_ 6l — B. HELLAND-HANSEN 



are striking- and cine to finctnations in the numbers of individuals of the different 

 year-classes. After a decline, a single }'ear may bring back once more the total 

 number of individuals to a high value: and these effects can subsequently be 

 traced recurring with systematic regularity throughout the numbers of individuals 

 which constitute the different groups. Compared with these effects the differences 

 of the intensity of fishing in the different years are very small. 



Dr. C. G. J. Petersen in his well-known treatise, "What is Over-fishing?"' has 

 described four different forms of over-fishing, viz: 



1. Over-fishing of an accumulated stock. 



2. Over-fishing of the mature fish to so great an extent that they 

 cannot render a sufficient number of eggs to supply the stock with 

 yoitng fishes. 



3. Reduction of the average size of the fish to such an extent that they 

 are not sufficiently saleable ("the relative destruction of immature 

 fish"). 



4. When the smaller sizes, which have no sale-value, are living in 

 certain ver}' restricted areas close to the shore they may sometimes 

 be destroyed there in large quantities by trawling for shrimps or 

 otherwise. 



In those parts of the North Sea which the "Huxley" and "Poseidon" 

 investigated, the mature haddock and cod are migrants. They come periodically 

 from other areas and subsequently disappear. Accordingly in these parts of the 

 North Sea the first form of over-fishing may be left of account. 



Whether or not the second and third forms affect the North Sea will never 

 be properly ascertained until we have had a corresponding examination of far 

 larger areas; namely everywhere frequented by the mature fish, whose progeny 

 are found in the waters we have investigated. 



The last form of over-fishing, does not concern the haddock and probably 

 only very slightly affects the cod. In the case of these two species we have 

 however all over the North Sea a destruction of young individuals that have no 

 value (unmarketable). But whatever form of over-fishing one might be afraid of, 

 it is clear that for finally disposing of this question it will be necessary both to 

 study far larger areas than the ones treated here and to continue the investigations 

 for a very long series of years, so that the large fluctuations shall not conceal the 

 results. 



For solving the problem a studj^ of the numbers will be far more important 

 than a study of the weights. Nearly all the market statistics up to now have 

 merely given the weights either of the total quantity caught or of the different 

 large trade-groups of each species. It might be imagined that one could convert 

 these weights into numbers by taking the proportional average between the 

 numbers and weights of each group. However a conversion of this kind would 

 be extremely doubtful for several reasons. For instance a single group of haddock 

 includes individuals of such dissimilar weights that a far greater weight may 

 correspond with a given number at one time than at another. To this we must 



1 Journal of the Marine Biological Association of the United Kingdom, Vol. VI (N. S.), p. 587. 



