Some interested people in the fisheries are only too ready to accept 

 the idea that the intense catching of fish from a certain limited area 

 would have no more effect than the taking of many pails of water from 

 the ocean o They think that the fish are spread equally over the sea 

 and therefore those that are caught will be replenished from the immense 

 oceans e 



This idea is absolutely wrong. As on the land, there are many 

 obstacles in the sea, generally of a hydrographic nature, which block 

 the free distribution of fish. For instance, in general, the different 

 fish species are related to water masses in which the temperature and 

 salinity remain within specific limits, and many species, especially those 

 which remain near the bottom, cannot occur in those areas where the 

 depths are greater than certain values, or where the character of the 

 bottom has changed. The right living conditions for a species may 

 appear in different places but often these places are divided from each 

 other by uninhabitable areas for that species. If this is the case, 

 then the individuals which inhabit one area will have no contact with 

 their like species in another area« They will form isolated populations' 

 which will have to reproduce by their own egg production and will not 

 be reinforced from other populations. 



Such isolated populations can be recognized in several ways. 

 Usually, small differences such as the number of vertebrae, the nvimber 

 of fin rays, the shape of the head, etc. are sufficient to designate 

 these populations as different races. A second possibility for differen- 

 tiation is given by the strength of the year classes, i.e., the number 

 of fish which yearly develop successfully from the roe. The reasons for 

 fluctuations of year-class strength are not yet sufficiently well known. 

 One assumption is that generally they are not brought about by great 

 fluctuations in the egg production, but by fluctuations in the mortality 

 among the hatched-fish larvae. This is probably the result of hydro- 

 graphic variations, which may form a situation where one year the larvae 

 would, and in another year would not be transported to a section in the 

 living area where sufficient nutrients may be found. Since it is 

 entirely possible that during the same year the hydrographic condition 

 would be good for one living area but bad for another, the picture of 

 the annual fluctuations in the mortality of the year classes often will 

 be different for one population than for another isolated group. 

 Finally, by studjdng the migrations of tagged fish, one can determine 

 if certain boundaries are not being passed. 



The North Sea from Dover Straits to 61° North Latitude and the 

 Skagerak is such a limited area for the plaice, sole, haddock, cod 

 and other economically less -important, bottom-fish species contained 

 in those waters. 



The Icelandic Waters and the Northeast Atlantic area of the 

 Norwegian Coast, the Barents Sea, Bear Island and Spitsbergen must also 

 be considered as areas with self-sustaining fish populations. 



