46 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



The discrepancies miglit have been explained 

 on the basis of annual fluctuations of growth, had 

 not the calculated lengths from fish of all the age 

 groups within each year class of unmarked lake 

 trout varied consistently about a lower mean than 

 those of the marked fish. Evidently, the larger 

 size of marked lake trout over unmarked fish of 

 the same year class is a real rather than apparent 

 difference, which suggests that the marked fish 

 may have derived a certain advantage from the 

 hatchery environment during their first summer 

 that carried over into later life. 



The lake trout of year classes 1944-46 from 

 area 8, caught in small-mesh nets, were decidely 

 smaller than the northern wild stock caught in 

 these nets. The average calculated length of the 

 southern fish at the first annidus was only 4.6 

 inches and the average annual increase in length 

 to the sixth year of life was 2.0 inches (table 22; 

 fig. 23) compared with a calculated length of 5.2 

 inches at the first annulus and an annual increase 

 of 2.4 inches for the northern fish. 



A major difference between samples of marked 

 lake trout from 2}^- and 4/2-inch-mesh nets was 

 the near absence of Lee's phenomenon in the data 

 for the fish taken by the latter gear (fig. 24). 

 These fish were subject to little or no selectivity 

 from the nets, for few of tlie marked lake trout 

 grew large enough to exceed the catching potential 

 of the large-mesh nets. 



Another factor in bringing about apparent 

 decline, even cessation, of the growth of marked 



lake trout with increase in age is believed to be 

 destruction by sea lampreys of the most rapidly 

 growing fish. Lengths, at capture, of marked lake 

 trout in age-group V were little greater than those 

 of fish a year younger; and lengths of fish in age- 

 group VI were actually smaller than those in 

 age-group V. A high percentage of the larger 

 specimens in age-groups V and VI (28 percent of 

 those caught in 1951) bore scars or open wounds 

 made by lampreys. Smaller fish were unscarred; 

 hence it is thought that lamprey predation is 

 most severe among larger lake trout 14 or more 

 inches long. It is possible, nevertheless, that 

 small lake trout which have been attacked by 

 lampreys die immediately so they do not come 

 into the nets with wounds as do the larger fish. 

 Hall and Eliott (1954) found also an increase of 

 scarring with increase in length of the fish for the 

 white sucker (Catostoinus commersoni) . They 

 showed that incidence of scarring was consistently 

 greater among suckers more than 10 inches long 

 than among smaller fish and near 100 percent for 

 fish 19 to 20 inches long. Thus the larger fish of 

 the younger age groups and nearly all in the older 

 age groups were being eliminated leaving only 

 small, slow growing individuals. 



Wild and hatchery lake trout of the same year 

 classes were subject to the same selectivity by the 

 nets and the same predation by lampreys. The 

 marked lake trout and the wild stock of year 

 classes 1944-46 were comparatively free from 

 atta(!ks by lampreys until they were about 14 



Figure 23. — (.'alculated lengths (sums of mean increments of growtli in inclies) of marked and unmarlsed lake trout 



(year classes 1944-46) caught in 2'i-inch-mesh nets. 



