150 



BULLETIN OF THE BUREAU OF FISHERIES 



That there was a considerable layer of this organic debris was shown by the fact that 

 the dredge was filled to the top. Alongshore there was less of this organic debris but 

 more fine mud. When this station was revisited on September 16, conditions were 

 very much improved. Much of the bottom deposit in the channel had disappeared 

 (result of heavy rains). No solid masses were seen rising to the top, but gas bubbles 

 still were very much in evidence along the shore. The bottom animals here are 

 mostly tubificids. In August there were 103,860 per square yard near shore and 

 31,680 per square yard in the channel. Associated with the tubificids in the channel 

 were 720 leeches per square yard. On September 16 there were 126,000 tubificids 

 per square yard along shore and 65,400 per square yard in the channel. The differ- 

 ence in the number of Tubificids in the August and the September samples is not 

 significant, perhaps, and may be due to natural fluctuation or local variations in 

 abundance. 



Conditions at station No. 8 (in the St. Croix River) are in marked contrast to 

 the conditions prevailing at No. 7. The first sample taken near shore on the Wiscon- 

 sin side consisted of pure sand only; but along the shore, on stones, May fly nymphs 

 and the snails, Pleurocera acuta and Physa sp.?, were abundant. One specimen of 

 Carnpeloma rufum and two of Heliosoma trivolvis also were taken. It will be 

 recalled that Carnpeloma rufum was very abundant near a sewer outlet at No. 2. 



The second sample at No. 8 was taken just off the peninsula (on the St. Croix 

 side), where the waters of the Mississippi and the St. Croix Rivers meet. In addi- 

 tion to the bottom fauna, the sample consisted of fine sand, cinders, pebbles, and live 

 aquatic vegetation. Each square yard of the bottom here contained 360 Tubificidse, 

 167 leeches, 3,240 Hyalella, 432 Planaria, and 72 May fly nymphs. One large mussel 

 and a few specimens of Carnpeloma rufum also were taken. 



The samples of stations No. 7 and No. 8 illustrate clearly the effect of environ- 

 mental conditions on the character of the fauna. Whereas the badly polluted waters 

 at station No. 7 contained large numbers of Tubificidse and leeches only, the less 

 polluted waters at station No. 8 contained not only these forms (which, however, 

 were much reduced in numbers) but also Hyalella IcnicJcerboclceri (a crustacean), 

 Planaria, May fly nymphs, a mussel, and the following snails: Carnpeloma rufum, 

 Pleurocera acuta, Physa sp.?, and Heliosoma trivolvis. Planaria and the May fly 

 nymphs are definitely known to be clean-water forms. 



The bulk of the sample taken in the channel at station No. 9 in August con- 

 sisted of empty shells of the small bivalve (Musculium transversum) . The animals 

 taken with these shells were as follows: 36 Tubificidse, 684 leeches, and 108 specimens 

 of Carnpeloma integrum per square yard. On September 17 three dredge hauls in the 

 channel here yielded 65 cubic centimeters of empty shells. The sample also con- 

 tained 240 Tubificidse, 2,520 individuals of Musculium near transversum, 492 leeches, 

 72 Carnpeloma integrum, 24 Hyalella, 12 Caddis fly larvse, and 48 dragon fly nymphs 

 per square yard. In the shore sample there were in August, in each square yard of 

 bottom, 1,980 Tubificidse, 18 chironomid larvse, 1,260 Hyalella, 54 specimens of 

 Asellus, 252 Carnpeloma integrum, 1,600 Musculium transversum, 90 Heliosoma 

 trivolvis, and 2,295 leeches. In September the shore samples gave 48 dragon fly 

 nymphs, 2,900 Tubificidse, 3,000 Hyalella, 288 Carnpeloma integrum, 48 specimens of 

 Anodonta imbecillis, and 1,776 leeches per square yard. The much smaller number 



