PLANKTON AS FOOD 



This method of fish farming is very effective indeed in fresh-water ponds in 

 the middle and far east, and yields very striking economic returns in ponds 

 specially built for the purpose and as an 'extra' in places like the rice paddy 

 fields. Can the same principle be used in the sea? Before attempting to answer 

 this, it is first necessary to consider the factors that contribute to making fish 

 farming a success and then weigh the difficulties in applying the same ideas 

 in the sea. The greatest successes have so far been in warm countries, and in 

 ponds or lakes where the water is either retained in the pond or changed very 

 slowly. The fertilizers are thus retained and are under proper control, the 

 process can be watched and the effects can be properly assessed. The fish 

 grown in these ponds arc largely herbivores, feeding directly on the increased 

 plant growth supplemented to some extent on herbivorous copepods etc., 

 at only one stage removed, hi the colder climates it has been shown that the 

 addition of fertilizers and lime can increase the amount of insect life etc. in 

 trout lochs and give a substantial increase in yield, but these fish are caught 

 primarily for sport and the economics of the cost of the treatment compared 

 with the actual food value of the increased yield are of only secondary 



significance. 



Most marine fish are carnivores, feeding at least at two or three stages 

 removed from the original plant production, and part of the food of the 

 bottom-living fish may be four, five or even more stages removed. As was 

 seen in Chapter lo, only lO per cent of food eaten becomes established in the 

 next stage so that a much greater total increase of plant growth is needed to 

 produce the same increase of yield of carnivorous fish as of herbivores. These 

 extra stages in the food chain also increase the chances of some link in the 

 chain getting out of hand so that the final result is not according to plan. 



Because of the size of the sea, it is advisable to consider the problem at 

 three distinct levels: the small almost enclosed body of sea water with a re- 

 stricted exchange of water with the outside, such as a shallow^ Cj^^*^ *-"^ '•^^ 

 Scottish west coast lochs in which preliminary experiments have been done ; 

 a relatively small but unrestricted area such as an open bay ; and the sea itself. 



The first factor to be considered is what happens to the added fertilizer, 

 will it be absorbed or will it be washed away? Experiments have shown that 

 the extraction of the fertilizer from the water is remarkably fast, taking less 

 than seven days, partly due to the immediate utilization of it by the plants 

 and partly because it is adsorbed by the mud particles (i.e. it adheres firmly 

 to the outside of the grains of mud) from which it is only slowly liberated. 

 This means that the fertilizer is largely retained /';/ situ and not washed away 

 as quickly as one might expect. 



In our almost enclosed body of sea water the losses are thus negligible, 

 but the fertilizers are nevertheless not used to our complete satisfaction. 



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