2.36 SPOLIA ZEYLANICA. 



Spawning Period. 



Although direct information regarding the spawnmg period of 

 the window-pane 03'ster has not yet been obta.ined, all the evidence 

 points to its occurring between January and March. This much is 

 certain, and it is hoped that in the early months of 1913 confirmation 

 of this point will present itself. In the chart on Plate 5, which 

 shows the rate of groAvth. I have assumed that the spatfall takes 

 place in February. The exact date may be expected to show some 

 variations in different years, according to the weather and other 

 influences. Large 2-year oysters were found to have ripe gonads 

 in January. Very few 2-year oysters were present in the lake 

 in Januar}^ of this year, and yet a very large spatfall took place 

 some time between January and May. It is just possible that 

 oysters are sexually mature at the age of 12 months. If this 

 be so the large spatfall of this year could be accounted for by the 

 presence of a considerable number of 12-months oysters on the 

 beds at the time when spawning must have taken place. After the 

 1911 oysters had been decimated by the rains, there was still a 

 sufficient number left to repopulate the bed if these oysters were 

 ripe. It is proposed to examine all these points in January 

 next. The floating larvae have not yet been found in the surface 

 water. 



In September the first-year oysters had small pale -yellow gonads, 

 and were in an extremely immature condition. The second-year 

 oysters had much larger gonads of an orange colour. It is almost 

 certain that these will spaAvn in Januarj^ or February next. 



It may be stated confidently that Placuna spawns only at one 

 period of the year, namely, in January or February. Willey 

 expressed the opinion more than once that all the oysters existing on 

 the bed at the same time were of the same age, and that fisheries could 

 not follow one another year after year as the oysters of different ages 

 came into full bearing. It may be true, as a matter of practice, that 

 after a fishery in any particular year insufficient oysters are left for 

 a fishery the following year, but this, I take it, is not due to the 

 absence of oysters of different generations, but to the reckless 

 methods of fishing, by which a bed is depopulated during one 

 fishing, irrespective of age. The fact is, that oysters of different 

 ages do occur at the same time on the beds. This is proved con- 

 clusively by the investigations of 1911 and 1912. The practical 

 difficulty still remains how to preserve the non-bearing oysters 

 during a fishery. O^ysters of different ages will be lying side by side, 

 and as we have seen a size limit is of little value. Nevertheless, 

 experienced divers can tell at a glance the different ages of the 

 oysters. In the event of oysters of two ages being present together 

 on the beds durmg a fishery, it would be necessary to instruct the 

 divers to return all the younger oysters to the water. Supervision 



