LARGE PLANKTON NETS 115 



will rise through a vertical height approximating to half the amount of wire hove in. 

 For example, if 2000 m. be hauled, the vertical ascent is likely to be in the neighbour- 

 hood of 1000 m. It is possible, however, hauling the same amount of warp, to fish a 

 much narrower horizon than this if the speed of the ship be kept down to about half 

 a knot while the net is coming in. With the ship moving so slowly ahead, the net, for 

 lack of sufficient forward force to give it the necessary lift, is strongly inclined to sink- 

 especially in deep water work when long lengths of wire up to several tons in weight are 

 used— and this downward tendency, acting against the oblique upward pull, greatly 

 reduces the angle of ascent, the horizon in consequence being kept within more reason- 

 able limits. It is of course obvious that oblique fishing from a very slow-moving ship 

 can only be carried out with fast-moving nets, travelling at speeds far in excess of 

 normal, for the usual oblique hauling rate of 10 m. per min. is too slow to overcome 

 the downward tendency of the warp and net, the latter in consequence sinking faster 

 than it can rise. 



In practice the speed of the ship is reduced as soon as the wire is all out, and hauling 

 begins after a lapse of about a minute, or if necessary several minutes, when it has been 

 ascertained from the steepening of the warp that the net has a strong downward tendency. 

 When working in deep water, 2000 m. is hauled before closing, the rate of haul depending 

 a good deal on the state of the weather. A maximum speed of 40 m. per min. has been 

 reached in calm conditions, but as a rule the average rate is about 35 m. per min., a 

 speed that can be maintained in winds up to force 3 or 4 without straining the gear. At 

 this rate the net can be hauled for an hour, long enough to produce an ample quantity 

 of material from the greatest depth at which the apparatus in its present form has so 

 far been used. The horizon examined rarely exceeds 600 m., for deep-sea work not 

 unduly wide, but with experience it can be kept down to as little as 500 m. or even 

 less (see p. 117 and Fig. 4). 



Although excellent results have been obtained by this new method of rapid oblique 

 hauling the actual rate of fishing does not at first sight appear to have undergone any 

 marked increase. At its fastest, that is, allowing 40 m. per min. for the net and two- 

 thirds of a knot at most for the ship, it cannot be higher than 60 m. per min., or about 

 2 knots — no more in fact than the speed at which the net is stated to have been fished in 

 the original horizontal experiments. The success that has attended the recent operations, 

 however, and the repeated failure of the early horizontal closing hauls can only have one 

 satisfactory explanation, that even at 2 knots the net travels more quickly now than 

 it did in the past, when in all probability the horizontal speed was less than it is 

 claimed to have been. It is impossible otherwise to account for the vastly increased 

 efficiency of the net. 



The increased catching power of the net is perhaps best illustrated by the presence 

 in recent hauls of unusually large deep-sea forms, including among the pelagic prawns 

 species of Acanthephyra 20 cm. long. So far it is true the occurrence of such forms has 

 been rare, yet by employing still faster nets there is no reason why they should not be 

 captured consistently, along with other and even larger animals which doubtless exist. 



