INTRODUCTION 



245 



Graphs for the annual cycle have been plotted from Klaehn's charts in Fig. 5. The three positions 

 have been chosen arbitrarily on account of their approximation to the centres of the most numerous 

 of our observations on the fishes of the shelf, in each of the three main latitudinal regions into which 

 our stations may be grouped. The curves show that the temperatures in our 'Northern region are 

 very slightly colder than those found (say) on the hake grounds south of Ireland. The range is similar, 

 but the winter temperatures nearly i^ C. lower. The temperature cycles plotted for our ' mtermediate 

 and 'southern' regions show a decrease with increasing latitude, as is to be expected; while the three 

 curves together show the diminution in annual range as one proceeds southwards. This is also well 

 illustrated by Klaehn (1911, pi. 34, ' Jahresamplitude der Wassertemperatur')- 



15-1 



14- 



13- 

 u 



o 



uj 12- 



D 



^ 1|. 



u, 9- 

 u 



■ c Northern Region 

 --X-- Intermediate Region 

 — Jk— Southern Region 



< 



in 



8- 



6- 



JULY 1 AUG I SEPT | OCT | NOV | DEC | JAN | FEB | MAR \ APR ] MAY | JUNE | 

 Fig. 5. Representative annual surface-temperature cycles for the plain of the shelf, after Klaehn (191 1). 



For consideration of the bottom temperatures, which are more significant in relation to our trawled 

 samples of fishes, we have only our own scattered observations to go upon, ^he actual %^^^^^^^^ 

 eiven in Appendix I. It is obviously impossible to show typical seasonal curves, like those derived 

 from Klaehn's monthly charts of surface temperatures, owing to the small proportion of these obser- 

 vatLs that fall within any one depth range over a reasonably restricted area. A single, partly hypo- 

 Ih Ucal, curve for the intermediate region is shown m Tig. 6. With this as a rough guide it is possibl 

 to perc ive some important features by simple inspection of the figures quoted in the Appendix. The 

 seasonal cycle appears to be centred later in the year m the bottom water, with maximum tempera- 

 Tre^i: Mich. I autumn and the first half of the winter there is much less dif^^^^^^^^^ 

 between surface and bottom water than at other seasons; it is rarely more than 2 C. whereas in 

 summer it usually amounts to some 4° C. This is due to intense mixing over the shelf m winter, 

 aradrdescribed'by Deacon (.937, P- SD- ^ spring and early summer even in the -rthern -^^^^^^ 

 some temperatures below 5° C. were recorded (lower than the winter values), but most of these were 

 in fairly deep water well ofTshore. 



