PLANKTON AS FOOD 



drums of moncl metal netting, revolving to reduce choking, and constantly 

 passing the plankton to the ends from which it could be continuously ex- 

 tracted in a concentrated form by pumps. Then it would need to be recon- 

 centrated to remove all the surplus sea water before being passed to the 

 drier otherwise the deliquescent sea salts would attract the moisture again. 

 All the salt could not be removed, however, and the dried plankton would 

 still have to be packed in airtight containers. To increase the filtering 

 efficiency the ship's propellers could be inside the stern of this plankton 

 tunnel. Such a ship could surely catch plankton ni quantity, but could it 

 do so economically? Because of its design and the special apparatus it con- 

 tains it might well cost twice the price of a conventional trawler to build. 

 Because of its shape and the resistance of the filters it would cost more in 

 fuel to run than a trawler. The fact that the delicate parts could only be 

 serviced or repaired in dry dock would reduce the available time at sea and 

 add heavily to maintenance costs. 



Assuming such a ship to be the same size as a trawler, the gape of the 

 'mouth' could scarcely exceed an oval 15 feet X 6 feet or about josqiaare 

 feet. This is seven times the area of a i -metre plankton net but it could 

 fish twenty-four hours a day at perhaps 6 knots and so catch 2,000 times 

 as much in a day as the plankton net catches in a quarter hour at 2 knots. 

 In an average area the i-metre net might be expected to catch j lb though 

 it would often be less; in a rich area the catch might reach 5 lb but would 

 average about 2 lb. In other words our ship could catch 500 lb a day 

 steaming to and from rich grounds where it might catch 4,000 lb a day. 

 For a twenty-one-day trip this could be 2,500 -j- 44,000 -j- 2,500 lb or 

 about 22 tons wet weight yielding about 2| tons of dry weight plankton. 

 A trawler on a similar trip will catch 500-2,000 kit or 30-120 tons of white 

 fish (wet weight) at less than half the cost and worth a great deal more 

 per ton in the market. This in home waters does not look like being a com- 

 mercial proposition. It does not mean that it never will be, and indeed the 

 prospects in the rich plankton area off' Peru (p. 112) offer much brighter 

 hopes — but the tish are easier to catch there too ! 



If a conventional type trawler towed several large plankton nets, say 

 six nets of 2 metres in diameter, the total cost of the dried plankton caught 

 would be about -£2,000 per ton and on only the basis of the amount of pro- 

 tein present would be about 20 times the price of fish. This ignores any 

 psychological factors and the serious possibilities of unknown toxins and 

 excess vitamins that might reduce its value or even make it dangerous. 



Out of the many millions ot tons of plankton in all the oceans of the 

 world what proportion could we hope to filter out? We should lose such 

 a large proportion of the minute organisms through our filters that we 



165 



