88 BULLETIN OF THE BUREAU OF FISHERIES. 



whether there is a sufficient food supply in water in which it is desired to promote an 

 abundant growth of mussels. 



Three bodies of evidence bearing upon some of these questions are presented in 

 the following pages. One is a summary of the observations by H. Walton Clark, which 

 have been published elsewhere in part; another is a table embodying the results of 

 Shira's studies of the 60 juvenile mussels taken in Lake Pepin (Shira, unpublished 

 manuscript); the third comprises previously unpublished observations made in 1916 

 by Franz Schrader," formerly scientific assistant in the Bureau of Fisheries. The last 

 will be given first since the studies were directed more particularly at the questions 

 just presented. 



OBSERVATIONS OF FRANZ SCHRADER ON FOOD OF MUSSELS. 



SPECIES STUDIED. 



Four species that were thought to be fairly representative were selected for investi- 

 gation: The river mucket, Lampsilis ligamentina, the Lake Pepin mucket, Lampsilis 

 luteola, the blue-point, Ouadrula plicata, and the spike, Unio gibbosus. The first named 

 is a typical river mussel, and one of the most important of all from the button manu- 

 facturer's point of view — considering the quantity and quality of the shells together. 

 Lampsilis luieola, a shell of fine quality, is predominantly a mussel of standing bodies 

 of water, and is found to comprise 31.5 per cent of the entire shell output of Lake Pepin. 

 Quadrula plicata, also a good button shell, is evidently equally at home in stagnant 

 and in flowing water. It is a member of a genus in general slow, ponderous, and heavy- 

 shelled. Finally, Unio gibbossus is a form of little commercial importance because 

 of its colored shell but is extremely common in some localities. Thus in Lake Pepin 

 13 per cent of the shells were found to be of this species, and it was thought that if 

 competition for food played an important part in mussel ecology, the presence of this 

 valueless form might be detrimental to the commercial species, especially when occurring 

 in such numbers as in Lake Pepin. 



FOOD CONTENT OF WATERS. 



The first step taken was to make a careful examination of the water. For this pur- 

 pose samples were taken from well-known mussel grounds. A water sampler operating 

 by means of valves that are closed through releasing the catch by a string was used. 

 The sample of water taken at from 2 to 4 inches from the bottom was treated with 

 formalin and the contents allowed to settle in the usual way. 



The solid matter thus obtained may be roughly divided into three groups: (i) Min- 

 eral matter; (2) organic remains predominantly from plants (detritus); (3) plankton, 

 chiefly green algae and diatoms. The proportions of these were extremely variable, 

 varying not only with the season but also with changes in the river level. Plankton 

 varied from less than i to more than 20 per cent. The remaining material comprised 

 chiefly detritus, for, except after thaws or rains, the mineral matter seldom exceeded 

 5 per cent of the total of solids. 



Regarding the plankton, it may be said that relatively few forms made up the 

 greater bulk. Thus, among green algas there were Scenedesmus, Selenastrum, Pedi- 

 astrum, Cosmarium, and Volvox, the latter especially in the spring. In August greater 



o Included with his consent. 



