— 13 — LATER STAGES: PLEURONECTIDS 



February to a maximum in July (Fig. 7 and App. Table 12). F, and D 2 though based on 

 much smaller quantities and densities also show the regular curve with a July maximum. 

 Bi and E also show a summer maximum in September and August respectively, and a 

 winter minimum. The area D 3 gives the most diverse curve with two large maxima in 

 May and June in the one case and October in the other. The catch falls off to zero in 

 January and reaches as high as 90 kilos, per diem in October. In its general features 

 and in the small figures upon which it is based this curve resembles that of the megrims 

 for the same area, though without the winter increase of the latter. 



It is doubtful how far the regular succession of summer maxima, from June to 

 September, in this species and in the witch may be regarded as expressing a shoaling 

 migration of the species. 



Generally speaking, the monthly curves for these species of flat fish are of certain 

 rather regular types. They may be classified into 



(1) Simple unimodal curves illustrating a regular seasonal change. 



(2) Double or bimodal curves, with two rhythmic seasonal changes. 



The unimodal curves are mainly in relation to the extreme seasons of summer and 

 winter, and may show a summer maximum such as those of plaice in the shallow water 

 areas, and plaice, megrims, lemon soles in the deeper areas, or a winter maximum, such 

 as those of plaice in the shallow water areas, and plaice, megrims, lemon soles in the 

 deeper areas, or a winter maximum, such as those of soles and plaice in most deeper 

 water areas, and witches in the comparatively shallow water areas for this species. 



The bimodal curves are in direct or inverse relation to the equinoxes. Small plaice 

 and dabs show well marked double curves with two more or less well-defined maxima 

 or modes in spring and autumn. The two maxima may vary and may, as in some of the 

 areas for dabs, only exhibit one of them. Certain of the curves for soles show a double 

 character of an inverse nature with maxima in summer and winter. 



So far as the shallow water species are concerned, the intensity of fluctuations is, as a 

 rule, greater in the inshore areas than in those further from shore, a feature which may 

 reflect the greater variations in physical phenomena which are known to be charactertistic 

 of the former. 



The general impression to be gathered is that the more common species of Pleuro- 

 nedidae undoubtedly show well marked variations in seasonal distribution. The primary 

 causes of these variations can only be determined by a correlation with biological and 

 experimental results and with physical observations. 



An interesting report upon the Sole, (3) has recently been published by Redeke and 

 Tesch, giving a general survey of the life-history and distribution of this species in rela- 

 tion to its fishery. 



The International statistics are employed to show the relative catches of this species 

 made by the different countries bordering the North Sea. Its capture in any appreciable 

 quantity is confined to the Southern and Eastern countries, namely, England, Holland, 

 Belgium, Germany and Denmark. A varying proportion of the total catch of each country 

 is obtained from the North Sea, from about 60% by England to 100% by Holland. Of 

 the total quantity taken in the North Sea, England claims 59%, Holland 16% and the 

 other countries 15 %. These figures are based on the average for 1903 — 7. 



