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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



and then surpass in weight the Lake Michigan 

 population at greater lengths. On the other 

 hand, although Lake Michigan yellow perch had 

 the lowest exponent value (2.811), they were so 

 heavy at the shorter lengths that the other Great 

 Lakes populations (Green Bay, Lake Erie, and 

 Saginaw Bay 1929-30) were unable to reach 

 their weights even at the greatest lengths. 



The unusually low value of n (2.811) in the 

 equation for yellow perch of northern Lake 

 Michigan probably can be attributed to the selec- 

 tive action of the commercial gill nets (2%-inch 

 mesh) by which they were captured. Gill nets 

 have been shown to take the relatively heavier 

 of the shorter fish and the relatively lighter of 

 the longer ones (Farran 1936; Deason and Hile, 

 1947; Le Cren 1951). 



The rapid rate of increase of weight with 

 length in Saginaw Bay collections of 1943-55 

 also gave them a weight advantage over fish 

 collected in 1929-30. Differences between the 

 two groups were nil or slight at the shorter 

 lengths, but among larger fish the 1943-55 col- 

 lections had substantially greater weights. At 

 10.5 to 13.5 inches the advantage ranged from 

 0.5 to 1.6 ounces. 



CALCULATED GROWTH 



Body- Scale Relation 



Most workers who have published on the 

 growth of yellow perch have given only the 

 average size of age groups, or, if they published 

 calculated lengths have assumed that the body- 

 scale ratio is constant at all lengths of fish. 

 Studies have been made, however, of the body- 

 scale relation of perch in Saginaw Bay (Hile 

 and Jobes, 1941), Lake Erie (Jobes 1952) and 

 Lake of the Woods (Carlander 1950). Particu- 

 lars on the findings for the two Great Lakes 

 stocks are given later in this section. Carlander 

 described the body-scale relation in yellow perch 

 of Lake of the Woods by two conic-section parab- 

 olas, one fitted to data for fish 50 to 150 mm. 

 long and the other to data for smaller (down to 

 19 mm.) and larger (up to 289 mm.) fish. 



Inasmuch as the curve of regression of fish 

 length on the radius of the key scale below the 

 lateral line, presented by Hile and Jobes (1941), 

 was based on plentiful materials, its use in the 

 present study for the calculation of growth from 

 measurements of scales from the same general 



area of the body could be considered valid. 

 Through a misunderstanding on the part of the 

 field collectors, however, the scale samples of the 

 1954 collections were removed from above the 

 lateral line, which made necessary a study of the 

 body-scale relation for scales from that part of 

 the body. Because the assembling of materials 

 for a body-scale study was necessary it was de- 

 cided to extend the study and make a redetermi- 

 nation of the relation for the key scale below the 

 lateral line. This extension permitted the com- 

 parison of two regression lines determined inde- 

 pendently for the same key scale and also the 

 comparison of the growth of the same fish as 

 estimated independently by regression lines for 

 scales from two positions on the body. (The 

 exact positions of both key scales are stated in 

 the section on materials and methods.) 



Key scales below the lateral line 



The graphic presentation of the body-scale data 

 from below the lateral line (fig. 11) indicated 

 that for fish over 70-mm. standard length the 

 body-scale relation could be represented by a 



20 



40 60 80 100 120 140 160 



SCALE RADIUS (MILLIMETERS, x 43) 



180 200 



Figure 11.— Relation between body length and scale 

 length of Saginaw Bay yellow perch (key scales from 

 below the lateral line). 



