.Poulsen (1930) detected little agree- 

 ment between the salinity in the Belt and 

 Kattegat in February-May and the number of 

 cod larvae, but rather high agreement 

 between the bottom salinity in November- 

 January and the numbers of fry in the fol- 

 lowing spring. This supports Jensen's 

 (1929): hypothesis that increased inflow of 

 Atlantic water, which may be diagnosed by 

 a salinity increase, is responsible for 

 good year classes rather than salinity per 

 se . .;.— 



Dahl (1909) aiK - . A. Dannevig (1919) 

 conducted laboratory experiments, observing 

 eggs and larvae in waters of different 

 salinities. While the two experiments were 

 designed for somewhat different purposes, 

 Dahl's to determine what salinities were 

 necessary to float eggs and larvae, and A. 

 Dannevig's to investigate moraality caused 

 by salinity, the results are comparable. 



Table 2. --Dahl's (1909) and A. Dannevig's (1919) experiments with 

 eggs and larvae in waters of different salinities. 



Schmidt (1931) was unable to demon- 

 strate any connection between variation in 

 meristic counts and salinity differential, 

 as he had found for temperature. 



Perhaps the most important effect of 

 salinity on the eggs and nearly helpless 

 larvae and fry of the cod is in the changes 

 of density of the water which may cause the 

 planktonic forms to float on the surface, 

 suspend at some intermediate layer, or sink 

 to the bottom. Bigelow and Schroeder (1953) 

 note that in wet springs runoff from the 

 land may dilute the surface of the sea so 

 that eggs- are suspended in intermediate 

 layers, while A. Dannevig (1947) points out 

 that in years when winds keep brackish sur- 

 face waters offshore the eggs can float 

 higher and '.get more light, which he feels 

 is conducive to hatching. 



An example of the suspension of eggs 

 below the surface is given by Dahl (1909), 

 who reports that at one station in the 

 Kattegat (for instance) he found very few 

 eggs or larvae at the surface in 6.32°/oo — , 

 and the great majority at 30 meters in 

 29.16°/oo. 



1/ Dahl (1909) as quoted here and below 

 spates salinities as densities in <5~ t 

 units, which are usually taken to mean 

 densities at temperature t, uncorrected 

 to standard densities at temperature 

 0" ( <fl) . However, since he mentions 

 no temperatures and in a footnote 

 defines : 



Salinity 



Eggs 



Larvae 



Author 



<f t = (Albs. sp. gr. -1) 1000, 



which in modern usage is more usually 

 represented as o , his O. has been 

 taken as <J~ in converting to salini- 

 ties. 



9.93 All die (34 hours) Dannevig 



12.47 High mortality All die (52 hours) Dannevig 



18.71 Eggs survive All die (138 hours) Dannevig 



22.36 Most suspended or Most sunk 



sunk 



Dahl 



23.61 More than 1/2 float 1/2 float Dahl 



24.04 1/3 float Most sunk Dannevig 



24.98 1/2 die (142 hours) Dannevig 



25.31 More than 1/2 float 1/2 float Dannevig 



26.10 All float All Float Dahl 



27.85 All float All float Dannevig 



29.99 All live (148 hours) Dannevig 



30.74 All live Dannevig 



Examination of the table reveals that 

 both eggs and larvae are damaged by salini- 

 ties below 13°/oo, but that eggs apparently 

 can survive from about 19°/oo upward. This 

 confirms the opinion (Anonymous 1932a) that 

 eggs can stand a wide salinity range. Lar- 

 vae, however, need a salinity over 25°/oo 

 to survive for more than a few days. A. 

 Dannevig's later work with Sivertsen (1933) 

 disagrees somewhat, for they kept larvae in 

 the laboratory at 22.41°/oo. With regard 

 to flotation, below 23°/oo buoyancy is re- 

 duced for both eggs and larvae, while above 

 26°/oo all eggs and larvae float. All of 

 these measurements apply to live fertilized 

 eggs. Rognerud (1889) found that dead eggs 

 did not float even in 37.>32°/oo, and Hen- 

 sen (1884) describes unfertilized eggs 

 which floated in 18.5°/oo and began to sink 

 at 17.2°/oo. There is a suggestion that 

 these measurements apply only to European 

 cod in the work of Ryder (1886) who claims 

 27.39°/oo fatally low for American cod 

 eggs. And the question of how cod eggs and 

 larvae survive in extremely low salinities 

 of the inner Baltic seems unanswered. 



