CONDITIONS OF LIFE 295 



justment of the action of the living tissues to the salinity of the 

 medium, and therefore fresh-water fishes, such as the carp or 

 perch, are killed when placed in sea- water, and conversely sea- 

 fishes such as the haddock or mackerel are killed by fresh water. 

 Many fishes, however, like the salmon and eel, are able to pass 

 from the sea into fresh water and vice versa in their regular 

 migrations, and many littoral fishes such as the sticklebacks 

 and grey mullets can live either in fresh or salt water ; in these 

 cases, however, the change must usually be made gradually. 

 There are some lakes in which there is such a large quantity of 

 saline and alkaline matter in solution that no fishes can live in 

 them, for example, the Dead Sea in Syria, and some of the 

 lakes in Utah, North America. 



The annual changes of physical conditions with the seasons 

 have their effects on fishes as on other animals, the resulting 

 migrations or changes of habit varying with the species and 

 with the habitat. Pelagic fishes within the tropics and bathy- 

 bial fishes are little affected or not at all, while littoral and 

 fresh-water species often exhibit remarkable annual cycles in 

 their habits. 



The ultimate cause of migrations is the seasonal change 

 of temperature, but the immediate cause may be reproduction 

 or food. Definite migrations are exhibited along the coasts 

 chiefly by gregarious fishes, for example, the herring. The 

 greatest change of temperature occurs in the shallow waters 

 near the land, and on the coast of Europe we observe in 

 summer a movement of fish from the sea towards the coast and 

 from the south to the north. Mackerel are unable to bear cold 

 water ; on our coasts they are found at the entrance of the 

 English Channel even in winter but in summer as the shore 

 waters become warmer, they approach nearer land and travel 

 up the Channel to the North Sea. They spawn a few miles 

 from the coast in May and June and then travel, in search of 

 the young of other fishes on which they feed, into bays and 

 estuaries, so that in August and September they are caught 

 with the seine drawn on to the beach. From November to 

 May there are no mackerel in the North Sea. The anchovy is 

 another good example of temperature migration ; it annually 

 passes up the English Channel to reach the Zuyder Zee and 

 the estuary of the Schelde, where it spawns in summer, and 



