THE POLLUTION OF THE SEA 27 



Such shellfish are not found in economic abundance on the 

 shore, nor beneath the shallow waters fronting the open sea. 

 The shallow water in the great bays and estuaries, such as the 

 Wash, the Tyne, the Thames, the Mersey, Morecambe Bay and 

 so on, are the great breeding and rearing grounds of these 

 animals. They require, as a most favourable environment, a 

 sea bottom just above and below the margin of extreme low 

 water of springtides, that is, shallow water possessing a moder- 

 ately high temperature in the spring, summer, and autumn 

 months. They flourish in water of low salinity (10 to 30 per 

 mille of total dissolved solids ; normal high-seas water is about 

 35 per mille), so that the fresh water brought down by rivers is 

 an important factor in their development and growth. The 

 low-salinity water is itself a factor, but it is also the case that 

 the relatively large proportion of dissolved organic matter 

 contained in river water, and derivable from the washings from 

 cultivated land, is both a direct and an indirect factor : directly 

 inasmuch as these shellfish are probably capable of assimilating 

 dissolved organic matter, that is, of living saprophytically, and 

 indirect inasmuch as the soluble organic matter encourages the 

 growth of diatoms and microscopic algae, and these organisms 

 then form the food of the shellfish. It is just such estuarine 

 water that contains a relatively large proportion of sewage 

 matters, either crude and recently discharged, or already 

 partially resolved by bacterial action into soluble " albuminoid " 

 substances, or even into salts of nitrous or nitric acids. Into 

 all the great estuaries and bays there open rivers which drain 

 land areas thickly populated and all of which are, to some 

 extent, charged with sewage matters. 



A relatively high proportion of sewage, even such a pro- 

 portion that the polluting matter is recognisable without resort 

 to chemical or bacteriological analyses, does no harm to mussels, 

 oysters, or cockles — indeed, the former molluscs, at least, grow 

 very rapidly in such sewage-contaminated water, and one may 

 often see large and " well-fed " mussels growing within a few 

 feet of the mouth of an outfall sewer. This is true, but to a 

 less extent of oysters and cockles also. The natural conditions 

 are therefore such that these shellfish tend to establish them- 

 selves in such areas of foreshore and very shallow estuarine 

 water as receive great volumes of fresh water coming either 

 from cultivated land or from the river water highly charged 



