366 DISCOVERY REPORTS 



year farther north. Some of the northern and intermediate fish probably do not proceed so far south 

 before they begin to move offshore again ; and indeed it can be shown that the mean lengths of southern 

 samples were significantly greater than those of fish from the intermediate region. We cannot deter- 

 mine the relative extent of meridional and on- and offshore trends of movement, owing mainly to 

 dearth of inshore data from the intermediate and northern regions early in summer, particularly from 

 the Golfo san Jorge; but it seems certain from the general distribution observed that both trends 

 exist together. 



The disposition of our catches of S. maculatiis in the southern region in summer, the time of 

 maximum concentration, is shown diagrammatically in Fig. 48, in relation to distance from the main- 

 land coast. This arrangement of the data gives some idea of what a practical fisherman in quest of the 

 species could expect, in addition to amplifying our ideas as to its general bionomics. 



It is very evident that the species was most plentiful close inshore, but well distributed up to 

 100 miles from the land. It was rare farther offshore at this season. The sex ratio was highest inshore 

 ('normal') and the proportion of males diminished as one proceeded seawards. The males of this 

 species are slightly (but in large samples significantly) smaller than the females, and it is just possible 

 that this difference has some small effect on their mobility, for we know that in other fishes where the 

 difference in size between the sexes is more pronounced, the larger females travel farther and faster 

 than the males. Here, however, the sexual dimorphism is so slight that one would expect that some 

 factor not yet determined, such as more rapid dispersal after spawning among males, must be mainly 

 responsible for the diminished sex ratios in samples of the offshore population.^ 



In Fig. 49 the percentage length frequencies of both sexes of Stromateus for the same area and 

 period are considered in relation to distance from the coast, the observations being grouped according 

 to the same distance intervals as were used in constructing Fig. 48. The inshore grouping showed 

 bimodal distribution, with modes at about 25 and 29 cm. for males, 26 and 3 1 cm. for females. These 

 probably indicate year-classes. 



There was an increased tendency towards suppression of the smaller mode as one proceeded off- 

 shore, until, at distances of more than 100 miles from the land, such few fish as were caught belonged 

 almost exclusively to the older (larger) year class. 



Within a brief period around midsummer 193 1-2, a series of observations were obtained that fall 

 naturally into two groups, serving to show the strong contrast in the population of Stromateus inshore 

 and that found offshore in the intermediate region, in respect of length-frequency distribution 

 (Fig. 50). The inshore grouping showed bimodality with the smaller length-class dominant; the off- 

 shore grouping was unimodal, with almost complete suppression of the smaller class. 



There are still some big gaps in our knowledge of S. maculatiis, for pressure of work in the field 

 upon other, more important species, made it impossible to collect routine data on the condition of the 

 gonads, for example. We have very little idea of its diet, though some were found to have fed upon 

 Parathemisto. Hake have twice been found to have preyed upon Stromateus, but we can only guess at 

 its other natural enemies in this region. 



The main features in the bionomics of S. maculatus that seem clear from our data are : a double 

 trend of seasonal movement, inshore in spring and summer with maximum concentration, and offshore 

 in autumn and winter with maximum dispersal ; and, superimposed upon this, a meridional move- 

 ment southwards in summer and northwards in winter. A schematic representation of these move- 

 ments is given in Fig. 51. 



1 Probably females have somewhat greater food requirements than males during the recovery period, with a resultant 

 tendency towards greater local concentration where food is plentiful (though such concentrations would not be comparable 

 with the spawning shoals). This would lead to the same result. 



