Table 1. — Earll*3 (1880) and H. Dannevig's (1895) time-temperatures 

 for the development of cod eggs. 



Author 



Dannevig 



Barll 



Earll 



Barll 



Earll 



Dannevig 



Earll 



Dannevig 



Dannevig 



Barll 



Dannevig 



Barll 



Dannevig 



Dannevig 



Dannevig 



Dannevig 



1 Reported elsewhere in the same paper as 9.3 days. 



reactions cannot be shown to apply to 

 developing cod eggs. 



*>-•!_ 



Bonnet (1939) found that temperatures 

 above 13* were invariably fatal to cod eggs 

 (contrary to H. Dannevig above), and claimed 

 in addition an increasing mortality from 

 temperatures of 6° up, although Jacobsen 

 and Johansen (1908) performed in their ex- 

 periments on the specific gravity of cod 

 eggs at temperatures from 12" to 20*. A. 

 Dannevig (1919) also hatched eggs up to 14°, 

 but concluded that the optimum is in the 

 neighborhood of 3* to 5°. Another opinion 

 of the optimum temperature is 2° to 8* 

 (Anonymous 1932a). Bigelow and Schroeder 

 (1953) place the optimum between 5° and 

 8.3*. Rognerud (1889) found that eggs 

 hatched in 5* produced strong and vigorous 

 larvae, but those hatched at 2.5° and 7.5° 

 did not. 



Schmidt (1931) concluded that the 

 temperature of development has a definite 

 effect on the meristie characters of cod, 

 that the number of vertebrae and fin rays 

 increases as the temperature decreases. 



Lethal and optimum temperatures for 

 fry differ from those for eggs. Here 

 again there are field observations such as 



those made by Schmidt (1926) when he found 

 no fry in Icelandic waters below 4* and by 

 Hjort and Petersen (1905), who found them 

 in temperatures over 5° in Norway; John- 

 stone (1906) notes that young cod, perhaps 

 beyond the fry stage, are found in the 

 North Sea in temperatures of 6° to 7*. 

 Tftning (1953) states that temperatures 

 below about 1.8" are lethal. Optimum tem- 

 perature has been stated as 7* to 9" (Anony- 

 mous 1932b), although A. Dannevig and 

 Sivertsen (1933) observe that fry survival 

 up to 16* in laboratory jars. 



In summary then, the maximum range 

 for the development of cod eggs to the 

 hatching stage is from -1° to 14°, but var- 

 ious authorities agree for the roost part 

 that the optimum is within a few degrees on 

 either side of 5°. Larvae, on the other 

 hand, can survive a range of similar magni- 

 tude but slightly higher, from about 2" to 

 16*, with an optimum of perhaps 7°. These 

 values are as might be expected for a fish 

 which is for the most part a late winter 

 and early spring spawner. It would be 

 interesting to experiment with the eggs and 

 larvae of one of the few groups of cod 

 known to spawn in the autumn to see if the 

 lethal and optimum temperatures vary. It 

 is difficult to see how the fry survive 

 the winter unless there is a marked differ- 

 ence in temperature tolerance. 



Salinity 



Alander (1952) describes a recent 

 increase in the abundance of cod in the 

 Baltic which has been associated with an 

 increase in salinity. During the years 

 1937, 1940, 1942, 1943 and 1947, salinities 

 in the Bornholm Deep were higher than usual, 

 reaching in one case over 18°/oo, and 

 these have produced good year classes. 

 (Salinities in the Baltic are low, ranging 

 ordinarily from 4°/oo to 16°/oo vs. an 

 oceanic average on the order of 35°/oo.) 



Jensen (1929) found slight correla- 

 tion between the salinity at Schultz* 

 Ground in the Baltic in June and the yield 

 of the fishery four years later, but he 

 felt that the effect was actually due to 

 an increased inflow of Atlantic water, re- 

 flected in the salinity increase. Poulsen 

 (1944) suggests that this influx and the 

 associated salinity change may influence 

 the migration of mature fish. 



