5 Plaice, turbot, dab, and brill in the Baltic

Transcription

1 ICES WGBFAS REPORT Plaice, turbot, dab, and brill in the Baltic The landing data of plaice, turbot, dab, and brill according to ICES Subdivisions and countries are presented in Tables The trend and the amount of the landings of these flatfishes are shown in Figure 5.5. Plaice and dab have the greatest proportions of the total landings of flatfish when excluding flounder. The highest total landings of plaice were observed at the end of the seventies (8289t in 1979) and the lowest around the year 1990 ( t). Since 1995 the landings increased again and reached a moderate temporal maximum in 2002 (2763t). After then the landings decreased to 1860t in In 2011, the landings of 1936t were on the same level as 2010 (1948t). The fluctuations are supposed to be caused mainly by immigration of plaice from the Kattegat into the western Baltic Sea. ICES Subdivision 22 is the main fishing area, and Denmark is the main fishing country. Subdivision 25 is the second most important fishing area where Poland and Denmark are the main fishing countries. In SD24-28, the plaice stock shows some positive trend. Stock trends from the Baltic International Trawl Survey (BITS) are shown for plaice in Figure 5.1. In the Baltic Sea three different stocks of plaice could be identified (WKFLABA, ICES 2010c). One stock in Belt Sea SD W, one in the Öresund SD 23 and one stock in Arkona, Bornholm, Gdansk and Eastern Gotland basins (Figure 5.6). WKPESTO (ICES 2012a) suggested for analytical assessment a separate stock constituted by Kattegat (SD 21), the Belts (SD22) and the Sound (SD 23), but this suggestion had not been finally approved by ACOM when WGBFAS met. Turbot occurs mainly in the southern and western parts of the Baltic Proper. Therefore, most of the landings are reported from ICES Subdivisions The total landings reported between 1965 and 1996 of turbot increased from 42t to 1210t. From that high level the landings decreased to about 500t in the 2000s. The total landings in 2011 of about 316t mean an increase by 20t compared to that of 2010 (285t). A successful turbot gillnet fishery started at the beginning of the 1990s in Subdivisions 26 and 28. This development was caused by fishermen having more interest in turbot. Since 1990 in all eastern Baltic countries turbot was sorted out from the flatfish catches due to the better price. For example, the Polish landings of turbot increased from 33t to 360t from 1999 to Swedish landings are taken mainly from a gillnet fishery that reached a maximum of 250t in Since then landings decreased and have been under 50t for the last five years. Denmark and Germany landed turbot from Subdivisions 22 and 24. Since 2000 these landings have decreased notably. The German landings in the last year were about 56t and remained at the same level as 2010 (57t). According to the reported landings, the turbot stock in the Baltic Sea has decreased since the mid 1990s. Due to the low stock level, fishery targeting turbot was totally closed for some years in the EEZ of Latvia and Lithuania. Available age data were compared during WKFLABA meeting. Results using sliced otoliths were remarkable better than using whole otoliths. These two ageing methods showed significantly different results. Applying the new method, the fishing mortality estimate declined by a factor of about two. WKFLABA did not make suggestions for turbot stocks in the Baltic Sea. Genetic information did not show any stock structure while tagging data indicated the existence of small local stocks. Fur-

2 260 ICES WGBFAS REPORT 2012 ther investigations, especially in the Eastern part of Baltic Sea are recommended. Stock trends from the Baltic International Trawl Survey (BITS) are shown for turbot in Figure 5.2. The total landings of dab increased to 1894t in Then they decreased to 1230t in In 2011 the landings of about 1270t show an increase by 230t compared to the previous year (1040t). During the years 1994 to 1996 the total landings of dab were over-reported due to by-catch misreporting in cod fishery. The highest landings were observed in Subdivision 22. The main dab landings are reported by Denmark (Subdivision 22 and 24) and Germany (mainly in Subdivision 22). The German landings of dab are mostly by-catches of the directed cod fishery. In 2003 a trawl fishery targeted to dab was started in Subdivision 22. The recent years increase in landings is due to an increase in the stock as also evidenced by the BITS survey data (Fig. 5.3). Dab occurs mainly in the western part of the Baltic Sea. According to WKFLABA conclusions, dab forms three stocks. One stock is identified in the Belt Sea (SD W), one in the Öresund (SD 23) and one in Arkona and Bornholm basins (SD 24E + 25). It is unclear where the border in SD 24 has to be drawn. Possibly, the Öresund stock should be merged with the Belt Sea stock. Hence, for dab 3 and not 2 (Temming 1989, Nissling et al. 2002) stocks are proposed in the Baltic (Figure 5.7). Stock trends from the Baltic International Trawl Survey (BITS) are shown for dab in Figure 5.3. The brill fishery is carried out mainly by Denmark (about % in the period ) and in Subdivision 22. In the period from 1970 to 2005 the total reported landings varied between 1 and 160t. It can be assumed that the total landings of brill reported for are overestimated due to species-misreporting in the landings of the directed cod fishery. Excluding these years, the landing averaged to about 25t between 1994 and A moderate increase of reported total landings was observed from 2001 (19t) to 2006 (56t). Since 2006 the reported landings varied between 56t and 105t. In 2010, the reported landing amounted to 57t, a decrease compared to the 82t of the previous year. WKFLABA did not find any data concerning genetic or tagging that could be used to illuminate the stock structure of brill in the Baltic, hence no suggestions for possible assessment units based on biological information were given. Stock trends from the Baltic International Trawl Survey (BITS) are shown for brill in Figure 5.4.

3 ICES WGBFAS REPORT Table 5.1. Plaice in the Baltic Sea: total landings (tons) by ICES Sub-division and country. (There are some gaps in the information, therefore "Total" is preliminary) Denmark Germ. Dem. Rep. 1 Germany, FRG Poland Sweden 2 Finland /SD (+25) (+25) 28 25(+24) continued

4 262 ICES WGBFAS REPORT 2012 Table 5.1 continued Total by SD Total SD From October-December 1990 landings of Germany, Fed. Rep. are included. 2 For the years and 1990 the Swedish catches of Sub-divisions are included in Sub-division For the years and 1990 catches of Sub-divisions are included in Sub-division Preliminary data 5 Danish catches in 2002 in SW Baltic were separated according to Sub-divisions 24 and 25

5 ICES WGBFAS REPORT Table 5.2. Turbot in the Baltic Sea: total landings (tons) by ICES Sub-division and country. (There are some gaps in the information, therefore "Total" is preliminary) /SD Denmark erm. Dem. Rep Germany, FRG Poland Sweden 2 Latvia Lithuania Russia Finland Estonia ( (+24) ( continued

6 264 ICES WGBFAS REPORT 2012 Table 5.2 continued Total by SD Total (+29) SD From October-December 1990 landings of Germany, Fed. Rep. are included 2 For the years and 1990 the catches of Sub-divisions are included in Sub-division 24 3 For the years and 1990 the Swedish catches of Subdivisions are included in Sub-division 24 4 Preliminary data Danish catches in in SW Baltic were separated according to Sub-divisions 24 and 25 In 2005 Lithuanian landings are reported for 1995 onwards

7 ICES WGBFAS REPORT Table 5.3. Dab in the Baltic Sea: total landings (tons) of by Sub-division and country. (There are some gaps in the information, therefore "Total" is preliminary) /SD Denmark Ger. Dem. Rep. 1 Germany, FRG Sweden 2 Total Total (+25) SD From October-December 1990 landings of Germany, Fed. Rep. are included. 2 For the years and 1990 the catches of Sub-divisions are included in Sub-division For the years and 1990 the Swedish catches of Sub-divisions are included in Sub-division Preliminary data. 5 In 1995 Danish landings of Sub-divisions are included.

8 266 ICES WGBFAS REPORT 2012 Table 5.4. Brill in the Baltic Sea: total landings (tons) by Sub-division and country (There are some gaps in the information, therefore "Total" is preliminary) Denmark Germany, FRG Sweden Total Total SD Preliminary data

9 ICES WGBFAS REPORT Number per Hour SD22-1 SD Number per Hour SD SD Figure 5.1 Plaice in Subdivisions (Baltic Sea). Catch per unit of effort (number/hour) of fish 20 cm from Q1 and Q4 BITS survey SD22 (upper) and SD24-28 (lower), from ICES DATRAS database. Averages from all (incl. 0 catch) daytime hauls standardised to gear TVL (1.4 * TVS) and weighted by depth stratum area (stratum 8: area 10-19m only). SDs are not sampled representatively by BITS and were therefore excluded from the calculations.

10 268 ICES WGBFAS REPORT SD SD Number per Hour SD SD Number per Hour Figure 5.2 Turbot in Subdivisions (Baltic Sea). Catch per unit of effort (number/hour) of fish 20 cm from Q1 and Q4 BITS survey SD22-SD24 (upper) and SD25-28 (lower), from ICES DATRAS database. Averages from all (incl. 0 catch) daytime hauls standardised to gear TVL (1.4 * TVS) and weighted by depth stratum area (stratum 8: area 10-19m only).

11 ICES WGBFAS REPORT Number per Hour SD SD Figure 5.3 Dab in Subdivisions (Baltic Sea). Catch per unit of effort (number/hour) of fish 20 cm from Q1 and Q4 BITS survey SD22-SD24, from ICES DATRAS database. Averages from all (incl. 0 catch) daytime hauls standardised to gear TVL (1.4 * TVS) and weighted by depth stratum area (stratum 8: area 10-19m only). 3,5 3 SD SD Number per Hour 2,5 2 1,5 1 0, Figure 5.4 Brill in Subdivisions (Baltic Sea). Catch per unit of effort (number/hour) of fish 20 cm from Q1 and Q4 BITS survey SD22-SD24, from ICES DATRAS database. Averages from all (incl. 0 catch) daytime hauls standardised to gear TVL (1.4 * TVS) and weighted by depth stratum area (stratum 8: area 10-19m only).

12 270 ICES WGBFAS REPORT Landings (metric tonnes) plaice turbot dab brill Figure 5.5. Landings of brill, dab, turbot, and plaice in ICES Sub-divisions ( )

13 ICES WGBFAS REPORT Figure.5.6. Approximate location of three identified stocks of plaice in the Baltic Sea. Numbers within circles refers to ICES SD.

14 272 ICES WGBFAS REPORT 2012 Figure 5.7. Approximate location of three identified stocks of dab in the Baltic Sea. Numbers within circles refers to ICES SD.