14 THE IMPOVERISHMENT OF THE SEA. 



the case with the June and August surveys), it is perfectly clear 

 upon examination tliat the averages for the month would have been 

 intermediate in value between those of June and August. 



Nothing could illustrate more forcibly than this table the great 

 differences between the two areas as regards the influence of the 

 seasons upon their productivity. The changes in the abundance of 

 each species are relatively slight in the case of the Forth, but are 

 exceedingly great in St. Andrews ]>ay. In the Forth the maximum 

 summer average is not four times the minimum winter average in 

 the case of plaice, nor elevenfold in the case of dabs ; but in the Bay 

 the maximum abundance in August exceeds the minimum abundance 

 in December a hundredfold in the case of plaice, and even two hundred- 

 fold in the case of dabs. The monthly catch of plaice in St. Andrews 

 Bay exceeds that in the Forth for each month of the year except 

 December, January, and February, the degree of excess rising to 

 fourfold in the height of the summer (August), and falling away to 

 three- and two-fold towards the earlier and later months of the year. 

 In the three winter months, on the other hand, the catch in the Forth 

 exceeds the catch in the Bay by as much as three, four, or seven fold. 

 Tlie figures for dabs present the same general features, but the excess 

 of the summer catches in the Bay over those in the Forth is not 

 quite so great, and tlie winter excess of the Forth catches over those 

 in the Bay is shown in five instead of three months only. 



With these facts before one, which refer, it must be remembered, 

 exclusively to the closed waters under discussion, it is easy to forecast 

 the general effect of combining the statistics of the two areas. With 

 a perfectly equivalent number of hauls the monthly average of the 

 combined areas will assume a mean character intermediate between 

 the average for the two areas taken separately ; but any deviation 

 from strict equivalence will raise or lower the combined average to 

 an extent depending on the nature of the seasonal differences between 

 the two areas for the month in question. Thus for April the combined 

 average for plaice would be Gl upon an ei^uivalent number of hauls 

 from the two areas ; but if two hauls in the Forth were combined 

 with one in the Bay the average would be reduced to 55 ; and if 

 the hauls in the Bay preponderated over those in the Forth to the 

 same extent the combined average would be raised to 08. For August 

 the changes introduced would be still greater ; with equivalent hauls 

 the combined average would be 191 ; with two hauls in the Forth 

 to one in the Bay it would be reduced to 154 ; with two in the Bay 

 to one in the Forth it would be raised as much as to 262. Con- 

 se([uently, monthly averages based on quite irregular combinations of 

 hauls in the two areas are fallacious and misleading, and it is quite 



