THE AGE OF FISHES 193 



ing southward. Arriving again at their point of origin, the 

 plankton now rises to the surface. Thus a regular cycle keeps 

 the antarctic surface supplied with plankton. 



Replenishment of plankton in tropical waters comes partly 

 from the cooling and sinking of Gulf Stream water in the 

 north and its return southward as a deep water flow. In the 

 Gulf of Maine the work of Bigelow, Huntsman, and Redfield 

 has shown that the breeding stock of plankton Crustacea is 

 maintained by a great circular eddy. This flow carries the eggs 

 during their development around a great slow circuit which 

 takes three months to complete. Johannes Schmidt of the 

 Danish laboratory has found that the Icelandic cod fishery 

 similarly depends on circular currents for its continued exist- 

 ence. When codfish lay their eggs they rise toward the surface 

 and hatch there. The water currents around Iceland therefore 

 carry the eggs of young codfish away from their spawning 

 grounds off the south and west coasts. If the growing fishes 

 were not returned to the spawning grounds there would be no 

 future spawning, but fortunately for the fishery the currents 

 complete a circuit around Iceland in a clockwise direction. 

 When the developing codfish have drifted to the north and 

 east coast they sink to the bottom, there to continue growth 

 and to feed like their parents on bottom life. When they 

 mature and the time comes to breed they return to the spawn- 

 ing grounds. 



Many fishes undertake local migrations every year. Herring, 

 menhaden, and pilchards — all plankton-eating fish — were 

 probably coastal or fresh-water fish originally. They are nor- 

 mally scattered about in fairly deep water, but when ready to 

 breed they gather in spawning schools in shallower depths. 

 Whereas the herring are cold-water fishes, their relatives the 

 shad are mostly found in warmer water, and their spawning 

 migration brings them into rivers in search of warmth and 

 oxygen. 



Thus we see a great dynamic interlocking of the currents of 



