MARINE BIOLOGICAL STATION AT PORT ERIN. 41 



that are attributed to the effects of the war. Unless we can rule 

 out these two former possible explanations the latter one — 

 that the variations depend on " intensity of fishing "—must 

 remain doubtful. Can one be sure that the falling-ofE in the 

 quantity of plaice landed from the North Sea in 1908-1914 

 would have continued, or become less, or become greater had 

 there been no war ? Fluctuations of much the same range 

 have occurred in the Irish Sea before the war, so that there are 

 undoubtedly factors that lead to fat years and lean years apart 

 altogether from legislative restrictions. 



What conditions rale the egg-production ? 



It ought to be possible to make an enumeration of the 

 mature female plaice in the pond and to calculate their mean 

 length. The total number of eggs spawned is known. A 

 certain number remain in the pond and develop in spite of the 

 care taken by Mr. Cregeen to skim the surface. This fraction 

 unenumerated may be taken to be the same from year to year. 

 What then are the physical conditions that may be correlated 

 with variations in the egg-production ? 



What conditions rule the successful development ? 



A female plaice can be " stripped " of her eggs. The 

 numbers of those that die after fertilisation can be estimated. 

 Upon what conditions will the variation in this fraction of 

 eggs failing to develop depend ? 



The duration of development : How does this depend on tempera- 

 ture ? 

 There are no data for the Port Erin or Piel plaice (though 

 an investigation might easily be made). But take the 

 following* data : — 



Temperature ... 31,32, 33, 34,35, 36, 3T, 38, 39 



Incubation period 50, 40, 35, 3] , 2ti, 25, 23, 21, L9 



Temperature ... 40, 41, 42, 43, 44, 45, 4G, 47 : °F. 



Incubation period 17, 16, L5, I t, 13, 12,11,10*5: days. 



This is tho best series of observations I know. It is from Manual of 

 Fish Culture, U.S.A. Fink Commission, p. iOG. 



