306 FOUNDERS OF OCEANOGRAPHY 



there in autumn. The deeper layer consists of " bank- 

 water " of salinity 32 to 34 per thousand, and as this 

 rolls in along the bottom as a series of huge undulations 

 it forces out the overlying fresher water, and so the herrings 

 living in the bankwater outside are sucked into the Kattegat 

 and neighbouring fjords and give rise to important local 

 fisheries. Pettersson connects the crests of the submarine 

 waves with the phases of the moon. Two great waves of 

 Salter water which reached up to the surface took place 

 in November, 1910, one near the time of full moon and the 

 other about new moon, and the latter was at the time when 

 the shoals of herring appeared inshore and provided a 

 profitable fishery. The coincidence of the oceanic phenomena 

 with the lunar phases is not, however, very exact, and doubts 

 have been expressed as to the connection ; but if established, 

 and even if found to be due not to the moon but to prevalent 

 winds or the influence of ocean currents, this would be a 

 case of the migration of fishes depending upon mechanical 

 causes. 



A correlation seems to be established between the Cornish 

 pilchard fisheries and periodic variations in the physical 

 characters (probably the saHnity) of the water in the 

 English Channel, and between Dutch anchovies and the 

 temperature the previous year ; also between the prevalence 

 of coastal water and the Norwegian fisheries. The summer 

 catches of mackerel on the south coast have been shown to 

 vary with the amount of sunshine earlier in the year — the 

 connecting link being probably the large Copepod Calanus, 

 upon which we know the mackerel feeds. The herring, again, 

 in our summer fisheries is apparently affected in its move- 

 ments by the temperature of the water, the catches being 

 heavier in seasons when the water is colder, up to a limit, 

 for the shoals break up and the fishery comes to an end 

 when the temperature falls below 54*5° F. 



The characteristics of the environment affect not merely 

 the movements, but also the nourishment and growth of 



