RAPPORTS. XLII Ci: MASÏERMAN — 12 _ 



Henk-ing on Market Statistics, from which that author provisionally inferred an offshore 

 migration of small females beginning in August. 



More observations on a more extensive scale and of a quantitative character will be 

 $eqüifed ^before these results can be definitely accepted. 



This report of itself .serves, to illustrate the extremely complex problems which are 

 involved in the migratory, movements of a species of fish. It may be attempted to give 

 a' certain amount of definiteness to the Subject by consideration of the different kinds of 

 ■movement which may be expected from the known facts. The life of the fish may be 

 said to date from the pelagic stage hatched at or near the spawning area, and in all 

 pelagic spawning types there must be some amount of passive movement or drift to the 

 nursery, which, . in the case, of the plaice, is thé littoral region. This nursery represents 

 the centre of distribution of the species, so far as the sea-bottom is concerned. In last 

 year's' report' it was shown, by the general statistics for the North Sea, that at least for 

 each of the shallow-water species plaice, .sole, turbot, brill and flounder, the nursery 

 constituted the area in which was present the densest mass of the species, as judged by 

 ■weight per unit of capturing power, and that from this central area the species gradually 

 spreads each season into neighbouring . grounds. In the case of the plaice this spreading 

 has been shown to be in direct relation to the size of the individual, and the general 

 movement may therefore be termed a growth-movement. The actual stimulus for this 

 movement on the part of each individual is not known. In any case it is symptomatic 

 in its character, and will always be the most marked in types like the plaice, in which 

 the nursery is far:removed, physically and geographically, from the spawning ground. A 

 second type of growth-movement is periodic in its character, being dependant on seasonal 

 changes., In the plaice such periodic movements are found to take place in the movements 

 of young plaice inshore in spring and autumn and offshore during the summer. 



At the attainment of maturity, the second set of movements or true migrations com- 

 mence. These are' definitely connected with reproduction, and are usually described as 

 spawning migrations. . In cases where the spawning and feeding grounds coincide there 

 :will be ilittle or no spawning migration^ and it will be most marked where the two are 

 ■widely apart. In the case of the plaice, in which the growth movement is determined by 

 size,')the spawning migration will be accentuated in proportion to the small size at which 

 niaturity is attained,, and hence will be greater in the male sex than in the female. 



It is possible to conceive of a species with little or no spawning migration. The 

 growth-movement would proceed until the average-size-at-first-maturity were attained, 

 •and spawning would then take place zn sitti. . The spawning area would then be a zone 

 of grounds to , which the fish of a certain size had reached in their growth-movements 

 seawards. In ensuing years fresh zones would be produced seawards for spawning fish 

 of a larger size, or these individuals might acquire a spawning-migration (reversing the 

 growth movement) returning at the spawning season to the spawning zone of the smallest 

 mature -fish. There are indications that the plaice may, so far as the female is concerned, con- 

 form to some extent to this type though concentration upon certain grounds undoubtedly 

 takes place. How far the grounds are selected in relation to the pelagic drift remains to 

 ibe shown. 



I . in both the growth-movements and the spawning migrations the general cyclic or 

 periodic movements will be to an unknown, extent masked by proximate movements of 



