PISCES 263 



attack man. Its flesh, while said to be poisonous, is probably never 

 injurious unless ptomaines have been allowed to develop. (See 

 page 243.) The saw-toothed piranha has powerful teeth and oc- 

 casionally attacks bathers. It is said not to fear the alligator. 

 Some of the catfishes have poison glands at the bases of their spines. 



Migration in Fishes. — Fishes apparently seek waters of a certain 

 temperature or acidity or salinity which are the optimum for their 

 developing eggs and furnish food for themselves and fry. Certain 

 races frequent the same streams year after year unless driven away. 



The fresh water fishes in many cases (trout) travel from the 

 larger rivers to brooks, where the temperature is cooler, to spawn. 

 Suckers have an annual migration very early in the spring. 



Salmon, shad, and sturgeons leave the salt water for fresh water 

 to spawn, but spend their life to maturity in feeding in the salt 

 water and migrating only from the deep to the shallow, more brack- 

 ish water inshore. When they ascend rivers salmon sometimes trav- 

 el over 2,000 miles to spawn. Apparently the young salmon return 

 to their ancestral spawning grounds whenever possible. Extensive 

 scale studies and tagging experiments of American and European in- 

 vestigators have proved that many individuals return after two or 

 three years to the very region whence they came as newly hatched 

 fish. King salmon have averaged forty-two miles a day for a 

 journey of 1,500 miles. In the case of the anadramous (up running) 

 fishes such as the salmon, it is the belief of many observers that 

 their remarkable ability to find their way back to the " parent- 

 stream " is inexplicable except as a mysterious instinct. Some 

 would hold that the ancient habit of migration to a region once 

 inundated by salt water has persisted through the centuries in 

 spite of the fact that the land has risen and that the animals must 

 now travel long distances in fresh water. Other investigators have 

 concluded that the olfactory and chemical senses, oxygen supply, 

 tactile and kinesthetic senses, temperature sense and the hormonic 

 stimulation of the developing gonads play an important part." 



" Consult Chidester, F. E. 1924. A critical examination of the evidence for 

 physical and chemical influences on fish migration. Brit. Jour. Exp. Biol., vol. 2, pp. 

 79-118. The author has recently conceived the idea that the iodin-fat balance may 

 be concerned in the migrations of fishes. For example, the eel, living in fresh water, 

 may migrate down to the salt water in order to secure the optimum iodin necessary 

 for it to mature its eggs or sperms. On the other hand, the salmon, and other ana- 

 dramous fishes, may find it necessary to migrate from the salt water to the fresh to 

 reduce the iodin content sufficiently so that eggs or sperms may develop under 

 physiological conditions determined for the race ages before. 



