The Spawning-Grounds of Plaice. 45 



** Goldseeker " records prove that the numbers of larval and post- 

 larval forms in the collections made dmring the period under observa- 

 tion are very few. Previous workers have commented on the paucity 

 of larval and post-larval forms. Within the Moray Firth, as well as 

 in other localities, where eggs with almost fully developed embryos 

 have been taken, the occurrence of larval forms is extremely rare. 

 It has been noted that there is a remarkable fall in the frequency of 

 eggs in the later stages of development in most of the areas of high 

 frequency for freshly spawned eggs. It has been suggested that this 

 fall may be due in great measure to their dispersal over a wider area 

 as they advance in development, and therefore a lower frequency of 

 those eggs which are advanced in development might be expected 

 at any particular locality. Undoubtedly the area of distribution 

 of eggs in the later stages of development is more extensive than that 

 of the freshly spawned ones. But this dispersal is not in itself suf- 

 ficient to account for the rapid decline in their frequency, for even in 

 the month of March there is no very obvious accumulation of earlier 

 spawned eggs that have now advanced in development. 



It has also been suggested that this loss is due to the high rate 

 of mortality amongst the eggs. Observations made in the spawning 

 season of 1914 gave a remarkable illustration of the importance of 

 this factor, when an enormous number of dead eggs appeared in the 

 February and March plankton. These dead eggs, though of slightly 

 higher specific gravity than living ones, do not immediately fall to 

 the bottom, but do so as soon as they are placed in a glass of fresh 

 sea-water in the laboratory. Doubtless the movement of the water 

 in the sea is sufficient to keep them in suspension for a time. These 

 eggs which sink to the bottom of a jar of sea- water are opaque, and 

 while many of them show no signs of the development of an embryo, 

 others have embryos in all stages of development. There is thus a 

 real mortality amongst the eggs, and death, in many cases at any 

 rate, is not due to the lack of fertilisation. It is impossible from 

 preserved material to separate eggs living and fertilised at the time 

 of capture from those already dead or unfertilised. 



While it is difficult to apportion the factors, dispersal, and mortality, 

 yet these, taken in conjunction with the fact that the present 

 communication is brought up to the end of March only, explain the 

 paucity of records of larval forms. 



The tables of occurrence of plankton stages of plaice in the 

 Northern North Sea from January to March show clearly that the 

 area of distribution of these young forms thus early in the year is not 

 more extensive than the area of distribution of newly spawned eggs. 

 Indeed, most of these plankton forms were taken in close proximity 

 to those areas which have already been shown to be favoured spawning 

 places. Even although our information regarding the larval and 

 post-larval forms appears meagre it is trustworthy, owing to the fact 

 that there is at this period no doubt as to the identification of the 

 forms. Confusion Mth the young stages of such a closely allied 

 form as the Common Dab {Pleuronectes limanda) is entirely eliminated 

 during these early months. While most of the specimens captured 

 were from 6 to 7 mm. long, none exceeded 7 '5 mm. In all cases 

 the yolk sac had disappeared, and in many of the prepared specimens 

 it was clearly evident that plankton feeding had begun, and although 



