51 



B. HELLAND-HANSEN 



number), and few in 1906 (about one fourth of the whole number). On the other 

 hand the average-number of cod varied very sHghtly (0-9 individuals per trawl- 

 hour in 1903 and 1904; 07 in 1905; and o-8 in 1906), while the percentages for 

 these years were between 12 and 21. 



We may take it then that the aggregate of cod is composed principally of 

 unmarketable and codling, and that therefore the fluctuations in quantity which 

 occur from one year to another are dependent upon these two groups. Variations 

 in weight on the other hand we find to be mainly due to the cod^ a point to 

 which we shall return subsequently. 



The curves in Fig. 32 clearly illustrate the differences in the distribution of 

 numbers. If we compare them with our observations on age which will be given 

 in the next chapter, we will see that the I group was exceptionally well re- 

 presented in 1906 but was not at all prominent in 1903; while again in 1904 and 

 1905 they were in quite considerable numbers. It seems accordingly as if in those 

 parts of the North Sea which we examined there was a strong accession of codling 

 spawned in 1905, but that those spawned in 1902 were in scanty numbers. At the 

 same time we have not found in these waters, as far as 1903 is concerned, that 

 the aggregate of cod was affected in equal degree by those influences which at 

 that time reduced so much the aggregate of haddock. 



We have said that cod is practically synonymous with mature fish, and we 

 notice that in the different years variations in number among the mature fish are 

 similar in character to what we observed in the case of this market-group. In 

 other words there were very slight variations from year to year in the 

 average quantities of mature cod throughout those parts of the 

 North Sea which were examined during 1903 — 1906. 



II. Length and Age 



In the catches of the "Huxley" and "Poseidon" in the North Sea the number 

 of trawled cod varied greatly. It has been previously mentioned that the average 

 number per trawl-hour was only 4-8; but in many of the catches not a single cod 

 was taken, and the largest amount for one catch was not more than about 60 

 individuals per hour. Under these circumstances curves depicting separate catches 

 would embrace so few individuals that we could not point to size-groupings which 

 we could thoroughly rely on for our study of the year-classes. There is no doubt 

 of course that in many of these curves some indistinct groupings might be traced. 

 Still on comparing them we should often find that these groupings differed in 

 situation on the different curves even although we took care to make our selection 

 from a given area and from approximately the same given period of time. 



Fig. 33 shows the distribution of sizes in five of the hauls that contained most 

 cod. Four of them are from the Dogger Bank (H) and one from the adjacent 

 area off Flamborough Head. These catches were made at different seasons. The 

 first curve from March 1903 shows an assortment of individuals ranging chiefly 

 between 30 and 47 cm. and we may infer that it represents two or more year- 

 groups which are indistinguishable. On each of the curves from June 1905 and 

 July 1904 there are groups mainly between 20 and 30 cm., but no groups centering 



