8 Part IID.—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 
1904, 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 larve 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 lberated 
numbering 363,250,000. The number of fry 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 hatching 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 
