CHARACTERISTICS OF MARINE FISHES 175 



12,201,984 immature eggs were present in a tarpon 80 inches long. Those eggs 

 produced in large numbers are quite small, generally only a millimeter (about 

 one-twenty -fifth of an inch) or so in diameter. Nature seems to have made pro- 

 vision for a greater mortality among the young hatched from minute eggs, which 

 are extremely weak and delicate in comparison with those born alive or hatched 

 from large eggs. If every egg produced by those species, like the cod, for example, 

 were to live and mature, it is not difficult to see that the ocean would become 

 overpopulated. 



Larval Development 



The larvae and postlarvae of fishes all diflFer more or less from their parents. 

 Young eels, as stated elsewhere, are flat, ribbon-shaped, and virtually transparent. 

 While going through a sort of metamorphosis they become shorter and rounder. 

 Most young fish do not have fins, but only "fin folds." They have relatively large 

 heads and eyes and are virtually colorless. Even in the postlarval and yoimg 

 adult stages they are often quite different in shape and color from their parents. 

 Frequently, large spines are present about the head, but they are lost during 

 development. Also, marked changes in the position and shape of the mouth, teeth, 

 gill rakers, and alimentary canal take place. Such differences seem to be associated 

 with a change in diet, for it was shown in an earlier section that nearly all marine 

 fish at first feed on small or microscopic organisms, even though many later be- 

 come carnivorous and feed on comparatively large animals. 



Effects of Fishing 



Overfishing has in many cases caused the fish to be less abimdant. In fact 

 there has been such a great reduction in some areas as to make fishing un- 

 profitable. The chances of exterminating a marine fish through overfishing are 

 quite remote. Although several species of birds and mammals have become ex- 

 tinct during the past several centuries, the writer is unaware of any fish that has 

 met a similar fate. It is evident, nevertheless, that more than a few species are 

 decreasing gradually although it is not always wholly due to overfishing by any 

 means. Too often man has polluted the shore waters and streams in which the 

 anadromous species spawn. He also has built insurmountable barriers, in the form 

 of dams, across such streams. The combined results of overfishing, pollution, and 

 the construction of barriers have caused the Atlantic salmon to be no longer of 

 commercial importance in the United States and the shad and alewives of far less 

 value in many areas than formerly. 



That some fishes can be restored, in a measure at least, to their former abun- 

 dance through wise measures of conservation is evident from the partial restoration 

 of the shad in the Hudson River. The elimination of pollutions, in part, and the 

 regulation of the fishery have brought this fish back in commercial abundance. 

 One of the most effective measures of conservation is the prohibition of fishing 

 during the spawning season. The prohibition of fishing on a part of the feeding 

 grounds for a period of years seems to be proving very effective in restoring the 

 Pacific halibut to its former numbers. Unfortunately, the life histories of many 

 of our commercially important fishes are still too imperfectly known to permit 

 the application of effective measures of control. 



