— 
SEA-FISHERIES LABORATORY. 135 
400 
300 
200 
100 | 
AnWIDO EF ODOOOnA NS HWHOEYrFPDA CHA DMANH OT 
for for) =) So Coal on 
@ @D a 7 for) or) 
ea a ~ et al = 
Variation in the numbers of plaice caught in the fish-trawl. 
(Data for the years 1898-9-10 are missing). 
We see at once from these diagrams that there has been 
a very regular variation in the abundance of plaice, of one to 
three years old, on the Mersey fishing grounds. Some time 
about 1890-2 there was a minimum of abundance, and there 
was another minimum about 1905. There was a maximum 
about 1895, and another maximum about 1910. Just about 
the present time there is another minimum. That is to say, 
there have been two complete periods of variation in the 
abundance of plaice on the Mersey fishing grounds between 
about 1892 and about 1915.* 
The variation seems to be very much the same for the 
plaice caught in fish and shrimp trawl nets, but if we take 
single years instead of groups of three years, and adopt certain 
precautions to get rid of accidental variations, we find that 
* Until we have other two or three years’ records the precise date of 
the latter minimum must be rather uncertain. 
