153 



(3) The larvae must have found suitable food in the plank- 



ton on the sea area where they metamorphosed. 



(4) Suitable wind-drifts, tides, etc., must have brought 



them to a nursery ground and not to some unsuit- 

 able part of the coast. 



(5) There must have been plenty of food on the nursery, 



with other suitable conditions. 



To test all these conditions is quite possible — and it 

 would be the most fascinating kind of research imaginable. 

 Just yet the resources for such investigation do not, of course, 

 exist and so we cannot offer any data. But even the inadequate 

 material we do have at our disposal may carry us some way 

 and so we have prepared the following Table 20. There are 

 marked differences between the prevalent sizes of plaice taken 

 in the shrimp trawl-net in different years, and these we are 

 considering. We have included all the measurements of plaice 

 caught in hauls with such nets (of 2-inch mesh) in the months 

 October to March of 1908-9, 1909-10, 1910-11, and so on. 

 By October the fish have ceased to grow, and it is April of the 

 following year before growth begins agahi. Therefore we can, 

 from a study of the relative abundance of very small plaice 

 taken during the winter months, endeavour to obtain a measure 

 of the numbers of plaice spawned, successfully metamorphosed, 

 and transported to the nursery grounds. Thus we have 

 prepared Table 20 in order to make such a measure of the 

 productivity of the various years for which we have data. 



Looking at the length-frequencies recorded in Table 20 

 we see two kinds of distribution : that of the years 1908-9, 

 on the one hand, and that of 1914-15, on the other. Small 

 plaice of less than 8 cms. largely preponderate in winters of 

 which 1908-9 is the type, while in the other kind of winter, 

 such as those of 1915-20, the small plaice are much less frequent 

 in the catches. It is hardly necessary to examine the small 

 fish taken in the shrimp trawl-net in order to find what is 

 their approximate age, for we can be pretty certain that 



