CULTURE OF FRESH-WATER MUSSELS. 



The second summer yielded one individual measuring 62.8 millimeters (2.47 inches) 

 in length, the maximum, and many over 55 millimeters (2.16 inches) in length. From 

 one of these were cut 16-line buttons 2 lines thick (see fig. 72). Although this is not a 

 favorable size for cutting, the fact that the shell in two seasons' growth is almost suitable 

 for commerciaUuse is of significance and far exceeds expectation. 



Growth during the third summer, when the adult stage was attained, determined 

 by the first breeding, reached a maximum length of 85 millimeters, weight 63.1 grams, 

 in the male, and a length of 77 millimeters, weight 66.5 grams, in the female (gravid). 

 Length, average male 79.1 millimeters, average female 71.5 millimeters. The growths 

 of the 1915 brood during their second summer compared with that of the 1914 brood 

 for their second summer show a very striking difference. Although the 1915 brood 



1915 



•Jon. Feb. Ho 



Mfly Jun< 



July 



Auj. Sept. Oct 



1916 



Nov. Dec, ijan. 



Feb. Mor. Apr. 



May June 



MM. 



"Temp 



70 



GO 



50 



40 



30 



FlG. 71. — Growth of a fresh-water mussel in relation to temperature: . mean monthly water temperature (F.) in 



the Mississippi River at Fairport, Iowa; , monthly increase in growth of a IrcsliwuUr mussel in its second 



year, in millimeters. Zero represents the line of no growth om i the coordinates represent the increase for each month 

 taken separately. (See p. 70.) 



began the second summer very much smaller, averaging n.6 millimeters in length, 

 compared with 25.7 millimeters for the 19 14 brood, at the end of the season the former 

 had increased 475 per cent while the latter had gained only 212 per cent. 2 This dis- 

 parity in growth brought the brood of 19 15 to a size — their second year — equal to 

 that of the 1914 brood at the end of their second year in the face of a large handicap. 

 This difference may be ascribed to difference in season which is, perhaps, the simplest 

 explanation. The summer of 191 6 had higher water temperature, higher water stages, 

 and less wind than usual. Flood stages, generally speaking, have been found unfavor- 

 able to plankton production as determined by Kofoid in the Illinois River (Kofoid, 1903). 

 The rapid growth this season occurred on falling stages but at an unusually sustained 

 high level. As this high level was not due to local precipitation, it would seem that 

 the conditions were consistent with (an assumed) high plankton production at the 

 point of observation. The absence of wind as an important cause of turbidity would 

 be favorable to the feeding of mussels. 



'The small size of the 1015 brood was due to a late planting and partly, doubtless, to a less favorable growing 9e " 



