Effect of trawling on animals of the sea bed 3 



the intervening crest. The beam trawls formerly used could generally get along safely, 

 presumably because the beam took the blow of the sand-crest. By 1939, beam trawls 

 were almost extinct, so the " bank " area provided another sanctuary. Had trawling 

 had an appreciable effect, the benthos should be different as one left the bank area. 

 On July 23rd and 24th, 1939, this was investigated near the northern boundary, as 

 located by echo-sounding, using Petersen's grab and the Naturalists' dredge. Two 

 lines of stations were completed {George B/igh L 1939, Sta. 14, Hauls 0-9, Sta. 15, 

 Hauls 0-6). 



Another line of approach was to try to damage animals of the sea-bed with a trawl 

 armed with a heavy tickler-chain. The idea was to choose a bed of some animal with 

 a fragile shell, as fragile as possible, and then tow over it as precisely as possible. 



Fig. I. Dredge hauls before and after trawling 



Tracks of hauls before using trawl and tickler chain are shown as whole arrows with the serial number 

 in a circle; hauls after, as broken arrows with unringed serial numbers, related to the position of a 

 buoy's anchor. " Before " hauls 7 to 10 showed a bed of Heart Urchins, which was also sampled by 

 " after " hauls 2 to 8 with moderate success, the wind having changed. The swinging positions of the 

 buoy have been taken into account. Its area of 180 yards radius limited trawling, probably to 

 something like the shaped area indicated. In the waist, the area would be trawled over on, the average, 

 about 5 times. Heart Urchins were broken, but not Mactra. 



Range judged visually was perhaps accurate to within 20 per cent; but the buoy's 

 position (on 90 fms. wire in 13 fms.) would alter with the tide. A trawl, which probably 

 fished at something over 20 yards width, was furnished with half-inch chain extending 

 for 100 feet between the otter-boards, shorter and therefore mostly ahead of the 120 

 foot ground-rope. This was towed up and down on the 11th, 12th, and 13th July, 1938, 

 crossing the ground 42 times; but owing to the error in the buoy's position due to 

 tide and the imprecision of trawling, these cannot be thought of as 42 exactly super- 

 imposed hauls 20 yards wide. Instead we must think of an area like that shown in 

 Fig. 1, representing fishing as close to the buoy's moorings as the ship dare go. The 

 waist of the constricted area is 1 50 yards wide, which allows for 7 to 8 strips side by 



