342 Part III. — Tirenticth Animal Report 



mean size being then 54 mm. During August growth is slower, the 

 apparent increase in length in the 31 days to the end of that month 

 being only4'3mm. In September the diminution in growth is more 

 marked, the apparent increase in the 29 days amounting to only 2'2 mm. 

 In the next thirty-three days to the end of October ^.he increment was 

 only 0"1 mm. The averages for the preceding year from 1st August 

 to 28tli December show the same diminution of growth in autumn, 

 although not with quite the same regularity, the average size of the 

 young plaice being somewhat less in that year, probably owing to a 

 lower temperature in the summer, or a later spawning. But if the two 

 series of hauls in August, Se])tember, and October 1899 be combined, 

 and the mean taken for the average and the period, the apparent 

 increments of growth are as follows: — In the 44 days from 7th Augu.st 

 to 19th September 2'5 mm., in the 30 days from 19th September to 

 18th October 1*8 mm., and in the 40 days to 27th November 0'8 mm. 

 At the end of December, it will be observed, the average size was 

 1*2 mm. less than in November, and that at the middle of March it 

 was less than in December by 0'8 mm., and than in November by 

 2 mm. 



These facts, I think, prove that the growth of this series of young 

 plaice, living on the beaches, is quite arrested in winter. It is possible, 

 indeed, that the fish actually diminish in length as the averages in- 

 dicate, for in certain experiments I made some years ago by keeping 

 plaice from thii^teen to eighteen inches long in tanks, in which the tem- 

 perature of the water followed that of the air, it was found they 

 usually diminished .somewhat in length and always in weight in winter, 

 from October until March.* Moreover, the period indicated — fi-om 

 November to February is characterised by the most rapid /cell in the 

 temperature of shallow water. 



The close relation of the temperature of the water to the growth of 

 the young plaice is well bi'ought out by detailed comparison, and this 

 connection, it is interesting to observe, extends to the spawning period 

 as well. It so happens that the period in which the greatest number 

 of plaice eggs are spawned coincides with the time when the temperature 

 of the sea at some distance from the land, where the spawning takes 

 place has just begun to rise. This in indicated in the accompanying 

 diagram (fig. 4), which also shows the relation between the mean 

 temperature of the shallow water in the various succeeding months, 

 and the average growth of the young plaice. The temperature of the 

 bottom water, in which the spawning plaice live, I'eaches its lowest 

 point in the year about the beginning of the second week in March, 

 after which it begins to rise. Very soon after this the spawning 

 attains its maximum, so that the development of the great bulk of the 

 embryonic fishes within the egg takes place in a slowly rising temperature, 

 and the transformation of the post-larval stages in a temperature some- 

 what higher, and rising more rapidly. When the settlement of the 

 swarms of small transformed plaice on the bottom is at its maximum 

 the temperature is rising most rapidly, and the greatest increase in 

 growth coincides with the greatest increase in temperature. It is note- 

 worthy that growth is less rapid when the temperature is more uniform, 

 althovigh higher, in July, August, and September; that it slackens 

 most when the temperature begins to fall, which is usually towards the 

 middle or end of August, and becomes arrested at the period when the 

 fall is greatest. 



The temperatures I have been dealing with are the mean temperatures 



* Eleventh Ann, Report Fisheri/ Board fw Scotland, Part iii., table, p. 193 (1892). 



