APPENDIX H': REDEKE 



— 10 — 



SO that they can scarcely account for the great periodic changes in the distribution of 

 the plaice. 



It is different with the temperature of the water. We certainly cannot detect the 

 causal relation without experiments, but we can certainly recognize a parallelism between 

 the annual changes of temperatui-e and the periodic migrations of the plaice. 



As already mentioned above, the eggs develop in the central part of the North Sea, 

 that is, the warmest and also distinctly the region of greatest salinity in winter and 

 spring. 



As they travel northwards, the temperature of the water gradually rises, so that in 

 April and May the coastal region and the central part of the North Sea have almost the 

 same temperature. From this time onwards, the temperature of the coastal region and 

 the Wattenmeer rises quickly under the influence of the warmth from the mainland, until 

 in midsummer it may mount to over 20°, whilst the central part of the North Sea 

 remains relatively cool and only gradually reaches its maximum-temperature of 16° in 

 August and September. 



We thus see, that as soon as the temperature of the coastal water begins to rise 

 over about 15°, the plaice in their second and third year travel seawards into the deeper 

 and cooler water. It appears as if the warm coastal water did not suit them. The plaice 

 is not the only fish which shows such a so to speak "dislike to heat". The flounder 

 does the same, as they are known to leave the warm littoral region in midsummer and 

 migrate far out to sea. 



In autumn, the temperature in the neighbourhood of the coast and in the Watten- 

 meer rapidly falls, whilst the deep sea still retains its relatively high temperature far into 

 October. It is in January and February, that it first reaches its lowest temperature and 

 this is the time when the larger plaice frequent, as is known, the deepest parts where the 

 water is always some degrees warmer. 



Concluding That we have been able to reach the results described above, has only been possible, 



I believe, because we have fished as uniformly as possible on fixed stations, and have 

 endeavoured constantly to compare the changes in the population of the bottom which 

 appear in the course of the year. 



As we chose the stations so that the majority are in the centre of the principal 

 fishing-grounds of our region, whilst others are near the coast where the fishery for prac- 

 tical reasons is not carried on or only exceptionally, and as we have also taken the 

 Wattenmeer into consideration, this small report may contain, even as it is, a not un- 

 important material to assist in the preliminary orientation of the occurrence of the plaice 

 in the southern North Sea and the fisheries questions regarding Holland which stand in 

 connection therewith. 



Our results will only be able to attain real value, however, if and when they are 

 shown to be in agreement with the observations of our neighbours who begin their 

 work where we leave off. 



