 Michigan indicates that they represent a true 

 geograpliical difl'erence of growth between the two 

 populations. 



GROWTH IN WEIGHT 



Weights were available for only 1,118 of the 

 1,319 marked lake trout, but these were sufficient 



