DISTRIBUTION AND GENERAL NOTES ON THE SPECIES 317 



commoner to the north, so that it would seem that we have here another instance of the influence of 

 size of fish upon movement. The larger fishes seem to range farther afield than the smaller ones. This 

 is in good agreement with our unavoidably scanty observations upon the feeding habits of Macruronus, 

 presently to be described. 



Numerous computations of mean lengths of Macruronus have been made besides those tabulated 

 here. From these it appears that the difi^erences in size between the sexes is small. Females are usually 

 larger by 1-2-7 ^^"^- ^^ mean length than males, but the diff^erence can only be shown to be significant 

 in large samples. The fairly constant sex ratios, with slightly fewer males in the deeper catches, and 

 the general slight preponderance of females in the trawl, suggest {a) that the slightly smaller males 

 may tend to migrate a little less than the females, (b) that the sex ratio is probably nearly normal near 

 the main locus of the species, the discrepancy being due to a higher escape ratio of the smaller males. 

 We have already seen how the much greater difference in size between the sexes seems to aflFect the 

 distribution of the hake population. 



Detailed notes upon the food of M. magellaniciis were made at eight stations only. At four of these 

 the fishes were small, the mean lengths being 34-4, 34-9, 35-0 and 44-5 cm. These fish were almost 

 entirely carcinophagous and had been feeding heavily upon euphausians and Parathemisto; one was 

 crammed with fish larvae, and a single specimen had taken a Munida. The other four records are from 

 stations where the fish were considerably larger, of mean lengths 50-4, 51-8, 52-5 and 55-6 cm. These 

 had been feeding exclusively and heavily upon Clupea fuegensis and Notothenia spp. (most being 

 undoubtedly A^. ramsayi). Squid have not yet been observed in the stomachs of Macruronus. It is 

 therefore probable that Macrtirofius exhibits, perhaps to a marked degree, that change over from a 

 carcinophagous to a fish diet that seems characteristic of many demersal fishes as they grow older (e.g. 

 hake). The point should not be stressed unduly in the absence of data as to possible seasonal changes 

 in diet. 



The facts that only the larger fishes were found to be ichthyophagous in summer, and that they 

 appear to migrate much more extensively than the smaller fishes, seem to find a close parallel in some 

 of Harold Thompson's observations on Newfoundland cod (1943, p. 86). He says: 'It is seen that 

 the volume [of food] increases rapidly with the size of cod, and it is probably this need for large 

 quantities of food (unobtainable all the year round in any one locality) which leads to cod making 

 increasingly great migrations as it becomes older; whereas small cod apparently find it possible, 

 within a limited inshore area for example, to find sustenance in the smaller crustacean species for the 

 first few years of their lives.' 



There is further evidence that the age at which M. magellaniciis attain a length of some 50 cm. 

 marks a critical period in their lives, in the relation between length and ponderal index. Among 

 females from the southern region values for K were highest among fishes of the 41-50 cm. length 

 class (weighted mean 7^=0-352). Fish of the 51-60 cm. class gave values some 4% lower (weighted 

 mean 7^=0-340) and two higher length classes gave lower values. If we had sufficient information to 

 plot a curve for this relationship it seems fairly certain that the point of inflexion would lie at around 

 50 cm. The fall was slight because the reliable data were collected in March, and this is too long after 

 the spawning period to give the best demonstration of this phenomenon. Earlier data are roughly in 

 agreement, but inadequate to show the efi^ect within convincingly narrow limits of time and space. 

 The implication is that it is at a length of about 50 cm. that sexual maturity is first attained. 



Thus it seems possible that sexual maturity, increased migratory movement (in search of food) 

 and a change in the nature of the diet, follow closely upon one another in that year when the fish reach 



a length of some 50 cm.^ 



1 Possibly their sixth year of life. 



