American Fisheries Society 209 



tnainly confined to within one hundred fathoms of the sur- 

 face, but the subsequent invention of the automatically clos- 

 ing net made is possible to demonstrate that plankton exists 

 at all depths of the sea. The extent and composition of 

 this great ocean life is not constant, but is dependent upon 

 seasonal variations, fluctuating from day to day, from 

 month to month, and from year to year. Its composition 

 though not its quantity, according to Hackel, varies with 

 the climatic zones from the equator toward the poles, while 

 the oceanic currents exert a marked influence on its move- 

 ments and therefore affect its density. The so-called "zoo- 

 currents" or "animal streams'' described by Hackel and 

 others are narrow currents in the sea crowded with masses 

 of swiftly moving plankton so dense that they appear like 

 "smooth, thickly populated, animal roads" extending con- 

 spicuously for miles through the otherwise wind-roughened 

 waters. 



Sometimes the animals composing the plankton are ex- 

 tremely varied and include hundreds of species, not only 

 foraminifera, radiolaria, and minute larval echinoderms, 

 mollusks and fish eggs, but also animals often attaining a 

 larger size, such as medusae, scyphomedus?e, siphonophores, 

 ctenophores. pteropods and small fishes, all carried about 

 at the mercy of the waves. At other times great stretches 

 of sea extending for miles may suddenly swarm with indi- 

 viduals of a single species in countless numbers, which in 

 the course of a few days or even hours will as suddenly dis- 

 appear. 



(b) The nekton, on the other hand, comprises the larger 

 swimming animals such as fishes and the cetacea which, 

 because of their size and strength, unlike the helplessly 

 floating plankton, are able to cope with the ocean currents 

 and swim largely according to their own will. These form 

 a group having a well-marked economic part in the ocean 

 life, since they are actively engaged either in preying directly 

 upon the plankton, which are thus consumed by the mil- 

 lion, or in devouring fishes which in their turn have fed 

 upon plankton. Thus the latter group is the most im- 



