HYDROGRAPHIC INVESTIGATIONS 67 



minimum in February, at a time when observation shows that 

 local species of fish, such as the plaice, arc not feeding. 

 Consequently the migrations of plaice at this time must be due 

 to some other cause than feeding. Johnstone says that there 

 are optimal conditions of temperature for such fish as the 

 plaice, and some at any rate of their migrations are due to the 

 fish seeking the regions where such optimum temperature is 

 to be found. The fish experiences a change of temperature, 

 and may move in such a manner as to make this change 

 minimal. In the summer plaice move out of water rising in 

 temperature into colder water; in winter they move out of 

 water falling in temperature into warmer water. 



The next figure (Fig. 13) shows the migration of marked 

 plaice from the Nelson Buoy fishing-grounds off the Ribble 

 estuary. These fish were marked and liberated in June, the 

 chart showing the recaptures made in the following October, 

 November, and December, the isothermal lines being for the 

 mid-month i.e., November. The broken lines represent 

 recaptures in October, continuous lines those in November, 

 and lines with dots those in December. The alongshore 

 migrations represent capricious or aperiodic movements ; they 

 occur mainly in the winter. The main migrations are into 

 Redwharf Bay, and also to the Bahama Bank off the Isle of 

 Man, in each case from water of about 9 C. to 11 C. The 

 details of the migrations, which cannot be shown on a single 

 chart, prove this. 



THE PROPERTIES OF SEA- WATER. 



Sea-water is distinguished from fresh water by its salt and 

 bitter taste, which renders it unsuitable for drinking or culinary 

 purposes. This taste is due to certain dissolved salts, of which 

 the chief are common salt, magnesium chloride, magnesium 

 sulphate, and calcium sulphate. Consequently sea-water is 

 heavier than fresh water; and it gives an alkaline reaction. 

 The amount of solid matter contained in sea-water is remark- 

 ably constant, especially if samples be collected far from land. 



