32 SCIENTIFIC SURVEY OF YORK AND DISTRICT 



northern and western parts are, of course, swifter, and here any many 

 good trout and grayUng streams, mostly preserved waters, which are 

 periodically restocked. The Yorkshire Fishery Board has a committee 

 for netting and restocking. The Board's coarse fish rearing ponds are 

 from five to seven miles to the north-east of York, and here fish, mostly 

 perch and roach, are bred to replenish the various waters. This year 

 (1932) the Board has established a trout hatchery at Keld Head, near 

 Pickering. 



The Ouse with its tributaries is an important salmon river, the salmon 

 ascending the Ouse as far as Aysgarth, and they enter the Derwent during 

 flood periods. Salmon parr have recently been caught in its tributary, the 

 Rye. The Report of the Yorkshire Fishery Board for 1931 shows that 

 823 salmon were caught in the Ouse during that year, the average weight 

 being 10 lbs. 3I ozs. 



It is noteworthy that in the two runs of the salmon up the Ouse, the one 

 in the spring and the other in the autumn, the fish take the left-hand fork 

 of the river at the junction of the Swale with the Ouse, thus passing into 

 the Ure. Though both rivers are free from pollution, the Ure is a salmon 

 river, the Swale is not. Mr. S. H. Smith, F.Z.S., of the Yorkshire 

 Fishery Board, is at present engaged in researches with a view to the 

 restoration of the Swale as a salmon river. A very full account of the 

 present position of this research may be found in the Naturalist, April 

 1932, and there is a further reference in the Report of the Yorkshire 

 Fishery Board for 1931. Briefly the argument is as follows. There is an 

 obstruction at Topcliffe Weir. After the building of the weir Swale 

 ' homing ' salmon would be unable to return, and so in time the Swale has 

 ceased to be a salmon river. If, then, salmon were to be reared in the 

 Swale now, and then found after a period of years to be returning to try 

 the Swale, the construction of a fish pass at Topcliffe would be justifiable. 

 The results of Mr. Smith's investigations will be awaited with great 

 interest. 



In connection with the restocking of the waters by the Board, Mr. Smith 

 is carrying out interesting experiments with selected fish, which are 

 measured and marked before being released. The results of these, 

 up to date, are to be found in the Board's reports for 1929, 1930, and 1931. 



On the occasion of the last visit of the British Association to York in 1906, 

 the Handbook, in dealing with the fish fauna of the district, said : ' With- 

 out doubt the greatest curse our fish fauna has to face is the constantly 

 increasing pollution of our streams.' It is satisfactory to find that since 

 that date the Yorkshire Fishery Board has got this matter in hand, and 

 considerable progress has been made in the prevention of pollution. 



The common frog and toad, the crested newt and the smooth newt are 

 plentiful, but there is no recent record of the palmated newt. 



The flatness of the district results in much stagnant water and sluggish 

 streams, conditions suitable for ensuring an abundance of the smaller 

 Crustaceans, but not for the fresh- water crayfish, which is only sparingly 

 distributed. Two very recent accounts of the prevalence of the crayfish 

 in Yorkshire are contained in the Naturalist for 1930 and 1931. 



The Mollusca are very richly represented. About forty-four fresh- water 



