Torsken i Oslofjorden - Undersøkelser utført fra 1936 til 1938 for Foreningen til fremme av fiskeriet i Oslofjorden innenfor Drøbak med støtte av Fiskeribedriftens forskningsfond
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1. From the autumn of 1936 to that of 1938 samples have been collected for the examination of trap-caught codfish in the Oslo Fjord inside of Drøbak. Trap-fishing takes place in the autumn, chiefly among a stock of young-fish in their second year, and in the spring among a more mixed stock of several year-classes which congregate for spawning. Calculation of the average lengths of the different year-classes showed that the cod in the Oslo Fjord grows well. At 1, 2, 3 and 4 years of age it is respectively 20, 33, 41 and 49 cm long. This growth compares equally well with that found in adjacent Norwegian and Danish waters where the growth of the codfish has been investigated. The codfish in the Oslo fjord spawns from January to the end of May, with a maximum spawning in February-March. Sexually mature miales occur down to a length of 26 cm. Sexually mature females down to 28 cm. 47 percent of the males and 33 percent of the females become sexually mature at an age of 2 years. All fish of 4 years and more, are sexually mature. 2. On investigation of the racial characters (number of vertebrae) of codfish from the insicle and outside of Drøbak no basis was found for determining whether the cod in the inner Oslofjord belongs to a local stock. The appearance of the otoliths indicates, however, that a major part of the codfish in the fjord inside of Drøbak have grown up there. It is possible that the cod migrate into the fjord as small young-fish, but there is no more support for such an assumption at the present moment than the presumption that the stock of fish in the fjord has both been hatched and grown up in the inner fjord. The marking experiments which have been carried out with fish exceeding 24 cm show that the cod is in a high degree stationary in the fjord. Only a few individuals have migrated from the fjord outwards past Drøbak or in the opposite direction. 3. For several years we have received statements from a trapfisherman in the fjord of the quantity fished and the gear used. With the aid of these statements and our own investigations we have calculated that from the time they are about 18 months old the cod in the Oslo fjord decline in number by 75 to 86 percent per annum, 25 percent of which must be ascribed to mortality while the remaining 50-60 percent is caught by trap-fishing. This annual reduction of the stock is 15-25 percent higher than in other Norwegian waters which have been investigated. There is therefore no doubt that the stock of codfish in the Oslo Fjord is exploited in a manner which is far from rational. 4. If we are to attempt to secure for the Oslo Fjord a better yield from the cod fishery the stock must be exploited more rationally than at the present time, and this can best be obtained by raising the size limit from that of 25 cm now in force to 30 or 35 cm. Calculations made on the basis of certain given assumptions show that a higher size limit will only cause a loss to the fisherman at the very outset when the new limit is put into effect and even during the same season it will prove profitable. In order to increase the stock of cod in the fjord we can investigate whether it will be lucrative to release fry or transplant unclersized fish from other waters to the fjord. Both these possibilities will be investigated in the future.
SeriesFiskeridirektoratets skrifter, Serie Havundersøkelser
vol 6 no 2