8 



Part III. — twenty-third Annual Report 



principal reason of the decrease was the difficulty in obtaining 

 large adult plaice in the preceding autumn and winter to replenish 

 the breeding stock in the pond, plaice of the class required being 

 then exceedingly and unusually scarce on the grounds from which 

 they are obtained. 



The floating eggs were observed in the water of the spawning- 

 pond about the middle of January, but they were then present in 

 very small numbers, and the first collection was made on the 26th 

 of that month, or three days later than in 1903. The last collec- 

 tion was on the 29th April, or more than a fortnight earlier than 

 in the previous year. This is, no doubt, partly to be attributed to 

 the smaller number of the spawners in the pond, as above 

 mentioned, but it appears to have been also owing to the relatively 

 greater intensity of spawning in the earlier part of the season in 

 1 904, nearly 28 per cent, of the eggs being collected before the 

 end of February, as compared with 18 per cent, in the same period 

 in 1903. As usual, the greater number of the eggs were obtained 

 in March, viz , 55*7 per cent., the percentage in that month in the 

 preceding year being 56*2. 



The duration of the period of development until hatching takes 

 place varies with the temperature of the water at the time. At 

 the beginning of the season, in January, when the temperatare is 

 low, the average time of incubation is about three weeks, while at 

 the end of the season, when the temperature is several degrees 

 higher, they hatch in about a fortnight. The larval fishes, after 

 issuing from the eggs, are retained in the apparatus for several 

 days until the yolk-sac is partly absorbed, and it is calculated that, 

 taking the two periods together — the time of incubation and the 

 period referred to subsequent to hatching — the eggs and larvae are 

 protected in the apparatus for about half of the time from the 

 spawning of the egg until the young plaice is transformed and 

 assumes the form and habit of the adult. 



Since the establishment of the hatchery, the total number of 

 plaice eggs dealt with amounts to 443,092,000, the fry liberated 

 numbering 363,250,000. The number of frv of other fishes 

 produced is as follows: — lemon soles, 5,727,000; turbot, 5,160,000: 

 cod. 4,010,000 : and other kinds, 2,000,000. 



Owing to the circumstance that the hatchery is worked in 

 conjunction with the Marine Laboratory, the expense of the 

 hatching operations at the Bay of Nigg is not large compared with 

 the number of fry produced, the annual expenditure in connection 

 with it being estimated at about £100. As previously stated, the 

 establishment was visited during the batching season by represen- 

 tative fishermen from the shires of Aberdeen and Argyle, to whom 

 the various processes adopted, as well as the fertilisation of the 

 eggs and the development of the fish, were explained. 



The Growth and Age of Fishes. 



During the last few years a considerable amount of attention 

 has been given to the study of the age of fishes and the rate at 

 which they grow, and a number of papers dealing with the growth 

 and age of the plaice, cod, haddock, whiting, and other forms have 

 appeared in the recent reports of the Board. It is a subject that 



