202 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



of graph is, therefore, the best general expression of the whole 

 experience. 



But it does not matter whether we take the results of the 

 trials with the fish trawl-net, or those made with the shrimp 

 trawl-net. Nor does it matter whether we take the average 

 catches per year, or the smoothed average catches per year, 

 or the half -ranges of the summed catches. Whatever way 

 we deal with the data the same result comes out : there is a 

 periodic fluctuation between 1892 and 1920. There are two 

 periods : — 



(1) There are maxima in 1896-7 and about 1910. 



(2) There are minima about 1905 and about 1915. 



(3) There must have been a minimum about 1890. 



(4) There appears to be a maximum about 1920-22. 



(5) When the fish- trawl and the shrimp- trawl data are 



examined it is seen that the minimum for the 

 former occurs about two years after the minimum 

 for the latter. 



Here, then, we have actual observations that point directly 

 to a national, periodic fluctuation in the abundance of plaice 

 on a certain ground — the Mersey nursery area. The evidence 

 is experimental, but the same results also come from examining 

 the variation in the quantities of plaice landed from year to 

 year. Allowing for various sources of confusion this same 

 periodic fluctuation is seen in the quantities of plaice landed 

 at Morecambe — a typical inshore fishing port. It is also to be 

 seen in the total landings from the English Channel and from 

 the West Coast. One cannot doubt that we have here evidence 

 of a natural periodic variation in the abundance of plaice on 

 the West Coast. Every ten or twelve years the fish becomes 

 unusually abundant. There was a minimum of abundance 

 (several lean years) just about the date of commencement of 

 war, but this cannot be more than a coincidence. 



