— 9 — REPORT ON THE MACKEREL 



quantitative methods the number of mackerel eggs per square meter of the surface 

 occurring in different parts of the respective waters, and at different phases of the spawning 

 time, and to state in this way the beginning, the optimum and the end of the spawning 

 period. As yet, such determinations — without being complete — have only been tried 

 in the North Sea, by German, Dutch, English and Norwegian investigators. 



Considerably less is known of the young forms of the mackerel. The larval 

 forms have been figured by different authors, the postlarval forms by Holt and by myself 

 up to a length of 18 mm; that is the size, they usually reach during their stay on the 

 spawning places in the North Sea. These, however, they seem to leave later on; and it 

 is remarkable, that the next following stages have never been detected in northern waters 

 until recently, when Kn. Dahl in the Risör Fjord (Southern Norway) caught some few 

 larger ones, measuring 19 and 43 mm., showing that it would be possible to get some 

 more of these young fish in the Norwegian fjords. By the courtesy of Dr. Hjort, I have 

 got these interesting forms and have figured them. But it will be necessary, to look for 

 similar samples in other waters, for instance near the Irish coast, and in the Channel, 

 where they have never been observed, even not near Plymouth. Only from the Mediter- 

 ranean these little fish are known in quantities, as they use to be marketed at Nice. 



Not before the material of these young fish is augmented, will it be possible to 

 consider the question of their connection with the stages of 7 or 8 cm average 

 length, which are better known and have been caught in the Risör Fjord and elsewhere 

 in quantities, during the months of July and August, at the same time as the smaller forms 

 mentioned, or to settle the question, whether they form a single or two age-groups. 



The task of age determination is not an easy one, as the most hopeful material 

 for this purpose, the bones, cannot be relied upon. The structure of the bones does not 

 allow one to see the age-rings with any sufficient clearness. The otoliths are a little 

 better, though not much; moreover they are rather small and oftentimes fail to reveal 

 their age with reliability; but they have to be used in connection with the scales which, 

 after the researches of H. Marukawa, from the Fisheries Institute in Tokyo, carried out 

 in my laboratory, give the best evidence, concerning the relative age of the bearers. It 

 seems, that the age of the scales is generally one year less than that of the otoliths, as 

 the scales are probably not formed before the second summer. For the settling of this 

 fundamental question, the available material of young forms was not at all sufficient ; and 

 especially fresh material has been wanting. Marukawa has however been able to discern 

 a series of about ten different age groups. The size of first sexual maturity, which in 

 both sexes is very nearly the same, about 30 cm, is probably reached in the fourth 

 summer, which ends with an average size of 32 cm ; but after this, growth seems to go 

 on very slowly, so that the end of the six following years is characterised by the average 

 sizes: 33-5, 35-4, 37, 38, 39-5, 41-5 cm. These figures are only preliminary statements, 

 as the number of individuals, that have been used for getting them, did not exceed 

 seventy in all. 



Some complementary information for the age determination can be obtained from 

 series of measurements of the total length. The material existing in this regard is 

 very bulky, especially E. W. L. Holt has kindly put at my disposal a very great number 

 of market-measurements ; Kn. Dahl has sent me some valuable series, embracing different 

 size groups, and I myself have tried to collect as much material as possible, getting 



