THE AGE OF FISHES 191 



rent. It therefore seems very likely that a sideways shift of the 

 Gulf Stream and of the delicate balance between warm and 

 cold currents had suddenly flooded the fishing grounds with 

 unusually cold water, and this great and rapid change from 

 more normal conditions almost annihilated the tilefish. Only 

 now, after seventy years, are they beginning to return to their 

 former numbers. 



No other stretch of salt water has received as much atten- 

 tion from scientists as the North Sea, but even there the inter- 

 action of life and water is so complicated that we cannot yet 

 predict the ebb and flow of fisheries with complete certainty. 

 We do know that the Gulf Stream sends waters around the 

 north of Scotland and then south into the North Sea, and 

 that where these meet water from the English Channel there 

 are great swirls and eddies. Where this happens rich bottom 

 water is drawn to the surface, and there we have the great fish- 

 ing grounds of the Dogger Bank and the Great Fisher Bank, 

 where plankton grows thick. 



Sometimes the natural effects of fluctuating water move- 

 ments have been beneficial; and in at least one case they help 

 us to forecast good and bad years in the fisheries. The herring 

 fishery south of England at the mouth of the Channel seems 

 to depend on the extent to which Atlantic water, bringing 

 enrichment from the deeper layers, is able to push eastward 

 and thus replace the less productive Channel water. The work 

 of F. S. Russell at the Plymouth Laboratory has given us a 

 rapid way of detecting these movements without the tedious 

 necessity of chemical water analysis, and thus predicting the 

 probable fluctuations in the fishery. He makes use of the fact 

 that water currents carry with them, in a sort of forced migra- 

 tion, the plankton animals living in them. In Atlantic water 

 — partly derived from the Gulf Stream — there lives one of the 

 tiny, fierce-jawed, torpedo-like arrow worms known to scien- 

 tists as Sagitta elegans. Its near relative, Sagitta setosa, on the 



