FEATURES OF GENERAL BIOLOGICAL INTEREST 385 



not previously studied or exploited, conform to tendencies recognizable elsewhere. (By this means 

 lesser divergencies, such as time-lag, usually become clearly attributable to the differing environmental 



conditions). To proceed : 



Many demersal fishes show seasonal movement shorewards in summer, and offshore to deeper water in 

 winter. Usually, but not always, the shoreward movement is a breeding migration. 



This is one of the best known tendencies of fish migration. Examples among Patagonian species are 

 furnished by fishes as diverse as hake {Merluccius hubbsi), spotted pomfret {Stromateus maculatus), a ray 

 {Raja brachyiirops) and Notothenia ramsayi. 



In a migratory species of ground-fish, the larger individuals tend to travel farther and faster than the 

 smaller ones. {Often the movement of immature individuals is so limited that they never proceed ' over the 

 edge' to the greater depths reached by the adults during the ' off' season). 



Very good examples of this are provided by the four species mentioned above. 

 A similar effect may be apparent interspecifically among the members of a taxonomic unit. Thus 

 hake migrate more extensively than the slighter Macruronus; and Notothenia ramsayi, the largest of 

 the Patagonian members of its genus-, migrates farther than any of the others. 



In temperate latitudes many migrating species of demersal fishes extend their range polewards in summer 

 and towards the equator in winter. This may result directly from the effect of temperature, but is ?iot {as 

 yet) clearly to be distinguished from secondary effects of the strength and direction of the currents {the factor 

 most strongly emphasized by Meek). Thus it is essential for fish with denatant pelagic larvae, inhabiting a 

 region like the Patagonian Continental Shelf, where the main current flows from south to north, to spawn 

 near the southern limit of their range {or so close inshore that the larvae are drifted by the inshore counter 

 current to the southward) if the species is to be maintained zvithin its ecological norm {cf. E. S. Russell, 

 1937. P- 321)- ^ considerable meridional trend of seasonal movement may thus be superimposed upon the 

 on- and offshore movement, and may even almost completely mask the latter. 



Considerable meridional trends of movement are shown by such fishes as Stromateus, Macruronus 

 and Genypterus in the Patagonian region. With Stromateus such trends are superimposed upon a strong 

 on- and oflFshore movement of the usual type, and seem almost certainly to be conditioned by the 

 currents as outlined above. Macruronus gave more indications of meridional than of on- and oflFshore 

 movement, and this may be partly due to its marked preference^ for moderate depths of water. Some 

 tendency to oflFshore movement was discernible, but the fish was never plentiful at such great depths 

 as those in which true hake are sometimes captured. It is very much a fish of the plain of the shelf, 

 and even the largest individuals rarely seemed to go far 'over the edge'. Since Macruronus sttms to 

 spawn in early summer, the southward movement cannot be regarded as a breeding migration. It is 

 perhaps a feeding migration, in which shoals of Falkland herring are the attraction. (Our largest 

 concentrations of this species were discovered near the southern limits of its range early in autumn.) 

 With Genypterus diflFerent factors are probably involved. The area investigated is probably south of 

 the main habitat of the species. Our catches certainly suggest an ellipsoidal path of movement similar 

 to that found in Stromateus, but the individuals concerned should probably be regarded as straggling 

 adolescents, and considerations of the necessity for contranatant movement before spawning do not 

 arise if this view is correct. 



The coastal elasmobranch Callorhynchus showed evidence of southward movement in summer. It 

 keeps so close to the 80 m. line (limit of the 'first slope') that such progress would be aided by the 

 inshore counter-current, but it may well be that temperature is the more important factor here. We 

 do not yet know where this species breeds. 



1 Using a teleological expression, in the interests of clarity and brevity. 



