364 



DISCOVERY REPORTS 



This shows, first, a 27% rise in average K values between December and the end of March, with 

 httle discrepancy between the sexes. Secondly, an apparent drop in K values in February, to a level 

 which was, however, still well above the December figures. This was found to be associated with a 

 drop in mean lengths (thin lines in Fig. 45), i.e. with an increased proportion of smaller fish in the 

 population sampled in February over that of the population sampled in January. 



It seems fair to conclude that most spawning takes place in early summer, and further (though at 

 this stage only as a working hypothesis) that larger fish spawn first and therefore recover condition 

 earlier than smaller ones. The smaller fish shoal inshore later than the larger ones, as will presently 

 be shown ; and any mature individuals among them are presumably correspondingly later in spawning. 

 But it is also quite possible that a proportion of the later, smaller shoalers are immature. If so they 

 may still tend to decrease the mean February K if, like adolescent hake, they show seasonal harmonic 

 variation in condition like mature fish, but on a lower level. 



1350- 



2 



hi 1-300- 



2 



1-250 ■ 



--0 

 [8:37] 



\^:46o] 



— \ — 



25 



50 75 iOO 



MILES FROM THE COAST 



125 



150 



Fig. 46. Stromateus maculattis: variation in average K with distance from the coast; southern region, summer. 

 Figures in brackets denote numbers of weighings, with the numbers of fishes measured in iUilics. 



The further testing of these hypotheses as to the movements of S. maculattis, from the available 

 data, depends mainly on considerations of size distribution (length frequencies) in relation to distance 

 from the coast and at diflFerent periods; but one further application of the use of K seems to help, 

 and may be described before we proceed to the other evidence. In Fig. 46 the mean K values of 

 Stromateus taken at different distances from the coast in the southern region, in summer, are shown 

 joined by a line (merely to guide the eye). It will be seen that the values increased sharply with 

 increasing distance from the land. This shows that the ofl'shore fish had had longer to recover after 

 spawning, for it is reasonably certain that the converse explanation (inshore population spawning 

 while that off'shore had yet to do so) could not hold, for we have already seen that the general rapid 

 increase of K values throughout the summer suggests that most spawning takes place early. The 

 point becomes even clearer when the length-frequency data are considered. 



