42 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



MEAN LENGTH AND RANGE IN LENGTH OF MARKED LAKE TROUT AT TIME 

 OF CAPTURE COMPARED WITH MEAN CALCULATED LENGTHS 



T — [ — 1 — I — I — I — r 



• MEAN LENGTH AT TIME OF CAPTURE 



X MEAN CALCULATED LENGTH 



Q l I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 



SONDJFMAMJJASONDJFMAMJJASONDJFMAMJJASONDJFMAMJJASONDJFMAMJJAS 



FIRST YEAR SECOND YEAR 



THIRD YEAR 



FOURTH YEAR 



FIFTH YEAR SIXTH YEAR 



Figure 20. — Mean length and range in length of marked lake trout, at time of capture compared with mean calculated 

 lengths obtained by adding annual increments of growth (assuming January 1 the date growth is completed). 

 Vertical broken lines give range of lengths, and dots the mean lengths at capture. Calculated lengths are shown 

 along the solid, diagonal line. 



year, 2.5 inches tlie third, fourth, and fifth jears, 

 and 2.4 inches the sixth year. 



Factors of Discrepancies in Estimates of Growth 



Several factors were considered as possible 

 causes of the discrepancies in the estimates of 

 growth made from the different age groups of 

 marked lake trout: (1) condition of the fish; (2) 

 sex differential in growth; (3) selectivity of nets 

 employed by the fishery; (4) selectivity of lamprey 

 predation. The effects of the first two were not 

 considered important. Nearly all fish captured 

 were taken during the summer months, thus 

 seasonal changes in condition were not a factor. 

 Combining data on the three year classes masked 

 the annual differences Sexual differences had 

 not developed on these fish, most of which were 



still immature; none cauglit was in gravid condi- 

 tion. The other two factors affecting estimates 

 of growth are discussed later. 



Selectivity of gill nets is an important factor 

 which would have a tendency to cause discrepancies 

 in estimates of the growth rate. Some marked 

 lake trout (43.4 percent) were caught in the 4/2- 

 inch-mesh gill nets of the whitefish and lake trout 

 fisheries and others (56.6 percent) were taken in 

 the 2j2-inch-mesh gill nets of the chub fishery. 

 The percentage of the total catch of lake trout 

 taken by the 4)2-inch-mesh nets decreased from 

 91.3 in 1947 to 17.5 in 1950. At the same time 

 the percentage caught in tlie 2}2-inch-mesh nets 

 increased from 8.7 to 82.5. During this period lake 

 trout were becoming so scarce that fishermen were 



