NATURE ADRIFT 



beginning always with the green plants of the phytoplankton. It has been 

 estimated that the annual production of green plants in the seas of the world 

 amounts to something like 150,000 million tons. This is, of course, only a very 

 rough estimate based on a few calculations in isolated parts of the ocean and 

 it is only possible to guess what is happening elsewhere. This figure is not too 

 different from production on the land. At each stage of the food chain about 

 90 per cent is lost so that 100,000 lb of green plant yield 10,000 lb of herbi- 

 vorous zooplankton, 1,000 lb of carnivore and 100 lb of fish if it feeds at 

 this stage, 10 lb or i lb offish if it feeds at one or two stages further re- 

 moved. The 150,000 million tons of green plants in fact yield some 30 

 milHon tons on the world's fish markets, a yield of 0-02 per cent or i in 5,000, 

 but this covers a large area of ocean where little commercial fishing is 

 possible, hi areas such as the North Sea, or the Icelandic shelf about 0-2 to 0-3 

 per cent of the yield is taken out as fish. This high figure is partly due to the 

 large proportion of herring which are mostly feeding on herbivores or at one 

 stage further removed from the primary production. It is also probably in 

 part due to the current systems which bring into these areas the middle 

 stages of the food chain which have had their primary plant stage outside. 



The word fisheries used in the title of this chapter includes the catching 

 offish as well as their abundance in the sea. Plankton in the form of jellyfish 

 can be a serious hindrance to successful fishing, and in Faroese waters, in 

 particular, they can be sufficiently numerous to spoil the fishery. After living 

 near the surface in the summer, the jellies, when they are quite big, go to the 

 bottom in August or September, or even as late as October. Here they con- 

 gregate in such numbers that they become a serious menace to trawling, and 

 catches arc so poor that the trawlers have to go elsewhere. We are not quite 

 sure how the jellies affect the fish; they may cause the fish to rise above the 

 bottom where the trawl is, they may possibly be driving the fish off the 

 grounds, but most likely they are just so numerous that the nets become 

 choked and heavy and cannot catch the fish with their usual efficiency. Some 

 skippers report that the nets become full of jellies almost as soon as they get 

 to the bottom and on hauling their sheer weight bursts the net. Later in the 

 year, about November, the grounds clear and fishing then becomes more 

 profitable. 



Stake nets around the shores for catching salmon also choke with jellies 

 and spoil the fishing. In the Baltic, fish traps can become so heavy with 

 Aiirclia that they break when lifted and the fishery for herring and sprat is 

 spoiled. The jellies penetrate so far into the fresher waters of the Gulf of 

 Finland that they affect the pike fishery there. They seem to be most prolific 

 in warm summers. 



This chapter and the preceding two chapters have tried to show how a 



136 



