35 



It will be seen that the averajie rate of increase in growth per 10 days vared 

 considerably according to the sex and size of the fish. Taking the whole period, 

 the mea 1 rate, in the case of males, ranged from "02 to "11 cm., the average being 

 about .07 cm., which is equivalent to an annual increment of about 2-5 cm., but it 

 will be noticed that, with two exceptions, all the males, when liberated, were of 

 adult size. The average rate of increase per 10 days among the females ranged 

 from "07 to "15 cm., the mean for the 8 cases being '106 cm., or at the rate of 3"7 

 cm. per annum. The majority of the females were under the adult size when 

 liberated. 



Two females, both liberated at 20 miles east of the Isle of May, were recovered 

 after an interval of between 4 and 5 years. The former (No. 1705) was taken in 

 March 1915, after 1566 days, not far from where it had been liberated, namely 

 25 miles E.S.E. of the Isle of May ; on hberation it measured 38'5 cm., and on 

 recapture, 53"1 cm., an increase of 14'6 cm., or at the rate of "1 cm. per lOdays. The 

 other female (No. 3032) was recaptured in June 1916, after 1821 days, 70 miles 

 N.E./N. of Longstone, Northumberland ; it had grown from 37 to 50"8 cm., an 

 increment of 13*8 cm., or at the rate of '07 cm. per 10 days. 



One fish was recaptured after more than 5 years of freedom. It was a male 

 (No. 1716) liberated at 20 miles east of the Isle of May on 28th November 1910, 

 and recaptured in February 1916, after 1892 days. This fish was got in London 

 in the market, and the place of the recapture was not ascertained. 



IV. The Intensity of Fishing. 



One important result of the marking experiments is the Ught they throw upon 

 the destruction of the fish by fishing operations. There is no reason to suppose that 

 the marked plaice are captured in greater numbers, or in less, than the unmarked 

 plaice which live around them on the fishing-grounds. The proportion of the 

 marked fish which were recaptured by fishing operations thus furnishes a clue to the 

 intensity of fishing, and gives an indication of the proportion of the plaice which 

 may ultimately be captured by commercial fishing. As already mentioned, in order 

 thoroughly to understand such questions, one would require to possess detailed 

 information as to fishing operations, both in regard to the amount or quantity of 

 fishing, and in regard to the fishing-grounds, and this is especially desirable when 

 the number of fish marked and Uberated and the proportion recaptured are small. 

 In these experiments on the east coast of Scotland, however, the number of plaice 

 marked was large, viz. 7166, and the number recaptured was also large, viz. 3735, 

 or 52" 1 per cent. The number actually recaptured must have been still greater, 

 for, in the first place, there is evidence that the mark is sometimes lost and the 

 fish recaptured with the signs of having been marked, though the mark is absent ; 

 in the second place, all the marked plaice whicR are recaptured are not returned, or 

 their capture made laiown. It is probable also that a certain number died after 

 having been returned to the sea, owing to injury received in the process of capture, 

 or in the process of marking. One feels justified in saying that fully a half of the 

 adolescent and adult plaice hving on the fishing-grounds along the east coast of 

 Scotland are ultimately caught in commercial fishing. 



In some cases, however, the proportion recaptured was much more than 52 per 

 cent. The experiments which give the highest percentage of recapture were those 

 made at the station 20 miles east of the Isle of May. In the 10 experiments at this 

 station, 961 plaice were marked, and 643, or 66*9 per cent., recaptured, the per- 

 centage of recaptured in different experiments ranging from 54*9 to 77*3 per cent. 

 At the neighbouring station in the Firth of Forth, between Ehe and Fidra, 823 

 plaice were marked and liberated, and 489, or 59*4 per cent., recaptured, although 

 trawHng, the method of fishing by which plaice are usually taken, is prohibited 

 in the Firth of Forth. Here the percentage of recaptures varied from 46"2 to 69-6 

 per cent. Of 200 liberated 3 miles from Fidra Light, at the entrance to the Firth 

 of Forth, 122, or 61 per cent., were recovered. In the experiments at Aberdeen 

 Bank, 1227 plaice were marked and liberated, and 741, or 60*4 per cent., were 

 recaptured, the percentage in the different experiments ranging from 40 to 70 per 

 cent. In the neighbourhood of Fair Isle, 940 were marked and hberated, and 461, 

 or 49 per cent., were recaptured, the percentage or recapture in the eight experi- 



