336 Part III. — Twentieth Annual Beport 



various expeditions and fi^om a consideration of Dr. Hugli Robert Mill's 

 investigations in the Clyde sea area, and Dr. Hjort's in the deep water 

 on the Norwegian coast, that the range is much less, and the maximum 

 and minimum retai'ded, in comparison with the shallower water near 

 the shore ; but it is not clear what relation the mean temperature for 

 the year in one region bears to that in the other. The most favourable 

 conditions for growth, so far as concerns temperature, are to be found 

 in the shallower inshore waters in summer, and in the deeper water 

 offshore in winter, and it is probable that this in part explains the 

 movements of fishes at these seasons — as, for example, the whitings in 

 Aberdeen Bay, referred to later. 



The greatest range in the temperature takes place in the shallow 

 water on the beaches, where it may fall 'm 32*^ F. or below it in winter, 

 and rise to about 70° F. in summer, and since the young plaice inhabit 

 this region it is of interest to note the relation that exists between the 

 changes in the temperature of the water and the rate of growth of the 

 fish. On page 343 will be found a diagram showing the rate of growth 

 of the plaice in the Solway in relation to the temperature of inshore 

 water in the different months of the year, fi'om which it aj^pears that 

 the greatest growth takes place at the time the gi-eatest rise occurs in 

 the temperature, viz. in May and June. After the temperature 

 I'eaches its maximum, growth begins to be retarded ; although the 

 temperature in September and October is higher than it is in May, 

 growth is much less rapid ; while in winter, growth is quite arrested. 

 Other examples may be found in connection with the whiting and 

 haddock. In the Firth of Forth, for instance, where the range between 

 the summer maximum and the winter minimiun is considerable, the 

 growth of the whiting in summer is rapid, while in winter it is greatly 

 retarded (PL XXI.). Off Aberdeen, where the seasonal range in tempera- 

 ture is not so great, the curve of growth is correspondingly flatter, 

 v/hile in the deep water ofi' the Shetlands, where the range is still less, 

 the rate of growth is much more uniform throughout the year 

 (PI. XX.-XXI.). Further illustrations of the close connection between 

 the temperature of the water and the rapidity of growth of fishes will 

 be found in the following pages. 



It is evident, however, that temperature alone does not explain the 

 difference in the rate of growth of one and the same species in different 

 localities. The plaice, for example, grows quicker on the east coast of 

 Scotland than it does on the west, and the same is true of the common 

 dab, and yet the temperature in the foi'mer i-egion is not so high. In 

 this respect the long rough dab furnishes a marked example. In the 

 Clyde its growth is slower than in the deej) water off the Shetlands, and 

 much slower than off Aberdeen or in the Firth of Foi'th. There is 

 obviously some other cause, or causes, which bring about the differences 

 in rate of growth i-eferred to, apai-t from temper'ature, or salinity, as the 

 above instances show, for the salinity of the water is lower in the Firth 

 of Forth than it is off the Shetlands. The observations made by Mr. 

 J. T. Cunningham some years ago on the variation in the size of plaice, 

 especially at maturity, from different parts of the North Sea are of 

 interest in this connection, * He found that those taken off the Dutch 

 coast were smaller than those from the English coast, or north of the 

 Frisian Islands, and resembled rather those obtained on the south coast 

 of England ; and he says, " It would appear that the Channel condi- 

 tions extend northwards along the Dutch coast, while the size of the 

 mature plaice, which is characteristic of more northern grounds, extends 



*Journ. Mar. Biol. Assoc. IV, 104. ' 



