| dc.description.abstract | 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. | |
