of the Fishery Board for Scotland. 



61 



Owing to the great natural fluctuations that occur, as shown, for 

 example, in the means for the second period, when no plaice fry were 

 added, one would not expect that the fluctuations in abundance in the 

 years when plaice fry were added would be closely related to the 

 number of fry liberated in any particular year, for the increase due to 

 this cause is liable to be masked by the extent of this natural 

 fluctuation. In the twelve years for which annual averages exist, four 

 of the six highest were in the first period of six years and two in the last 

 six. The highest of all was in the year when the greatest number of 

 plaice fry were added, viz., 1901, when the average was 174 per hour 

 and when 51,350,000 fry were liberated. The second highest was in 

 1905, when no fry were added, the average being 112 per hour. The 

 third highest was in 1896, viz., 111*4 per hour, when 4,100,000 fry were 

 added ; the fourth highest w r as in 1898, viz., 95*6 per hour, when 

 19,200,000 fry were placed in the loch; the fifth highest, 53" 1, per 

 hour, was in 1900, when the number of fry liberated was 30,590,000 ; 

 the sixth highest, 37 3 per hour, was in 1903, when no fry were added. 



The period of thirteen years over which these experiments have 

 extended is considerable, and ought to go far to equalise the 

 natural fluctuations ; and I think it is reasonable to conclude that 

 the greatly increased average abundance of the young plaice in the 

 first six years was mainly due to the liberation of the 142,880,000 fry of 

 the plaice in those years, and that, on the other hand, the decrease in the 

 abundance of young plaice in the last six years was mainly owing to the 

 fact that no plaice fry were added to the waters of the loch in that 

 period. On theoretical grounds alone it would be an astonishing thing if 

 the addition of the immense number of plaice larvae mentioned to the 

 waters of a long, narrow, and confined loch like Lochfyne should 

 produce no increase in the numbers of young plaice a few months 

 older than the larvae added. And if that is the effect in Lochfyne, it will 

 also be the effect elsewhere, though the natural fluctuations may conceal 

 it. With regard to the extent of the influence of the liberation of the 

 fry on the abundance of the young plaice during these experiments, 

 the difference in the averages in the two periods shows that 

 the plaice were more than doubled in number.* There is one 

 consideration, however, that ought not to be lost sight of. 

 On the East Coast, and in the North Sea generally, there have 

 been complaints of the diminution of plaice in recent years, and 

 this was proved to have occurred in the Firth of Forth from the trawling 

 experiments of the " Garland." There is not sufficient information 

 with regard to the Clyde area to show whether the same change is 

 occurring there, but if it is — if the adult plaice are decreasing and have 

 been for some years decreasing— it might account, in part at least, for 

 the reduction in the average catch of the young plaice in Lochfyne in the 

 second period of the experiments. It would therefore be desirable to 

 have a third period in the experiments, namely, a series of years during 

 which large numbers of plaice fry were added to the waters of the loch, 

 as in the first period, and to ascertain the effect of this on the abundance 

 of the young plaice on the beaches. 



* The above conclusion with regard to the result of the experiments in Loch- 

 fyne — that the number of plaice on the beaches was more than doubled by the 

 addition of the artificially-hatched fry — has been confirmed by an elaborate 

 mathematical investigation of the fluctuations in the different years, which was 

 kindly made by Miss R. M. Lee, of the Marine Biological Laboratory, Lowestoft, 

 after my paper had gone to the printer. — T.W.F, 

 E 



