SEA-FISHERIES LABORATORY. 171 



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These investig-ations and results, Avliich might be 

 extended with advantage to other channels and estuaries, 

 are of considerable importance. They indicate that even 

 in an estuary which was proved to be polluted the flood 

 tide water may be clean. A practical application of this 

 fact is the possibility of cleansing polluted shell-fish in 

 the flood tide water intercepted in shore ponds or tanks. 



The numbers of organisms in 1 c.c. of water may 

 vary also on different parts of a shell-fish bed as the result 

 of amount of shelter, exposure to prevalent winds and 

 many other local factors which must be studied on the 

 ground by an expert and which make every shell-fish bed 

 an independent problem to the marine biologist. Con- 

 sequently, it is sometimes possible to collect samples of 

 relatively pure and of highly polluted mussels from 

 neighbouring parts of the same bed. Mr. Johnstone has 

 dealt with these considerations in some detail in a recent 

 paper (Journ. of Hygiene, February, 1910) from which I 

 quote the following paragraph : — 



"It is clear that much may depend on the precise 

 conditions under which the sample is taken, and upon 

 the precise spot. If this is so, then great caution is 

 necessary in applying the results of analysis of a sample 

 of shell-fish purchased from a market stall or shop, to the 

 general locality from ivhich the molluscs are said to have 

 been taken. Xot only so but the Eeport must, in justice 

 to the fisherman, consider the length of time which has 

 elapsed since the shell-fish were taken from the sea, and 

 the conditions under which they have been stored 



