| dc.description.abstract | 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. | |
