234 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



grounds, diminished from 1908 to 1913, and so did the average 

 quantity of plaice caught per day's absence from port of the 

 average steam trawler. 



Of course the whole question is a rather academic one : 

 what the owner of a steam trawler has to consider is the 

 average cost of catching the average cwfc. of fish and then the 

 average price obtained when it is sold. 



The Impoverishment of a Fishery Region. 



For the moment we deal with plaice of a definite range 

 of size — say, 20 to 25 cms. (small plaice). Suppose that the 

 productivity of a certain fishing region is " indefinitely great," 

 no matter how many plaice are caught there would still be 

 plenty left — that would be what we mean by " indefinitely 

 great." So many small plaice are produced that a certain 

 fraction must die from want of food : now if, say, l/10th are 

 caught by the fishermen that would mean that about the 

 same number would not die, but would survive to take the 

 place of those that had been caught. Probably some localised 

 fishing regions are like this — they are " overcrowded " grounds. 

 On the whole, however, such an area as the Irish Sea is not 

 an overcrowded one, where the productivity is indefinite^ 

 great, for the fact that the abundance of plaice undergoes 

 periodic changes shows either that the quantity of plaice food 

 changes, or that the quantity of baby plaice spawned, hatched, 

 and transformed changes. Probably the latter is the case. 



Has there been an Impoverishment of the Irish Sea Plaice 

 Grounds ? 

 To answer this question we have to consider both the 

 commercial statistics and the results of experimental trawlings. 

 The quantity of plaice landed from year to year depends on 

 the catching power and the natural productivity of the grounds. 

 The statistics of catching power are not very accessible (if they 

 exist), but it is probably the case that it has not decreased 



