Joint Meeting of the Baltic-Belt Seas and Tlie
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1. Danish Investigations on the Stocks of Cod, Plaice, Flounder, and Dab in the Central Baltic and the Fishery for these Species in the Western Baltic By A a g e J . C. J ensen I. The Danish Fishing Methods Fishing for cod and flatfish is done mostly by cut flounder, and plaice in the western Baltic and along ters of 5 to 35 gross tonnage. Those of 5 to 10 gross the Baltic shores of Seeland, Møen, and Falster, and tonnage have motors of about 25 to 35 h.p., and those cod are also taken in traps, but is is not possible to of 15 to 35 gross tonnage, motors of 60 to 70 h.p. A ascertain from the Danish statistics the quantities few have engines of 100 to 150 h.p. and power is landed by the different methods. gradually being increased. Between 20 and 60 gross The trawls are made almost exclusively of cotton tonnage (t) the length in metres is approximately twine, and the cod-ends are frequently made of nylon. 8.5 m + 0.2 X t. The length of the groundrope varies between 60 to 80 The cutters are mostly used for trawling but the feet between the dan lenos. In recent years nearly all majority of the small ones in the Bornholm region the trawls used in the central Baltic have been fitted fish with long-lines during the summer when the cod with rollers (bobbins) of 10 to 15 cm in diameter, move too quickly to be taken by the trawl. Some of and 1 litre metal floats are increasingly being used. the larger trawlers in the central Baltic also use long- Most of the trawls used are cod trawls which also take lines for cod in season, as well as for flounder on the flatfish. Plaice trawls or flatfish trawls on the other Middle Bank. Hand-lines for cod are used from boats hand, generally take only a few cod and these are and also from a few cutters up to about 15 B.R.T., used locally in the western Baltic but very seldom at while the Danish seine is used, chiefly for flatfish, Bornholm. The mesh size (knot to knot in mm) is as from a few cutters in the western Baltic and in the follows:— Møen region. Net fishing is carried on near the coasts God trawl Plaice trawl Cotton twine for flatfish. W ings................... 60-80 60 No. (12/15)-12/18 Pound nets take a considerable amount of cod, Cod-end............... 40-45 45-50 No. 12/45 II. The Cod (a) Spawning A‘ Biol°^ (b) The growth-rate Observations in recent years confirm that the spawn The growth-rate is shown in Figure 1. It fluc ing for cod in the central Baltic is from April to tuates from year to year but on the average it does August, but a few fish may spawn earlier or later not seem to have varied very much during the last 20 than this. The spawning place at Bornholm is mainly years. the Bornholm Deep, but spawning takes place also to the west of the island, and it has been found that in (c) Migrations and interchanges of stocks some years in the Bornholm Deep the spawning may The stock of cod in the western Baltic forms a part be hampered by the low oxygen content. Spawning of the Belt Sea stock, and is renewed by eggs and begins when the cod has reached a length of about larvae carried in by the currents from the Belts each 30 cm. year. Yet the cod which migrate into the area will 15 cm earlier years (Strubberg, 1922) showed only a slight _1926 o tendency for the younger cod to migrate towards the 1931-38 + Baltic proper. J950-56 - A certain number of eggs and larvae must be brought into the Baltic proper by the currents, as con siderable numbers are sometimes found at the en trance to the Fehmambelt as shown by Kandier (1952), and an exchange of adult cod between the Baltic proper and the western Baltic, Belts, and Kattegat has been shown by recent Danish tagging experiments. The stock in the Møen region must be influenced to an appreciable degree by these migrations. The stock in the central and eastern parts of the Baltic I • HI • 3! • proper must, on the other hand, be mainly independent. Danish tagging experiments in recent years have Figure 1. Length of the age-groups of cod in shown that cod from the central Baltic may migrate the Bornholm region. to the Belt Sea, but the recoveries from that region have been very few in proportion to those re mostly stay there or in the Little Belt, for recent covered in the Baltic proper. Also, the occurrence of tagging experiments in the western Baltic and the a high percentage of bullhead cod of the 1950 year- southern Little Belt have shown that nearly all cod class, which were observed at Bornholm in 1951 and irrespective of their size are recaught in the same the following years (Aa. J. C. Jensen, 1952, 1953), waters in which they were tagged. but not found in the western Baltic, the Belts, or the In the eight samples from Bornholm and the Baltic Kattegat, confirms that almost no cod migrate be proper, east and north-east of Bornholm, investigated tween the central Baltic and the above named areas. by Johs. Schmidt (1929), the mean number of fin Besides the regular migrations between the spawn rays in the second dorsal fin (D2) was between 17.2 ing grounds in the deeper parts of the sea and the and 17.9, but in samples from Møen, the western feeding grounds at lesser depths, irregular migra Baltic, the Belts, and the southern Kattegat it was be tions due to hydrographical factors can influence the tween 18.5 and 18.6. A sample from southern Scania fishery. In some years, for instance, fishing at Born showed a count of 18.2, intermediate between the holm is mostly done to the west of the island or on means for the two regions. Poulsen (1931) also found the Middle Bank, when the oxygen content in the a mean count of 17.4 fin rays at Bornholm and 18.4 Bornholm Deep is low, and differences in the quality to 18.6 in the western Baltic and the Belt Sea, and of the cod may sometimes occur rapidly, indicating 18.7 in the Kattegat. It is evident that the racial char irregular migrations. The recent Danish tagging ex acters of the cod at Bornholm and further to the east periments have shown that irregular long-distance and north-east are different from those of the cod in migrations in the central Baltic are frequent, and the Belt Sea and Kattegat, but that intermixing be Alander (1948) has also found that such migrations tween the races may take place in the western part of in the Baltic may occur. They have been further con the Baltic proper. Poulsen also found that the mean firmed by the tagging experiments of Mulicki and number of fin rays in a sample from the southern Otterlind. Little Belt increased from 18.3 (22,) in the II-group to 18.7 (67) in the IV-group, and from 17.15 (39) in the II-group to 17.6 (47) in the III- to IV-groups in (d) Fluctuations in the size of the stock one of the samples from the Bornholm region, and These have been mentioned in a previous paper (Aa. concluded that this was due to an immigration into J. C. Jensen, 1954). The stock density may have been the Baltic of the young cod. The small number of cod greater during and just after the war than in recent investigated (shown in brackets) makes the conclu years. It is said that in the 1940’s the long-lines often sion doubtful, but so far it seems to have been con had a cod on each hook, but today the number caught firmed by a Danish tagging experiment in the Sound is considerably less. Trawls have been improved and where one or two cod have been recaptured at the the horse-power increased but on board the trawlers entrance to the Baltic proper. On the other hand, there are also signs of a decline. none of the many recaptures from the recent Danish In the Bornholm region the size of the stock fluc taggings of cod in the Belt Sea have been taken in tuates very much from one year to another, due to the the Baltic proper, and the similar experiments in irregular migrations, but the causes of the long-period 16 no. cm. 15 25 35 45 55 1 1 1 1 1 1 1 1 1 1 1 ' 100 1949 1923 i f 200 1925-28 " W/ " 1 Æ .7 7 , 100 1950 ~ 7777 1931 -39 _ n=qq_cca=L ‘ 1 ..............'.....................................1......1 1----- 1----- 1----- 100 1951 1949-50 - V///Å __ 100 1952 - Tim 1951-56 mnn][mnn I i m ij 11 m 11111 |ITO^TTTTTTr|.. | [ CM: 20 30 40 50 60 1953 Figure 2. Percentage length composition (5 cm groups) of catches taken in the Bornholm region from July to Septem ber by Danish research vessels. In 1923 and in 1951 to 1956 by trawl and in the other years by eel-tog. 200 changes are shown to be mainly hydrographical, al 100 ! ! 1954 though fishing must also have some influence (see Section B 3).