Dakin and Latarchk — The Plankton of Lough Neagh. 29 



The Nansen uet has been used in preference to the Hensen nets, chiefly 

 because of convenience in handling. The whole aim of the work has been 

 to corn-pare a series of catches made throughout a year. To this end the most 

 important point to keep in view was the taking of catches in a uniform manner 

 with the same net. 



In several cases we notice that horizontal hauls of a net have been used 

 for a study of the periodicity of plankton (no mention whatever is made of 

 methods in the Eeport of "W. West & Professor G. S. West). This is a very 

 unsafe method, accentuated when used by non-scientific workers. On several 

 occasions on calm days we found at Lough Neagh that the plankton was 

 concentrated in the upper few inches of water. On another day, perhaps 

 within twenty-four hours, the same plankton was scattered anywhere between 

 the bottom and surface by rough weather. Imagine the great difference 

 between two horizontal hauls taken at these times. A vertical haul would 

 have given true pictures of the plankton present. 



Again, even on the calmest days it is impossible to tow a net so that it 

 remains in the same layer of water. It would be quite possible on a calm 

 day for two boats making horizontal hauls close to each other to have entirely 

 different catches both in quantity and relative composition. This could rwt 

 occur if vertical hauls were made. Hence, for the study of variation in 

 plankton (periodic), vertical hauls must be made. 



The Nansen Net. 



The Nansen net is a closing net of bolting silk made by Andersen, of 

 Christiana, Norway. It has been used largely by Herdman in the Irish Sea ; 

 and one of the authors, who made the catches at Lough Neagh, has had 

 considerable experience with it in marine work. Four definite sizes are 

 manufactured — diams. 35, 50, 75, and 100 cm. ; and we have therefore 

 a uniform set of nets here if workers buy from the maker. The net 

 is conical in shape, with a mouth having a diameter of 35 cm. The 

 length from mouth-opening to end of bag is 228 cm., but this is not all 

 composed of filtering-tissue. 



From the net-ring (which supports the mouth), extending down for about 

 58 cm., is a cylinder of non-filtering sailcloth. To this mouthpiece the actual 

 net is attached. It is 170 cm. in length. The silk used (No. 20) runs 

 seventy-two meshes to the centimetre ; at the end of the silk bag there is a 

 brass filtering bucket (fig. 2, B), which is attached by a bayonet-joint to a 

 brass ring fixed to the silk, and also connected with the net-ring by three 

 ropes. To these ropes, which are continued below the bucket, a heavy weight 



[£2] 



