The losses of organic material that occur at each stage of a food chain are thought generally to amount to some 80-90 per cent. On this reckoning, 1 kg of phytoplankton provides about 100 g of herbivorous zooplankton, which in turn yields 10 g of first-rank carnivore, 1 g of second-rank carnivore, and so on.
Most of the food that man takes from the sea comes from food chains involving several links, and therefore the harvest can be only a small fraction of the primary production. Some of the most plentiful pelagic fish are first-rank carnivores, but the majority of the most popular species for human food feed at later stages of the chain. Cod, for example, feed largely on other carnivorous fish or on carnivorous benthic animals. It is, therefore, apparent that far larger quantities of food could be obtained from the sea if it were possible to collect the earlier stages of food chains than can ever be provided by fishing. Instead of catching fish, why not directly harvest the plankton itself and process it to extract the food materials?
The practical difficulty of collection presents a major obstacle to obtaining large quantities of food in this way. Usually, plankton is dispersed in a very large volume of water, and even in the most productive areas enormous quantities of water would have to be filtered to obtain plankton in bulk. If the smaller organisms are to be retained, and particularly if the aim is to collect the phytoplankton, very fine filters would be required and the process of filtration could therefore proceed only very slowly. It seems unlikely that direct harvesting of the plankton from the open sea could be carried on economically, except perhaps in a few areas where there are very dense aggregations of the larger zooplankton such as krill (see below).
In recent years, there have been a number of experiments to investigate the possibilities of mass culture of marine phytoplankton. There seems little doubt that methods can be developed for culturing diatoms in large, shallow, seawater tanks enriched with plant nutrients. In dense fast-growing cultures, availability of carbon dioxide becomes a limiting factor; but if the culture tanks are sited near industrial installations, washed flue gases can be used to supply the carbon dioxide for photosynthesis, and waste heat to maintain the optimum water temperature. In this way, a rapid growth of phytoplankton can be maintained, and it might be possible to develop continuous culture methods similar to those now used in brewing or the preparation of antibiotics. However, although diatoms are rich in protein and oil, there are considerable difficulties in the separation of the plants from salt water and the subsequent extraction of the food materials from the cells. These processes are fairly efficiently performed biologically, and it seems likely that mass cultures of phytoplankton will find their chief usefulness in association with the rearing of young fish or the culture of some of the popular species of bivalve molluscs.
The Southern Ocean at times contains enormous numbers of a small shrimp-like crustacean known as krill (Euphausia superba) (see Figure 2.11). This is the food on which the great baleen whales including the largest of all, the blue whale, depend almost entirely. They grow quickly to a huge size simply by sieving this crustacean from the water.
At 5 cm long, krill can be captured using fine-meshed nets. The harvesting of krill probably represents one of the only remaining ways in which we can significantly increase the harvest of the world's oceans. The stock of krill is variously estimated at 50-100 million metric tons and is probably greater now than it was when whales still abounded in these waters. Krill is now harvested by ships from a number of nations including Russia, West Germany, Poland, Japan and Taiwan. The present total annual harvest is around 300 000 tonnes and the stock could most probably sustain a much larger harvest in the region of many millions of tonnes. Shoals of krill are detected using sonic techniques and there are further prospects of using satellite sensors to track the distribution of shoals.
However, there are problems to be overcome, not the least of which is persuading people to eat them! At present, most krill is converted to protein meal on board factory ships (mostly Russian). Krill does not keep well and can only be efficiently exploited by technically advanced fishing fleets. The problems of suitably processing krill into acceptable products for human consumption have yet to be fully solved but it can certainly be used for animal feeding. Whether the returns will justify the high costs of maintaining ships in distant Antarctic waters remains to be determined.
If krill stocks were to become heavily exploited, there would also be environmental costs. In addition to whales, many other animals also feed on krill, including seals, penguins, other birds, squid, and many species of fish. Excessive human harvest of krill could lead to diminished numbers of these predators and could prevent the hoped-for recovery of the baleen whale populations. To monitor the situation a degree of international accord has been provisionally reached on measures to set annual quotas and to assess the effects of krill harvesting in order to establish safe limits.
As whales can collect krill so efficiently, various suggestions have been made for devising 'artificial whales'. These might be constructed as atomic submarines with gaping bows opening to revolving filter drums, and provided with means for the continuous removal, processing and storing of the filtered zooplankton. At present, such ideas remain in the realms of Jules Verne.
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