SCIENCE AND THE SEA FISHERIES 227 



Only a few instances of the possible connection of the fisheries 

 with the salinity of the sea water can be given here. The summer 

 herring fisheries of the British Isles commence in May off the outer 

 Hebrides and, the Orkneys. In June the fish are found off the coast 

 of Aberdeenshire ; further south in successive months. There is 

 reason to believe that the movement of these herring is connected 

 with the movement of bodies of water of certain definite temperature 

 and salinity, but the connection has by no means been proved. 

 The anchovy fisheries of the Zuyder Zee are probably influenced very 

 largely by variations in the hydrographical conditions, and the 

 arrival of the winter herring off the Norwegian coast coincides 

 with the appearance of the so-called " bank water " of medium 

 salinity and high temperature. The cause of the water movements 

 in the North European seas is to be sought for in the fluctuations 

 of the Gulf Stream. This Gulf Stream circulation undergoes an 

 annual or, at any rate, a periodic contraction and expansion, which 

 affect not only the fisheries, but the climate of these islands. This 

 pulsation of the Gulf Stream has been investigated by the inter- 

 national commission. In addition to the Gulf Stream water there 

 are two important constituents of our northern seas, the cold 

 bottom water from the Arctic and fresh water from the rivers ; 

 but of the three the first is by far the most important. 



The biological side of marine investigations now demands our 

 attention. Organisms in the sea — ^whether animal or vegetable — 

 may be divided into three groups, according to their mode of life : 

 drifters, swimmers and fixed organisms. To the drifters the technical 

 term " Plankton " is applied. The plankton consists for the most 

 part of microscopic animals and plants which float about at the 

 mercy of wind and tide. To the group of swimmers (Nekton) belong 

 fish and marine mammals, which are capable of moving against the 

 tide. To the fixed organisms (Benthos) belong creatures like 

 oysters, mussels and " sea-lilies." Upon the microscopic plants 

 of the plankton all marine life ultimately depends, since they 

 alone can manufacture organic from inorganic materials, so it will 

 be seen that the determination of the constituents of the plankton 

 and its seasonal variation is of considerable importance. Many 

 pelagic fish, e.g. the herring, sprat, mackerel and pilchard or sardine 

 are direct plankton-feeders, while other fish, the demersal group 

 to which the flat-fish and cod family belong, are only indirectly 

 dependent on it. The plankton-feeders possess comb-like structures 

 — ^the gill-rakers — attached to the gills, and these structures act as 

 food strainers. 



For the purposes of respiration water is taken in through the 

 mouth and passed out over the gills, and during this process the 



