344 BULLETIN OF THE BUREAU OF FISHERIES 



with the actual. Owing to the absence of herring of years I and II in my material, 

 I could not determine definitely whether corrections for these two factors would also 

 give more accurate calculated lengths for the first and second years of life. 



From the preceding study it is concluded (1) that the structural characters of 

 the scales of the lake herring {Leucichthys artedi Le Sueur) are so clearly recognizable 

 as to permit their use by the scale method, and (2) that the fundamental assmnptions 

 underlying the scale method are warranted. The scale method, therefore, may be 

 applied with confidence in a study of the life history of the lake herring. In this 

 life-history study, which comprises the second major part of this paper, all computa- 

 tions of body lengths and increments are based on the diameter measurements of 

 scales. No corrections were made in these calculated values. They are regarded 

 as approximately correct for any age-group under consideration. 



TIME OF FORMATION OF AN ANNULUS AND FACTORS INVOLVED IN IT 



I have recently (Van Oosten, 1923) presented data that, 1 believe, definitely 

 establish the causal relationship between the growth of scales and the formation of 

 annuli in the whiteiish {Coregonuf< dupeajormis). This was done by a study of scales 

 taken at monthly intervals from whitefish segregated and kept living at the New York 

 Aquarium. The growth changes in the scales were followed from November to July. 

 It was shown that the annulus was completely formed some time in April or March, 

 at the time when rapid scale growth was resumed. The data on pages 313 to 322 of 

 this paper indicate that scale and body growth are closely correlated. Any factor, 

 therefore, that can retard the growth rate of the body may have primary influence 

 in the formation of annuli on scales. To hold a factor responsible, it must be shown 

 that this particular factor was altered previous to or synchronously with the change 

 in growth rate, and that no resmnption of rapid scale or body growth occurs until the 

 change in the factor is reversed or its effectiveness is lost. It is possible for more than 

 one factor to be active at the same tmie and for the factors to vary with the years or 

 even with the seasons of the same year. 



With this criterion in mind, I found that in the adult whitefish of the New York 

 Aquarimn temperature and sexual maturity apparently assumed primary significance 

 in the formation of annuli. My conclusions w^ere recapitulated on page 407, as follows : 



It has tlius far been shown that the scales of the aquarium whitefish ceased growing some time 

 in August or September and resumed growth in April or March(?) ; that se.xual maturity was reached 

 some time between September and March or FebruaryC?) ; that the lowest temperatures of the aqua- 

 rium water occurred in January to March, inclusive; and that the amount of food required by the 

 fish was less for these months of the year. It was suggested that food could only have had secondary 

 significance in the formation of annuli, since the reduction of food was caused by some other factor, 

 which affected the appetite of the fish. It was further suggested that since reduction and increase 

 in food consumption occurred synchronously with the decrease and increase in temperature, respec- 

 tively, and since the scales resunied their growth at the time of a rise in temperature in April, when 

 sexual maturity could have had no influence on growth, temperature must be considered a primary 

 factor in the formation of annuli. Lastly, since the se.x products began their development at approxi- 

 mately the time at which a retardation or cessation of scale growth occurred in late summer, when 

 the environmental factors of food and temperature were known to have been constant, it appears 

 reasonable to a.ssert that sexual maturity is also a primary factor in the formation of annuli in scales. 

 If sexual maturity is not such a factor, then it must be conceded that the retardation or cessation 

 of scale or body growth, and consequently the formation of annuli, is cau.sed by some unknown 

 physiological factor or factors of annual recurrence. 



