— lO — 



occurrence of the fish that it will only be observations which can afford to neglect 

 the size of the catch that will be able to enlighten us as to the density of fish 

 at different periods. 



2) Even if the values of the total numbers recorded by us do not in 

 themselves correctly represent the general values, some relations among the 

 different averages will yet probably agree tolerably well with the actual 

 conditions. I have in my mind for instance some such question as this: What 

 proportion does the number of mature haddock bear to the number of immature? 

 Or again: What proportion do the different market groups bear to each other in 

 the different years? Questions like these can be solved if the different sub-divisions 

 of the whole average-number per trawl-hour are correct in the proportion they 

 bear to one another. In other words if the material is so far representative that 

 for instance the percentage which the mature haddocks in our catches bear to 

 the whole average-number per trawl-hour equals the percentage in the aggregate 

 which really exists. In this respect our material seems to be fairly reliable, since 

 many of these proportions give practically the same results with other tests and 

 with different groupings of the material. 



For investigations of this nature we require measurements of the different 

 individuals, and we have as previously mentioned no less than some 70000 length- 

 measurements for haddock and about iiooo measurements for cod. 



3) These length-measurements can however — as is well known — serve 

 also to solve other very important problems. According to the method introduced 

 by Dr. C. G. Joh. Petersen we may expect to find groupings round fixed lengths, 

 which represent the different year-classes. This is the most important question, 

 from a biological point of view, that measurements like these can help to elucidate. 

 Besides being able to trace the development of the different size-groups with the 

 advance of time and within the different areas, we can — if Petersen's method 

 is correct — find the age represented by the different market-groups and the age 

 when maturity begins. His method too will enable us to study the contribution 

 made to the aggregate by the different years. The material we have here 

 considered will very possibly present us with some valuable results of this kind: 

 but it must not be forgotten that the present work is first and foremost a 

 methodical study. 



While engaged upon this statistical material I have frequently made use of 

 the following valuable literary works: Fulton's papers in the Report of the 

 Scottish Fishery Board (especially his papers in Reports No. XXI and XXII) 

 Heincke and Henking in the General Report on the Work of the Period, July 

 1902 — July 1904, Appendices E and F' in Rapports et Procès- verbaux. Vol. Ill, 1905; 

 D'Arcy Thompson, The Statistics of the Aberdeen Trawl-Fishery 1901 — 1906, with 

 Special Reference to Cod and Haddock, in the present report on Commission A's 

 work during the period 1903— 1907 (Appendix 2); D. Damas, Sur la biologie des 

 gadïdes de l'Atlantique Nord, present report's Appendix 3. To these two last works 

 in particular it will often be necessary to refer in what follows, and we shall then 



