of the Fishery Board for Scotland. 119 



ill Scotland.* By the end of 1896 most of tlie deep-sea Scottish trawlers 

 were equipped with the new gear ; a year later the beam-trawl was 

 employed only by the small paddle-boats that worked near the coast, 

 and in 1898 the otter-net had entirely replaced the beam-trawl. In 

 1899, when I desired to make comparative experimental hauls with both 

 nets, I found that beam-trawl gear could not be obtained. 



Experience showed that the new net not only was more efficient than 

 the beam -trawl in capturing a larger aggregate quantity of fish, but that 

 its success was more mai'ked in taking round fish than flat fish. It was 

 therefore desirable to determine, if possible, the relative efficiency of the 

 two apparatus, in order to aid in interpreting the official statistics of the 

 quantities of fish landed by steam -trawlers over a series of years. These 

 statistics showed, concurrently with an increase in the number of steam- 

 trawlers, a rapid rise in the quantities of cod and haddocks in the years 

 following the introduction of the otter-trawl, and a relatively slight 

 aggregate increase in the quantity of flatfishes. It is obvious that these 

 changes might not be due entirely to the use of the otter-trawl, but 

 might be also caused by the fishing being conducted to a large extent on 

 different grounds in the years compared. 



The methods adopted in order to ascertain the comparative action of 

 the otter-trawl were partly experimental and partly statistical. The 

 experiments aimed at determining the distance between the boards 

 when the net was working on the bottom by (1) measuring the angle of 

 the diverging warps at the towing-block, (2) stretching a measured and 

 varying length of net-twine between the boards before shooting the 

 gear, and noting whether it came vip ruptured or intact, (3) tying 

 surface-bladders to each board when fishing in shallow water and 

 measuring the distance between them. 



Theoretical considerations make it clear that the distance between 

 the otter-boards must vary under difierent conditions. The boards act 

 in a kite-like manner, diverging from one another as they are drawn 

 through the water, and thus opening and keeping open the mouth of 

 the net. An increase in the vessel's speed may cause the boards to 

 move further apart, or diminished speed may cause them to come closer 

 together. They will also move closer when the net ofiers increased 

 resistance, as by a heavy bag of fish, the presence of boulders, much 

 mud, weed or jelly-fishes, &c., or when the ground-rope or the net 

 catches on the bottom. The action of the net depends upon a balance 

 of forces and the distance between the boards, that is, the width of the 

 mouth 01 the net varies with variations in the equilibrium. In the 

 beam-trawl, on the other hand, the width of the movith of the net is 

 constant, and is determined by the length of the beam between the 

 irons. The distance between the otter-boards, while varying as indi- 

 cated, is, moreover, normally much less than the length of the headline 

 extending between the boards, and to which the upper part of the net 

 is attached. At first it appeal's to have been thought that the headline 

 would be di'awn taut, oi' almost taut, by the lateral pull of the boai-ds, 

 and the net was then made square at the top on this assumption. But 

 it was found that the square of the net fell back and hung slack, and 

 within a year after the introduction of the otter-trawl, the net, instead 

 of being made square at the top, was made semi-circular or crescentic 

 by the introduction of top wings on either side. The net- makers allow 

 about one-third of the headline as forming the semi-circle between the 

 boaiTls, so that on this supposition in an otter-trawl with a headline of 

 120 feet the distance between the boards (the width of the mouth of 



■"Compare, Cutininghara, Jour. Alar, Biol. Assoc, iv., 117. Annual Reports Fishery 

 Board for Scotland, xiii., p. 143; xiv., pp. vii., 32, 33; xv., pp, 39, 155, 162; xvi., pj). 

 iv,, 41, 185 ; xvii., p. 183. 



