RAPPORTS. XIII Cl: MASTERMAN — 14 — 



Spawning Grounds. 



The question of location of spawning grounds becomes one of necessity for under- 

 standing the life cycle and distribution of fishes. A special aggregation of adult individuals 

 for the purpose of spawning tends to become the seat of the chief fishery for the species 

 as presenting the opportunity for a maximum catch of grown individuals with a minimum 

 expenditure of labour. From these grounds also must start the pelagic drift of eggs and 

 young to the nursery. 



Various methods have been proposed for the location of spawning grounds. So far 

 as the plaice is concerned, taken as a typical example of flat fish upon which a consider- 

 able body of work has been done, it was indicated in the Report of the English Board 

 that the detailed length and weight determinations could be employed for the indication 

 of spawning grounds by a study of the curves showing relation of length to weight in 

 fish of differing lengths. The depression of the curve during the spawning season not 

 only seems to indicate the locality but the duration or season of spawning, for each par- 

 ticular area. The method has not been employed for all the data, but it is probably 

 capable to extensive application. 



A further method has been suggested, namely, by the employment of general stati- 

 stics and the determination, by their means, of the seasonal occurrence of the large cate- 

 gory of the species investigated. In last year's Report it was shown that, for the whole 

 year's records, the distribution of "large" plaice differed very markedly from that of plaice 

 of all sizes. The features of the latter were determined chiefly by the preponderating 

 quantities of "small" plaice in certain areas. The area indicated (Fig. 7) as that with 

 greatest density of large plaice (36 kilos per day) lying in the southern centre of the 

 North Sea, probably covers the normal habitat of the large plaice both for spawning and 

 feeding purposes, as it reflects the characters of the whole year. 



The location of spawning grounds of the plaice has been made possible by each of 

 the three English authors whose reports are here reviewed. A Chart of the southern part 

 of the North Sea is here given with the spawning grounds of the plaice indicated in dif- 

 ferent ways, according to the results of each of the authors. Lee (black lines) identifies 

 the large areas of the Flemish Bight, the headquarters of the Lowestoft to that of Rams- 

 gate winter plaice fishery, extending from the latitude of Lowestoft to that of Ramsgate, 

 and within 15 to 20 miles of the Continental coast from Ymuiden to Ostend. This locality 

 is identified partly by direct observations by fishermen of the occurrence of spawning 

 plaice, and partly by the statistical returns. Hefford (dotted lines) defines an area not 

 dissimilar within the Flemish Bight but not reaching so far in towards the Continental 

 coast. The difference may be due to the system of areas adopted by each worker. 



In a further work of Lee upon Grimsby returns, now in the press, three northern 

 areas to the S.E. and S.W. of the Dogger are defined as spawning areas, though not so 

 markedly as those in the Southern Bight. The spawning areas as defined by Hefford in 

 this region cover much the same grounds. Both authors obtained negative evidence with 

 regard to the region between Lat. 52° 30' N. and Lat. 53° 40° N. 



Wallace obtained samples (a) from the Flamborough grounds and (b)' from the Southern 

 Bight, south of Lat. 53° N. (see Chart, square dots) finding in each case strong evidence 

 of spawning. Further evidence of spawning grounds was obtained at the extreme 



