Papers in Marine Biology and Oceanography. Suppl. to vol. 3 of Deep-Sea Research, pp. 1-6. 



Effect of trawling on animals of the sea bed 



By Michael Graham 

 Fisheries Laboratory, Lowestoft, England 



Summary — Damage to fish food species trawled over in the main area of the North Sea plaice, cannot 

 be serious; otherwise there would be a noticeable diflFcrence where trawling is impossible, as close to 

 light vessels or among the under-water sand dunes. 



Direct attack, covering the ground some five or six times over on the average, did break full-grown 

 Heart Urchins, Echinocardium cordatum, and possibly swimming or paddlcr crabs (Portunus dcpuraior), 

 but appeared not to damage Ophiura albida, nor any of the fragile-shelled plaice food animals: 

 razor shells, Mactra or Tellina. Those forms were not very abundant, but all the 15 specimens taken 

 of fragile animals (other than urchins and paddlers) were undamaged. Such large urchins as were 

 damaged were not plaice food. 



Doubtless Sabellaria habitations (ross) would be broken and laid low, but they would probably 

 soon be reconstructed. 



Trawling, even with a tickler chain, seems again to escape the so viable indictment. 



INTRODUCTION 



Many trawlermen are sure that their work alters the bed of the sea. Some say that 

 it improves it, increasing the growth of animal forms growing up from the sea bed by 

 clearing out old structures. Others say that trawling, especially with tickler chains, 

 harms the food of fishes by breaking protective shells and structures. 



The complaint of damage is an old one. In 1376 the Commons petitioned the King 

 of England " that the great and long iron of the wondyrchoun runs so heavily and 

 hardly over the ground when fishing that it destroys the flowers of the land below 

 water there ". In the 19th century the beam trawl came in for similar criticism, which 

 Dr. W, C. Macintosh, acting for the Royal Commission of 1863, disposed of. 

 However, trawls have become heavier and tickler chains more common, in certain 

 fisheries almost universal. One such fishery is the English one for plaice in the 

 southern North Sea; and in 1938 it seemed worth while to devote a little research- 

 vessel time to finding out whether tickler chains had any marked effect. 



Doubtless trawling with heavy tickler chains breaks up and flattens structures made 

 of sand-tubes by Sabellaria, and similar comparatively fragile highly projecting 

 structures, when these are not so strong as to prevent trawling because of the frequency 

 of tearing. But Sabellaria tenements seem to be annual growths, run up fairly quickly, 

 so that it may not be assumed that much permanent damage is done. 



The fishing skipper of the George Bligh advised that it would be foolish and not 

 really relevant to experiment on Sabellaria grounds, which he thought to be limited 

 compared with the whole plaice area; so the decision was made to confine the enquiry 

 to the usual clean, sandy ground frequented by plaice. 



METHODS 

 The first line of approach was to assume that there exists a small sanctuary about 

 three-quarters of a mile in radius centred on each light vessel, within which trawling 

 would be very rare, for fear of fouling the lightship's moorings. If trawling had made 



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