A Note on the History of Mark-Recapture Population Estimates

Dahl writes thus: 'The main difficulty which had to be solved was to obtain a satisfactory estimate as to the number of trout present in these tarns. Starting from the idea which originally prompted Dr C. G. Johs. Petersen in his famous experiment with the plaice of the Limfjord in Jutland, I endeavoured to employ the marking of fish as a means of estimating the stock. Accordingly I constructed a seine-net, about 25 fathoms in length and 4 fathoms deep, of a mesh which could retain all trout down to about 7 or 8 cm of length. With this seine I made a number of hauls, capturing between 100 and 200 trout in each tarn. These trout were now weighed and marked by cutting off the adipose fin and then evenly redistributed in their respective tarns. The tarns were then again immediately renetted and the number and weight of trout in each tarn calculated by means of the relations between the number of marked and unmarked fish captured. The theory of calculation is shortly this. If I have marked 100 trout and distributed them evenly, and if I then fish and capture 150 trout, of which 50 prove to have been marked, I have taken half of the marked fish. Considering the marked fish as representative of the stock I should consequently have taken half of the stock of the tarn and the total number contained should be about 300 fish. In other words, if I divide the number of marked fish liberated by the number of marked fish recaptured, I obtain a coefficient of capture, and by multiplying the total number or weight of fish taken with this coefficient, I obtain an approximately correct estimate of the total number and weight of fish present in the water.' This quotation is taken (with the present Editor's permission) from pages 18 and 19 of the 1919 English paper which in these paragraphs appears to be a direct translation by Dahl from his Norwegian. As Dahl is likely to have had an intimate knowledge of the work of Petersen and his colleagues it would appear that this quotation confirms that Petersen did not in fact