174 BRITISH FISHERIES 



sought for.^ Pelagic fishes, of which the herring 

 and mackerel are the best-known types, live 

 generally in the open sea, and frequent water of all 

 depths along the shallower part of the continental 

 slopes, At certain periods shoaling movements 

 occur, and it is then that the great fisheries are 

 held. These two fishes are plankton-feeders, 

 subsisting on the pelagic life of the sea, Crustacea 

 (copepods and schizopods) usually. Such plankton- 

 feeding fishes are furnished with appropriate 

 feeding organs ; in these species the gills are pro- 

 vided with "gill-rakers," comb-like structures 

 attached to the gills, which act as strainers. 

 When the creature takes water into its mouth, any 

 small organisms, such as copepods, become strained 

 out by the gill-rakers, and are then brushed off by 

 the tongue and are swallowed. These fishes are 

 caught by appropriate fishing apparatus — mackerel 

 by drift-nets or hooks, and herring by drift-nets. 

 The drift-net is simply a vertical wall of netting 

 of great extent which drifts in the sea, and herring 

 or mackerel striking against it become enmeshed. 

 On the other hand we have the bottom-living 

 species, like the sole, plaice, cod, etc., which live 

 at or near the bottom of the sea, lying on, or 



1 From the point of view of the naturalist, fishes may be divided 

 into shore and deep-sea species. The former inhabit the seas within 

 the loo-fathom contour line, and the latter live in the great ocean 

 depths — down to 2000 fathoms or more. There is, of course, no 

 absolute distinction between these two categories. Probably the 

 deep-sea forms have originated from shore or pelagic species. 



