LIFE HISTOKY OF LAKE HERRING OF LAKE HURON 295 



number of the circuli in each growth zone, and (4) the distances separating the anniili 

 fell below the normal. 



He considered next a series of scales taken periodically from carp reared in pond 

 and aquarium and observed that the rate of growth of the scales, the number of circuli, 

 and the distances between the circuli varied directly with the rate of increase in body 

 weight. The aquarium fish grew more slowly than those of the pond. Further, to 

 eliminate racial differences he took two carp of the same brood and of equal weights 

 and placed one in an aquarium and the other in a pond, at the same time removing 

 a few scales from each fish. At a later date more scales were removed and compared 

 witli those first taken. Those from the pond specimen showed an increase in scale 

 surface with widely separated circuli, while those of the aquarium fish showed little 

 increase in scale surface and closely spaced circuli. He ascribed the difference in 

 growth to the richer supply of plankton food in the pond water. 



That other factors may be involved in scale growth he illustrated by the fol- 

 lowing: The water in a pond containing carp was accidentally allowed to evaporate. 

 Some time after renewing the water supply in this partly dried pond the scales of a 

 surviving carp were examined. It was foimd that the abnormal condition was 

 registered on the scales by the formation of dark, closely approximated circuli, the 

 normal condition by clear, widely separated circuli. It was assumed that the carp 

 were unable to acquire their customary food. Again, an individual was found to 

 be greatly emaciated, and on examination a swelling was located in its anal region. 

 Hoflbauer found that the scales indicated very clearly when the growth processes 

 were first disturbed. 



The oldest carp examined were at the end of their third year. The author believed 

 that the reading of the scales from carp older than three years became increasingly 

 difficult with age as the transparency of the scales diminished with their increased 

 thickness and size, rendering the circuli, especially those of the first year, less dis- 

 tinguishable. He also found that all the scales of a fish were not equally reliable 

 for age determination, as some developed more or less sharply defined accessory 

 annuli. However, if a large number of scales were examined some would register 

 the true age. 



In later papers Hoflfbauer (1901, 1904, 1905, and 1906) supplied further evidences 

 to support his hypothesis. He extended his observations on the carp and applied 

 his method to several other species of fish {Carassius ca?-assius, Micropterus sahnoides, 

 Perca lucioperca, Esox and Salmo). 



Reibisch (1899) attempted to apply Hoft"bauer's method to the scales of the 

 marine plaice (Pleuronectes platessa) but failed. At Professor Hensen's suggestion, 

 Reibisch then turned to the otoliths, or "ear stones," and discovered the year rings 

 in these structures. 



The observations of Hoffbauer were repeated, and his conclusions were examined 

 critically by Walter (1901), who studied the scales of the carp in commercial ponds. 

 He granted the general truth of Hoffbauer's hypothesis but maintained that unless 

 the finer structures of a scale were known age determinations would be erroneous 

 in a large percentage of cases. Thus, for example, of 24 determinations that he made 

 13 were found to be incorrect. He found that Hoffbauer's criteria for a true annulus 

 997(30—29 3 



