38 



DEPARTMENT OF THE NAVAL SERVICE 



other spheres, that where regularity of the seasonally varying physical conditions in 

 animals or plants ceases, the annual period disappears, or becomes irregular and 

 indistinct. 



The influence of temperature upon the daily intensity of the spawning ma.-v 

 easily be observed in a hatchery, at any rate whei-e a spawning basin is employed. 

 Also the degree to which spawning depends upon temperature is in particular most 

 easily noticeable where the latter factor ranges beween 0° and + 5°, to a lesser extent 

 -when between + 5° and + 10°. Other influences may make themselves felt, as for 

 instance of the amount and more or less regular distribution of the food, the fish 

 spawning largely when well fed. This may, however, ix)ssibly be due to purely phy- 

 sical conditions, at any rate, as long as the amount of food consumed is sufficient for 

 nourishment, as the expansion of the stomach will exert a pressure vipon the ovaries. 

 Furthermore, the salinity of the water may be supposed to have some effect though 

 this will scarcely be of any importance as long as the variation involved is not too 

 great. 



■ Temperature centigrade] 

 - Eggs in liters 



2% 2Ji ys % '^3 % =% ^i 54 » '5^ 



Fig. 23. 



The curves here shown are based upon daily observations at the hatchery of 

 Flodevigen during the spring of 1909, and indicate the fluctuations in temperature 

 and spawning throughout the essential part of the spawning time. In order to present 

 the clearest possible survey, I have here marked off, not the daily values, w-hich vary 

 considerably, but a mean value calculated each day from the observations of that date 

 plus those of the two previous and the two subsequent days, i.e. a movable mean extend- 

 ing over five days. 



"V. — Possibilities of development, axd relative fkequexcv of the different stages. 



In the foregoing chapters, we have dealt with the spawning of the different 

 species in relation to depth, temperature and season. It now remains to consider how 

 the development of the eggs in the separate species takes place, and what chances 

 there are for their being hatched. 



From the figures in table II, we obtain the following numbers of eggs and larva? 



for the six most commonly occurring species : — 



Ova. Twarvse. 



Ctenolabrus ' 434 318 



Scomber 5,575 47 



Drepanopsetta 2,369 47 



Gadus callarias and G. a-rjlefinn^ 15.988 293 



Onos 2,975 92 



Merluccius 8,8 21 



It will be seen that the gadoids here predominate, and among these, as we have 

 already seen, the cod is of most frequent occurrence. Ova of Merluccius are also very 



