172 MARINE PRODUCTS OF COMMERCE 



numbers, only to return the following spring. It is not known definitely whether 

 these migrations are induced by temperature or whether they are undertaken for 

 the purpose of seeking better feeding grounds during cold weather. Fish are 

 probably influenced by both. Northward and southward migrations to find a 

 comfortable temperature and food also take place, but quite surely less extensively 

 than was formerly supposed. There is some evidence which tends to show that 

 more commonly fish merely migrate offshore to deeper water, rather than going 

 on long northward or southward journeys. 



It is not at all surprising, in view of the great extent of the watery home of 

 marine fishes, that some of the powerful and rapid swimmers go on long journeys. 

 It would not be surprising if some of the mackerels and mackerel-like fishes, such 

 as tunas and swordfish, crossed oceans. In fact some slight evidence that bluefin 

 tunas, Thunnus ihynnus, cross the Atlantic was obtained some years ago by a 

 European investigator. 



Geographical Distribution 



It is obviously much more difficult to determine the geographical range of fish 

 than that of land animals, whose courses or trails can actually be traced. It is 

 perhaps correct to say that the exact range of few marine fishes has been de- 

 termined. Nevertheless, not many distribute themselves widely as it is not neces- 

 sary for their well-being. Fish are generally prompted to rove by temperature, 

 water pressure, the presence of the proper food, the presence of enemies, and 

 other factors not yet well understood. Recent critical studies have shown that the 

 individuals of one species usually do not roam from one extreme to the other 

 of their range. For example, it has been found that there are several geographical 

 races of the anchovy, Anchoa mitchilli, and that those from the opposite extremes 

 of the range differ so widely that the taxonomist would be justified in classifying 

 them as two distinct species if specimens from the intermediate localities were 

 not available for comparison. However, with specimens from many intermediate 

 localities at hand for study, it was possible to show that there is complete inter- 

 gradation. Furthermore, it was shown that difi^erent sections within the range, as 

 New England, the Middle Atlantic States, the South Atlantic States, the Gulf 

 States, and Yucatan, all had distinctive races. This discovery seems to indicate 

 that the individuals of this species of anchovy, at least, do not roam widely, 

 even though they seemingly could do so if they had the urge. 



It is chiefly among the pelagic fishes of the mackerel and mackerel-like groups, 

 which are powerful swimmers, that the widely distributed species occur. How- 

 ever, several of the larger species of shark, bluefish, common mullet, etc. are 

 reported from widely separated seas. Even the common Atlantic herring has a 

 wide range as it occurs on both sides of the North Atlantic, ranging from northern 

 Labrador to Cape Hatter as on the American side. Yet, this herring can scarcely 

 be classed as a fast and powerful swimmer. A particularly poor swimmer of very 

 wide distribution is the porcupinefish. There are widely distributed species, also, 

 among the deep-sea fishes, such as the lantern fishes. It is quite probable that 

 further studies will show that most of these widely distributed species are com- 

 posed of geographical races or subspecies; there may even be more than one 

 species under the same name. With some exceptions individuals of these various 

 groups do not travel from sea to sea. 



