NATURE ADRIFT 



ranges from lo to 70 per cent, so that a flow meter is still necessary. If the 

 organisms one is to count are not very abundant and the sea at that part is not 

 very deep, a vertical haul may give too small a sample for satisfactorv results. 

 A vertically hauled net cannot make use of all the bright ideas that have gone 

 into the making of the high speed tow net, and an oblique haul is therefore 

 usually made. This means that a high speed net, with all its advantages, can be 

 hauled at a steady speed from the bottom to the surface whilst being towed; 

 the resultant catch is then equivalent to a very elongated vertical sample with 

 the actual water-flow properly assessed by the flow-meter. 



When attempting to do accurate quantitative work at known depths 

 some sort of a closing net is necessary to prevent the capture of plankton 

 from other depths during shooting and hauling. If the sample from a 

 vertical haul is likely to be big enough for the purpose the problem is easier 

 to solve. The net can be lowered vertically and if it is weighted at the tip 

 (Fig- 3 ; 3)> it will sink end hrst and so not catch anything on the way down. 

 After hauling for the necessary distance, a special weight, called a messenger 

 (Fig. 4; lb), can then be attached to the line. This is merely a weight, usually 

 of brass, so constructed that it can be clipped over the line so that it cannot 

 come off, yet is free to run easily down it. The messenger, on arrival at the net, 

 triggers off the closing mechanism (Fig. 4; if). The simplest of these is 

 merely a throttling device, so that the net instead of being hauled by the 

 bridles is now hauled by a loop round the elongated canvas collar of the 

 net itself (Fig. 4; id). Another method is to use a hinged ring hauled by four 

 bridles; to close it the two bridles attached near the hinges are released 

 putting all the strain on the other two, thus closing the opening like a lady's 

 handbag. Horizontally hauled nets must be sent down closed, opened at 

 the right depth and then closed again. To do this, a system of metal doors is 

 used. These are held by a spring across the mouth of the net until released 

 by the first messenger, when they swing through 90 degrees to a horizontal 

 position no longer impeding the water-flow. After the desired distance has 

 been covered, a second messenger is sent down which further releases the 

 catch and the doors swing through a second 90 degrees to close the net again. 

 One example often used is the Clarkc-Bumpus net (Fig. 4; 2). 



Various modifications of these patterns have been adopted for special 

 purposes. A plankton net can be fixed to a sledge and fitted with doors that 

 are kept closed by a light spring but which open as the apparatus is drawn 

 along the sea bed. Such a net captures only the plankton found in the few 

 inches immediately above the bottom (Fig. 4; 3 — the Bossanyi net). There are, 

 too, varieties of detail in the construction of plankton nets of all kinds, 

 developed in different countries for slightlv different purposes. 



In 193 1 Professor Hardy of the University of Hull (now Sir Alister 



18 



