Omplantningene av danske rødspetter (Pleuronectes platessa) til Oslofjorden og Skagerak 1934 og 1936
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1. A short review is given on the transplantation of young plaice (Pleuronectes platessa Lin.) in Europe, from the first experiments in Denmark, about 1890, to the large transplantations in later years. From the North Sea an annual immigration of young plaice reaches the Lim Fiord and other parts of the western coast of Denmark, where it regularly results in overcrowding and in bad growth. From these localities a series of transplantations have been carried out during the period mentioned, to other coast lines of Denmark (especially the Belt Sea), where the living conditions are better. These transplantations were mostly very successful, and after the first time of trial they are now performed annually on a large scale. Later on transplantations were also carried out from the Danish west coast to Swedish and Norwegian coastal waters. This work deals with the results of two transplantations carried out from the Danish west coast to Norway, that is the Oslo Fiord and the Skager Rack during the years 1934 and 1936. The transplantation areas are shown on fig. 1, (app. VI). 2. Analyses and Markings. The quantities and distribution of the transplanted plaice are given in table 1 and 2, p. 17-18. Analyses of size, age and sex are shown on the tables 3-7, p. 19-22. Seen in relation to their age the transplantecl plaice were relatively large, especially in 1934. (Av. size 21.5 cm, av. age 2.57 years. In 1936 resp. 21.5 and 3.12). Out of 342,000 released fish 1700 were marked during the two transplantations. (Av. size of marked individuals in 1934: 23.4 cm, in 1936: 24.1 cm). Since it was found desirable to obtain information about all recaptures, fishermen were paid also for those that were undersized (less than 27 cm). 3. The Recaptures. The localities for the transplantations in the different areas are shown on fig. 2-8 (app. VII-XIII). Numbers and percentage of recaptures are shown in tables 8-10, p. 25-26. The size of the percentage is discussed. It is stated that the size of the recaptures alone is no expression of the success of the transplantation. The distribution of the recaptures in the different months after the transplantation will be more illustrating. This distribution is given in the tables 11 and 12, p. 29-30, and is also shown in the diagram, fig. 9 (app. XIV). The relatively large variations in the number of the recaptures from month to month, should be seen in relation to the variations in the fishing intensity, which again are connected with the seasonal migration of the plaice. A summary of table 11 and 12 is given in table 13, p. 31. It is later shown that the plaice did not seem to reach the fixed minimum measure for plaice (27 cm) until the end of the 7th mointh in 1934 and not until the 12th month in 1936. On this basis table 13 is constructed. The Inner Oslo Fiord shows the best recaptures (14.8 %) of plaice above the fixed minimum measure. 4. The Migrations. The transplantation areas and the migrations of the plaice are shown on the maps, fig. 1-8 (app.VI-XIII)(Arrows = males, black points = females. The figures beside indicate number of months since release). The migrations in the transplantation areas are discussed individually and totally. An attempt is made to express numerically the extent of the migration. The area around each station of release was divided into three concentric zones, A, B, and C (radii less than 5 km, between 5 and 15 km and more than 15 km respectively). Table 15, p. 39, shows the numbers and percentage of the total recaptures in each zone. It will be seen from the total figures that the majority of the plaice recaptured are caught in zone A, but the figures vary from one station to another. Otherwise, the maps show that the farthest migrations took place in a southward direction, towards Danish, and partly towards Swedish and English waters. The longest distance noted is from Kragerø (St. IV) to a locality 32 miles ESE Spurn Light, England. During 620 days this plaice (a male) migrated a distance of at least 800 km, i.e. 1.3 km per day average, possibly much more. Table 16 (app. I) summarises in a comparative manner male and female recaptures. Generally males migrate farther than females. This matter is discussed. 5. The Growth of the Plaice. The growth is shown in table 17 and 18 (app. II-III) (monthly average increase in length). By means of these tables the diagram fig. 10 (app. XV) is constructed. The curves are discussed and compared with a corresponding curve from the Danish transplantation in 1928. 6. Economical Aspects. An estimate of the cost of the transplantations is given and compared with their assumed income. The relation between length and average weight of marked and recaptured plaice is shown in table 19, p. 48, and in the diagram, fig. 11 (app. XVI), where it is compared with STRODTMANN's ideal curve. By means of the average monthly weight, the weight of the quantity of fish caught each month is calculated, as is also the total quantity (table 20 and 21, app. IV-V). The latter is also calculated in another way (right part of the two tables), based upon the supposition that all undersized plaice will be thrown back. The second method of calculation gives reduced figures when compared with the first. The benefit derived from the transplantations may also be assessed directly from analysis of the fishermen's catches, since the transplanted plaice can be distinguished from the local ones. Unfortunately few examinations of this kind have been made, but the existing ones supply good results. As far as Oslo Fiord is concerned, both sexes of the transplanted plaice have been observed to spawn there. On a whole there is reason to believe that the transplantations have given a good result, better than some of the calculations seenm to show. The Oslo Fiord area seems to be specially suitable for transplantations. The results for this area are definitely the best.
SerieFiskeridirektoratets skrifter, Serie Havundersøkelser
vol 9 no 15