RELATIONSHIP WITH FISHERIES 



filters the plankton with a sieve made of a series of brushes, which are the 

 hairy fringes of the baleen plates (Plate XXXV). Its main area of feeding is 

 in the Antarctic during the southern spring and summer where the planktonic 

 krill [Eiipliaiisia superha) is abundant. A blue whale stomach may contain 

 tons of euphausids. An idea of the richness of this food is obtained from its 

 rate of growth. The calf is about 26 feet long and weighs 12 tons at birth 

 after one year's gestation, and during six or seven months lactation its weight 

 increases at about half a ton a week until it starts to feed for itself. After two 

 years it may weigh 60 or 70 tons. 



Although most of the larger whales are more or less universally distri- 

 buted in the oceans, one of the more abundant in the northern areas and in 

 part responsible for the whaling of these areas, is the sei whale. Here in the 

 northern waters the main food is another euphausid, the 'northern krill' 

 [Megaiiyctipliaiies norvegica, Fig. 22; 5). The smaller toothed whales in this area 

 are mostly fish feeders and not therefore the concern of this book. The harp 

 seal and some sea birds too are keen on euphausids, notably the fulmar in the 

 north and the short-tailed shearwater in Australia. 



Adult fish living on the bottom are also dependent on the plankton, not 

 so directly as the larvae and the pelagic fish, but nevertheless just as truly. 

 We have already seen that the young stages of very many bottom-living 

 animals — the food of the bottom-living fish — are planktonic. It is thus clear 

 that their abundance and their distribution are directly linked to the plankton. 

 Because the food supply of these bottom-living animals is in turn largely 

 derived from the plankton, their survival and growth is also dependent on it. 

 The richness of the food affects the growth rate of the fish ; for example a 

 five-year-old haddock living on the moderately good food supply of the 

 central North Sea grows to a length of 31-6 centimetres (just over 12 inches), 

 but a five-year-old haddock from the rich feeding Icelandic waters grows to 

 about 55 centimetres (nearly 22 inches) which is about five times the weight 

 (see the final paragraph in Chapter 2) . 



Recent changes in climate are affecting the temperature of the northern 

 seas and ice has gradually been receding. This has meant a northerly exten- 

 sion of areas in which fish — especially cod — can live. When such changes 

 take place, a re-colonization of the sea floor by animals suited to the new 

 conditions occurs and these, of course, are the food for the fish in these new 

 grounds. Although it is a gradual change, it is not so gradual that small 

 bottom-hving animals could crawl fast enough to keep pace, and rc-colon- 

 ization depends on the transport of planktonic larvae by the currents actively 

 associated with the climatic change. 



Plankton is thus of fundamental importance to the fisheries through a 

 shorter or longer food chain according to the type of food the fish eat, but 



135 



