THE VARIATION, RACES AXn MIGRATIOyS OF THE MACKEREL. 249 



V. Skx. 



§ 1. Proportion of the Sexes. Out of a total of 918 mackerel caught 

 in British waters 423 were males and 495 females, forming a percentage 

 of 46 males to 54 females, or a proportion of 117 females to every 

 100 males. 



This excess of females over males is slight compared with the 

 preponderance of this sex which Fulton has shown to exist in the case 

 of most sea-fishes with pelagic eggs. (8th Eeport Scottish Fishery 

 Board, 1890, p. 849; 9th Eeport, 1891, p. 247. Partially quoted in 

 Cunningham's " ^Marketable Fishes," 1896, p. 76.) In this respect the 

 mackerel comes nearest to the cod, in which the proportion of females 

 to 100 males is 133. 



The explanation which Fulton suggests of the general preponderance 

 of females in species of sea-fishes is founded on the great difference in 

 bulk between the ripe ovary and testis, which often leads to obvious 

 differences in the degree of distension of the abdominal cavity in the 

 two sexes during the breeding season. The necessity for the annual 

 production of a certain minimum number of eggs, coupled with the 

 difficulty experienced by the females of many species in carrying their 

 proper quantity of ova, has accordingly led in some cases to a relative 

 increase in the size of the females, in others to an increase in their 

 relative number, or even, as indeed is generally the case, to an increase 

 of the females under both these heads. The duration of the spawning 

 season is also affected by the same factors. 



As the general size of the female mackerel is shown in a subsequent 

 section to be only slightly in excess of that of the male, we are 

 probably correct in attributing the relatively slight preponderance of 

 females in the mackerel to the relatively large body-cavity which this 

 species possesses, as compared with the body-cavity of a gadoid or 

 fiatfish. The small size of the mackerel's egg also obviates the necessity 

 for a large preponderance of females. 



The large size of the body-cavity in the mackerel is probably, in its 

 turn, connected with the active pelagic habits of this fish, as it would 

 be a manifest impediment to vigorous movements if the abdominal 

 region should become so distended in the spawning season as it is 

 in the more lethargic cod and llatfi.sh tribes. As the same feature 

 is also found in the herring and pilchard, it would appear to be a 

 general phenomenon among pelagic and so-called " migratory " species. 



§ 2. Segregation of the Sexes. It has been maintained by Couch 

 {British Fishes, vol. ii., p. 68) that the sexes of the mackerel become 

 much divided during the early migration. Out of 20 specimens taken 

 indiscriminately on one occasion during jNIarch he counted 16 males 



