MIGRATION OF FISH 97 



in frosty weather they ought to be more 

 fortunate in working the trawl in those 

 areas. 



The greatest compelling motive in the 

 migration of fish is the necessity of find- 

 ing a suitable place in which to deposit 

 the spawn. We see that the herring, a 

 pelagic or surface-water fish, migrates into 

 the shallow North Sea waters so that the 

 eggs will have a suitable habitat for develop- 

 ment. Nature by supplying eggs in count- 

 less quantities to each fish does away with 

 the need for parental care ; if one in 10,000 

 reach maturity it will be sufficient. It does 

 not follow that because fish spawn a 

 great number of eggs the mature reproduc- 

 tion will be great. The skate deposits 

 only a- few eggs, but these are so protected 

 that the young have a much greater chance 

 of survival. The salmon would have no 

 chance of surviving if its eggs were laid on 

 the seabed, for the number is so small 

 20,000 that with the decreased chances 

 of survival in the ocean, the fish would soon 

 become extinct ; hence the need for its 

 journey into rivers where greater protec- 

 tion is afforded, 

 7 



