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8 . The reduction of the stock of plaice in the southern Horns Reef Area through intensive trawling in later years.

By

A age J. i. J e n s e n .

TTow far the stock of plaice has been reduced on tion between the 10 m. and the 20 m. contours as J-J- the nursing grounds in the southern Horns Reef these stations are evenly distributed, more or less, Area m a y be studied by examination of the results of over this area1). quantitative hauls made by “Kommissionen for Dan­ The experiments of 1934, 35 and 36 were made in marks Fiskeri- og Havundersøgelser”. In the following early June, when the stocks reached the 25—30 m. review the reduction of 1934—36 has been compared contour. The total density has been calculated by to that of 1925—27. means of catches within each zone of depths, viz. less The experimental hauls have been made by the than 10 m., 10—20 m. and 20—30 m.; these means 50-feet Ottertrawl with a fine-mesh cod-end ; the hauls have been added together after weighting by 7 3-l-2 to were of 30 min. duration (in 1934—36) at stations account for the respective size of zone. — Experiments mentioned in Tab. 1, — in 1925—27 some stations at less than 10 m. depth are rarely numerous, also in were fished for 15 min. — some for 60 min. Until 1935, some years they are less reliable on account of rough the hauls were made from the “Dana”, during 1936 weather. Comparisons of means, therefore, have been from the “Biologen”. Hauls were made from 7—9 m. made below between 10—30 m. areas as well as 5— depth as far out as the youngfish-stocks were present. 30 m. areas. The 1925 and 1927 experiments were made on April The quantities of plaice within each age-group in 2nd to 4th and March 28—29th respectively, when 1925—27 have been published in the paper cited above, the stocks reached the 20 m. and the 13 m. contours1). for 1934—36 they are given in table 1 (10—30 m. area). For purpose of quantitative comparison the means *) Medd. f. Komm. f. Havundersøgelser. Ser. Fiskeri. have been calculated of numbers caught at each sta­ Bd. VIII Nr. 6, 1928.

Table 1. Number of Plaice caught per hour in the Southern Horns Reef Area, 10—30 m zones (calculated with due regard to the size of the depth-zones). — In brackets: year-classes. No. of I Stations II III IV V VI VII ...... 14 309 187 139 121 137 236 3 (1933) (1932) (1931) (1930) (1929) (1928) (1927) — 1935 ...... 14 286 290 78 40 20 16 24 (1934) (1933) (1932) (1931) (1930) (1929) (1928) — 1936...... 14 236 240 94 16 6 0 4 (1935) (1934) (1933) (1932) (1931) (1930) (1929) — 41 — (IH, 8)

Table 2 shows the 2-year reduction 1925—27 and in growth rate seems to be so small that it can hardly 1934—36 for each year-class. It will be noticed that influence either the distribution as to depth-zones or the 1924 year-class was more than 5-doubled from the emigration. The increase in length from one age- 1925 (as I-Gr.) to 1927 (III-Gr.), agreeing with the group to the next one found from the fishing experi­ fact that the I-Gr. exists mainly at, depths of less than ments, however, does not show the growth, especially 10 m. While at this time of the year the III-Gr. when considering the enormous reduction of stocks preferably occurs at 10 to 20 m. depths. during the later years ; but marking experiments made Correspondingly, the 1923-class is almost doubled by Dr. Tåning in 1934 show that the growth of plaice by passing from the II-Gr. stage in 1925 to the IV- of initial length 25—28 cm. was about 2 cm. only, Gr.-stage in 1927. 90 % of the plaice which in 1925 from April to October of that year. belonged to the III-Gr. still remained in 1927 at the The intensity of trawling on the nursery-grounds V-Gr. stage, a reduction of 10 % by fishing and for small plaice and sole is the obvious cause of the emigration. Similar causes, but emigration in parti­ enormous reduction. In 1925—27, the Horns Reef cular, brought about a reduction to some 12 % of area was left to be worked by Danish Fishermen with the I V-Gr. of 1925 to 1927 (VI-Gr.). the saving Danish seine and thereby subject to a size-

Table 2. Percentage of the various year-classes present in the Southern Horns Reef Area after two years. From I- From II- From III- From IV- to III- to IV- to V- to VI- Group Group Group Group From the spring of 1925 to the spring of 1927 (zone 10—20m.).... 5481) 185 91 12 From June 1934 to (zones 10—30 m .)...... 30 9 4 0 1) This high figure is partly due to the fact that the I-Gr. is badly represented in catches so early in the year at depths of more than 10 m.

3. Percentage of the various year-classes present in the Southern Horns Reef Area after one year. F’rom From From From V + From I- II- to III- to IV- to VI- to to II- V- VI+VII- Group III- IV- Group Group Group Groups zones 10—30 m. June 1934—June 1935 ...... 94 40 29 17 11 June 1935—June 1936 ...... 84 32 21 15 11 zones 5—30 m. June 1934—June 1935 ...... 74 33 27 17 11 June 1935—June 1936 ...... in 38 19 14 11

A distinctly different picture is painted by corre­ limit of then 22-5—25-7 cm. — The number of plaice sponding figures in 1934—1936. The I-Gr. of 1934 is was then reduced by 10 % only from the III-Gr. — reduced to 30% as III-Gr. in 1936 (1925/27: 548%, to the V-Gr. stages, a percentage which has to-day see footnote to Tab. 2), the II-Gr. to 9 % (as against increased to 95 %. 185 %), the III-Gr. to 4 % (as against 91 %) whilst the IV-Gr. of 1934 in 1936 as VI-Gr. is practically extinct. Table 3 shows the 1-year reductions 1934—35 and Table 4. Average length of age-groups II 1935—36; it will be noticed that these reductions are and III in the Southern Horns Reef Area very similar, that of the latter period slightly greater in 1925, 1927 and 1934— 1936 (corrected for though than the former one. The number of plaice measurement to the nearest full centimeter below). entering the year as III-group is reduced by 70 % in Depth zone „ . Group n i-Group one year and older plaice are subject to much more serious reductions. The similarity of all reductions Spring 1925 ...... 10—20 13-4 16-8 calculated in two successive years makes the results 1927 ...... 10—20 13-9 19-6 obtained seem trustworthy. This change in reduction of stock, from 1925 to June 1934 ...... 5—30 17-2 19-5 1936, is hardly due to changes in growth-rate; table — 1935 ...... 5—30 14-1 19-7 4 shows that the average length of the same age-group — 1936 ...... 5 -3 0 14-9 18-5 has increased slightly in later years but the increase