326 Part III. — Twentieth Annual Rcjjort 



VI. RATE OF GROWTH OF SEA FISHES. 



II. 



By Dr. T. Wemyss Fulton, F.R.S.E., Scientific Superintendent. 



(Plates XIV.-XXI.) 



During tlie autumn of 1900 and throughout the greater part of 1901 

 investigations were made on board steaui trawlers fishing from Aberdeen, 

 the employment of which at intervals within the territorial waters was 

 authorised by the Fishery Board. Tiie general results, so far as they 

 concern the numbers of the various species of fish taken on the different 

 grounds in the different months, are given in another part of this Report 

 (p. 92). In the course of the work opportunity was taken to measure the 

 fish brought on board in certain of the hauls, comprising most species of 

 the food fishes, and a very large amount of material was thus obtained 

 for the determination of their rate of growth. In the present paper the 

 observations made on the rate of growth of the plaice, common dab, long 

 rough dab, whiting, and haddock are given. The treatment of the 

 measurements of the other species has had to be postponed. 



In the course of the enquiry it became evident that the method 

 employed was likely to furnish information of importance, not only 

 regarding the rate of growth of the food fishes, per se, but also with 

 respect to other subjects relating to their natural history and to certain 

 fishery problems. Among these may be mentioned seasonal migrations, 

 the distribution of the young and of the older generations, the influence 

 of temperature and of locality on growth, the determination of the size 

 and age at maturity, and the natural mortality, or death-rate, in a given 

 generation of the different species. With respect to the impoverishment 

 of certain grounds by overfishing, it is also obviously a matter of import- 

 ance to have an ample series of accurate measurements of the fish 

 frequsnting the grounds at different seasons in different years, since one 

 of the first signs of decrease in the abundance of fish is a diminution in 

 their average size, the larger and older individuals being removed in 

 greater proportion than the smaller. This has happened, for example, 

 with the plaice at the Dogger Bank and at Iceland, and I am informed by 

 skippers of trawlers that they do not now get so many large witches, or 

 pole-dabs, and megrims on the north-eastern grounds, off the Shetlands, as 

 Avhen these grounds began to be first trawled over, some five or six years 

 ago. Moreover, the average size of a species may normally differ in 

 different localities, and in different parts of the same sea, as the North 

 Sea (examples of which are given in the present paper), and this difference 

 in average size may not be due to difference in the seasonal temperature. 

 I think it probable, therefore, that a series of similar observations made 

 in different regions, e.g., of the North Sea, would furnish much useful 

 information on the questions above referred to, and it may be desirable 

 to describe in some detail the methods and treatment employed in the 

 course of the enquiry. 



Methods. 



The first requisite is to get a sufficiently large supply of fish at the 

 same time and place, and for this purpose the use of a large trawl is 

 necessary, as well as special nets. The otter-trawl of an ordinary steam 

 trawler is well adapted for the work, but owing to the smallest meshes, 



