67 



same length (85 — 95 m.m.), aud which no doubt were allofthem 

 produced by the spawuing of the previous suminer. On May 25 

 '88 a whüle basket of »zeobliek" only contained smal! fishes of 

 about 90 m.m. — amongst them only a couple of young herrings 

 of 55 m.m. were met with and these probably were the offspring 

 of a herring which spawned in the Northsea in winter. 



Whereas I consider the spawning in the Zuiderzee of a race 

 of spring-herrings as a well established fact, that young herrings 

 born in the Northsea should mingle with such as are natives of 

 the Zuiderzee , is only a supposition. Strongly in favour of this 

 supposition is the circumstance that, together with little Zuiderzee- 

 herrings, little sprats also are sometiraes caught in the kuilnets : 

 as the sprat never spawns in brackish water these specimens un- 

 doubtedly must have entered that sea frora the Northsea — and 

 might not young herrings have accompanied them on this visit? 



Of course we lose sight of the young herrings which leave 

 the Zuiderzee : we are quite ignorant of their habits after they 

 have entered the Northsea. Having spent a couple of months , 

 perhaps about a year in that sea, they try to reach the Zuider- 

 zee again , to spawn in the water that has seen their birth. 

 That the herring enters the Zuiderzee there to spawn , that they 

 fulfill this function in spring aud prefer doing so in brackish 

 water, has been determined — as I have tried to demonstrate — 

 by direct observation. Ho wever, Heincke's researches on the her- 

 ring-races of the Baltic having opened quite a new way for those, 

 who are interested in the study of this fish , I was anxious to 

 ascertain whether application of his method would yield for the 

 Zuiderzee-herrings a result as decisive as he arrived at for those 

 of the Baltic. My researches in this regard were not unsuecessful, 

 and therefore I beg leave to occupy a little space on their des- 

 cription. 



Heincke has succeeded in establishing the differences between 

 the two races of herrings of the Baltic ; the oue which spawns 

 in spring in brackish water, the other which has great resem- 

 blance with the Northsea-herring, aud spawns in autumn in water 



