COUNCIL - SEPT. 1910 - APPEND. D — 70 — 



Special lectures on biological subjects were given by Prof. Henking and Dr. 

 Knut Dahl. 



Prof. Henking gave a lecture on the methods of statistical investigations in 

 fisheries work, of which the following is a brief summary: 



Whilst hydrography and even biology have for some years possessed well- 

 founded methods, this had not been the case with fisheries statistics. Every new 

 contribution to this subject will therefore be welcome, as also the results which 

 have been obtained from the German plaice measurements for the year 1909 with 

 the aid of the physicist Dr. Eucken (Berlin). From the series of measurements of 

 plaice of each single size from the entire German fishing-fleet and from a curve based on 

 the measurements, it was shown, that this curve of capture falls into an upper 

 portion, which may be called the frequency-curve (curve of quantity), containing 

 the large fish down to a maximum point (the largest quantity at a definite size in 

 cm.), and into a selection-curve containing the small fish on the lower side 

 of the maximum. The possibility of escape through the net and the selective 

 action of the fishermen have the result, that we cannot obtain a rehable picture 

 of the natural fish-stock from this portion of the catch. The use of small-meshed 

 nets gives supplementary information here and leads one to conceive the possibi- 

 lity, that the frequency-curve of all stages of the plaice in a sea would probably 

 have a fairly straight course. 



The meaning of the "limits of the average size of the catch", the "median" 

 and the "interval within which the median lies" were explained in detail on the 

 basis of the material dealt with, and the question was then discussed, what 

 requirements the measurements must fulfil in order to be representative of 

 the total catch. For the steam-trawlers the following method had proved commen- 

 dable. First a representative sample of each of the commercial classes from one 

 voyage of a trawler is measured. How large the sample must be in order to be 

 representative can be determined mathematically. From this we can obtain the 

 total catch of the voyage of this steamer. Numerous voyages from the different 

 areas and regions are dealt with in the same manner (the number of the voyages 

 to be analysed each month can also be determined mathematically), and we can 

 then make conclusions with regard to the total landings. — As a test whether the 

 voyages measured can be taken as at all representative of the total, we may com- 

 pare the average catch of all the monthly landings with the average of the trawlers 

 measured. This important new fact was emphasized, namely, that the catches of 

 the trawlers are very different in their nature according as the catch is large 

 or small. 



