RELATIONSHIP WITH FISHERIES 



therefore vary according to the distribution of suitable hydrographic con- 

 ditions and the abundance of food, so that in turn this has its effect on the 

 disposition of the fishing. This effect is not only that herring search for food 

 but also that they will avoid distasteful water. On page 1 1 3 reference was made 

 to 'baccy juice' when the diatom Rhizosoleiiia is abundant and that herring 

 avoid such patches. Avoidance of dense phytoplankton seems fairly charac- 

 teristic even if we have no evidence of a distasteful effect. A rich patch of 

 diatoms may occur just because there are too few herbivores to graze on 

 them and there may be few herring there due simply to the lack of copepods 

 etc. 'White water' due to the presence of coccolithophorcs (p. 42) is an 

 indicator of the mixed conditions that are good for plankton production, 

 so that 'white water' is considered a good omen for herring fishing. But not 

 all 'white water' is due to coccolithophorcs, some is due to minute globules 

 of herring oil making the water milky or even to the presence of brushed off 

 scales. These would obviously indicate the presence of fish and perhaps 

 the more direct relationships have helped to give rise to the suggestions 

 about the other ! 



Plankton conditions can thus affect not only the food supply but the 

 movement of herring, and so affect their availability to the fishermen inde- 

 pendently of their actual numbers. Pilchard feed more directly on the 

 phytoplankton than the other fish of the herring family and so have an even 

 more direct dependence. 



One of the biggest fish in the seas around the British coasts is the basking 

 shark which reaches about 30 feet in length. Although it is a true shark it is 

 not a ferocious fish and it feeds almost entirely on plankton. This it gets by 

 cruising slowly along with the wide gape of its mouth open, rather like a 

 plankton net, and the plankton is filtered off by a series of special combs of 

 'gill-rakers' attached to the gills. The food is mostly the rich oily Calamis 

 and euphausids. Basking sharks may be seen cruising at the very surface on a 

 calm warm sumiy day with the dorsal fin out of the water (Plate XXXIV). 

 They are not at all aggressive and only cause damage to fishing nets when they 

 accidentally get entangled — can you blame them ! A sudden swish of the 

 huge tail can easily upset a rowing boat but this would only happen if the 

 shark had been suddenly disturbed. Strangely enough they lose their gill 

 rakers in the autumn and 'hibernate' on the sea bed until the new ones have 

 grown in February. 



Whales are not fish, of course, but they deserve a paragraph here. Whales 

 are of two mam types, the toothed whales like the sperm whale, and the 

 w^halebone or baleen whales which feed on plankton. The blue whale, the 

 largest mammal that has ever existed and which may reach nearly 100 feet 

 and weigh 120 tons, is a plankton feeder. Like the other baleen whales it 



133 



