28 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Discrepancies between increments of growth 

 were also small. At the second check the differ- 

 ence was 0.1 inch between marked and unmarked 

 fish from all nets in areas 1-6, but was nil between 

 unmarked or wild stocks from the northern and 

 southern parts of the lake. At the third check the 

 increment of growth of the unmarked fish in 

 areas 1-6 was 0.1 inch smaller than that of the 

 marked fish from the same areas and 0.2 inch 

 larger than that of the unmarked fish in area 8. 



With regard to the central check on the scales 

 of the naturally reared lake trout, two assumptions 

 are possible. First, that these fish did in fact 

 form a 0-mark during their first growing season; 

 under this assumption these data exhibit no 

 particular conflict with those for planted lake trout. 

 Second, it may be assumed that naturally reared 

 lake trout do not complete a 0-mark, and hence 

 that the calculated lengths for the first three 

 checks on the scales describe the fish at completion 

 of their first, second, and third growing seasons. 



A corollary to this thesis, namely, that the 

 average length of the unmarked, wild fish from all 

 nets in areas 1-6, at the end of their first year 

 (3.5 inches), was about the same as that of the 

 marked, hatchery fish at formation of the 0-mark 

 (3.7 inches), might be accepted without mis- 

 givings as the hatchery and naturally propagated 

 lake trout spent much of their first year in different 

 environments. If this corollary is accepted, 

 however, it follows that the increment of growth 

 in length of the wild stock in northern Lake 

 Michigan during their second growing season 

 would be only 2.1 inches which is considerably 

 less than the growth indicated for this group 

 during either the first or third (2.7 inches) years. 

 A growth of 2.1 inches the second year would be 

 0.7 inch less than the growth made in the same 

 environment by the hatchery fish in their second 

 year and 0.6 inch less than the growth made by 

 the marked liatchery fish between their introduc- 

 tion into the lake in September at a length of 3.2 

 inches and formation of the first annulus when 

 they were 5.9 inches long. 



The growth made by the wild stock between 

 formation of tlie first two checks on their scales, 

 nevertheless, was very nearly the same as that 

 made by the marked fish between formation of 

 the 0-mark and the first annulus. It would be 

 expected that the wild stock would grow at about 

 the same rate as the introiluced fish after Sep- 



tember, but if they did, and the first check on the 

 scales were the first annulus, they could not have 

 grown any the fore part of the season. The length 

 of the wild stock at the end of the second year 

 would be 5.6 inches or 0.3 inch shorter than 

 the marked stock at the end of their firs t year and 3 . 1 

 inches shorter than the marked fish at the end of their 

 second year. To justify this relationship, it isneces- 

 sary to assume that the wild stock grew erratically 

 during their first or second year. The rates of growth 

 in later years were about the same for the marked 

 and unmarked lake trout. Although it cannot be 

 stated categorically that the central check on 

 the scales of the unmarked fish was not the first 

 annulus, neither does it seem reasonable to assume 

 that it is. The evidence strongly favors the 

 belief that the first check on the scales of the 

 naturally propagated lake trout is a 0-mark 

 formed during the first growing season. This view 

 is supported further by the appearance of the 

 check itself (pattern, and location on the scales). 

 See figures 15 and 16. 



The contribution of data on lake trout from 

 southern Lake Michigan to the problem of the 

 0-mark is greatly limited by the lack of recoveries 

 of planted fish from this area for comparison with 

 the wild stock. Nevertheless, the much smaller 

 increment of growth before formation of the first 

 check on the scales of lake trout in southern than 

 in northern waters makes it difficult to assume that 

 the 0-mark of these naturally propagated fish is a 

 first annulus. If this assumption is made, it is 

 necessary to believe that these fish were only 4.8 

 inches long at the end of two full growing seasons 

 or 1.1 inches shorter than tlie marked fish from 

 northern Lake Michigan at the end of one year 

 (5.9 inches). Alternatively, if it is assumed that 

 the first check is a 0-mark, the calculated length 

 at that point is somewhat smaller than that for 

 the northern fish at formation of this check. 

 vSubsequent growtli is only slightly less for the 

 soutiiern than the northern fish. This growth 

 pattern follows closely that of the marked fish. 



If the hypothesis, that most or all naturally 

 reared lake trout do form a 0-mark on their 

 scales during their first growing season, is ac- 

 cepted, the question then arises as to the extent of 

 error that this structure might introduce into the 

 work of a competent and careful scale reader 

 who is not aware of its existence. The only 

 objective information on this point comes from 



