46 



DEPARTMENT OF THE J^'ATAL SERVICE 



On the 29th June, pumping was renewed, but as the young were considerably 

 fewer, (and always at the bottom), the experiment was brought to a close. The tem- 

 perature of the air had, during the last part of the time, been very high, with sun- 

 shine and slight wind. 



This experiment shows therefore that the young can thrive — and even grow very 

 rapidly — at temperatures up to 20°. The increment of growth from May 25, when the 

 length was 3 to 4 mm. until June IS, amounts to about 20 mm. or about 0-8 mm. per 

 day. 



From the measurements available for growth of young in a free state, the latter 

 would seem to grow less rapidly, presumably about 0-5 mm. per day. 



Among other causes which might be imagined as affecting the development of 

 ova, we may reckon the salinity of the water. 



If cod spawn in water of lower salinity than that which corresponds to the specific 

 gravity of the eggs, then these latter will necessarily sink to the bottom and be des- 

 troyed. And again, if eggs should by any means be transferred from water of their 

 own sx>ecific gravity to considerably fresher water, the mortality here likewise would 

 be very great. 



As an illustration of the manner in which fresh water can affect the ova, the 

 following data from an experiment which I carried out in March, 1909, at the hatchery 

 may be quoted. 



About twelve hours after fertilization, eggs were distributed in four glasses con- 

 taining water of different siieeific gravity, and at approximately the same temperature. 

 The glasses were placed in a cold room — temperature of air about 0° C. The water 

 was not changed, only aerated from time to time with a large pipette. 



The cleavage and the development generally were observed under the microscope. 

 After the lapse of twenty hours some samples were taken and transferred to fresh sea 

 water in order to ascertain whether the development could there proceed normally. 



In the case of sample IV, this was repeatedly done. 



The result of the experiment will be seen from the following table: — 



A similar experiment with newly hatched young in water of: — 



I Sp. gr. 1-0242 at T = 2-0°C. 



II " 1-020 " 2-2 " 



III " 1-015 " 2-5 " 



IV " 1-010 " 2-5 " 



V " I'OOS " 2-7 " 



gave the following results: — 



I. All the young suspended; after the lapse of an hour some few at the bottom, 

 after four hours the majority near bottom, but all living. After the lapse of 103 

 hours, some few were dead, but the remainder stood out well until the experiment was 

 concluded after 148 hours. 



