282 SCIENCE PROGRESS 



of fish must be mentioned. The first concerns herring, and is 

 of pecuhar interest since the rather intricate causal connection 

 has been elucidated. The work was done by Pettersson 

 {Quart. Journ. Roy. Meteorol. Soc, 38, 191 2). Herring are not 

 found in the Baltic because the water is not sufficiently salt. 

 They migrate into the Kattegat however, with an undercurrent 

 of Salter water which exists beneath the less dense surface 

 layers. This undercurrent is oscillatory and the movement 

 shows itself by changes in the level of the deep Salter water. 

 The crests of the submarine waves occur at intervals of about 

 thirteen and a half days, that is, the undulatory movement has 

 the tropical lunar period of twenty-seven days and depends on 

 the moon's declination. That the herring move with the salt 

 water is shown by the fact that the dates of the greatest herring 

 catches are those on which the crests of the waves of Salter 

 water occur. But this is not all. During the last century and 

 a half the years of maximum and minimum herring fisheries 

 are found to coincide with the years of maximum and minimum 

 declination of the moon, which are 9-3 years apart. It may be 

 presumed that the fishery has this period of i8-6 years because 

 of a variation of the Salter undercurrent with the same period. 

 The herring fishery has also a longer period which can be traced 

 back 1,000 years. The maxima are iii years apart. This is 

 the greater sunspot period, but since 6 X i8-6 = iii'6, the 

 moon's declination may again be the real cause. 



The second case of a lunar effect on the migration of fish is 

 the following. Dr. Johann Schmidt informs me that Danish 

 eel fishermen, who catch silver eels with nets in the shallow 

 watef as they migrate out of the Baltic in the autumn, expect 

 to obtain better catches in the interlunar periods than when 

 there is moonlight. In Egypt, too, the eel fishermen look 

 forward to the best catches when there is no moon. Mr. G. W. 

 Paget, Egyptian Government Director of Fisheries, suggests in 

 explanation of this a direct effect of the light of the moon in 

 stopping the migration of the eels, basing his opinion on success- 

 ful experiments carried out in Denmark on artificially stopping 

 eel migrations with acetylene lamps. The Italians, too, light 

 fires when they want to interrupt a migration in order to clear 

 their nets. 



I should be grateful if any readers of Science Progress 

 who know of popular beliefs in lunar influence on animals or 

 plants would communicate them. On closer investigation some 

 may prove to be as well founded on fact as the case of the sea- 

 urchins. 



