Flounder in SD 24-25

ICES WGBFAS REPORT 2011 575

Annex 7 – Stock Annex – Flounder in SD 24-25

Quality Handbook ANNEX:_Flounder in SD 24-25 Stock specific documentation of standard assessment procedures used by ICES. Stock: Flounder SD 24 and 25……. Working Group: WGBFAS Date: 19.04.2011

A General

A.1 Stock definition The stock structure of the flounder in the Baltic Sea is not yet clear. Stock identifications differ between studies relying on migration patterns (Aro 1989), spawning be- haviour (Nissling et al. 2000), or microsatellite analyses (Florin and Höglund, in press). Migration studies indicate that there are several rather distinct flounder stocks (populations). Flounder is regularly distributed in all parts of the Baltic Sea, except in the northern Bothnian Bay, the eastern-most part of the Gulf of Finland and the deepest areas of the Gotland Deep. According to Aro’s migration studies (Aro 1989), there are at least three stocks in the southern Baltic (ICES Subdivisions 22-26), three in the central and north eastern Baltic (ICES Subdivisions 27-28), one in the Åland Sea, one in the Archipelago and southern Bothnian Sea, and two in the Gulf of Finland. The migrations between the mature flounder stocks are quite sparse (Aro 1989). The natural boundaries of the stock in the southern Baltic (ICES Subdivisions 24 and 25) may be drawn from the southern part of the Öland Island to Rozewie on the Polish coast in the east and the Darss Sill in the west. Spawning takes place in the Arkona basin, the Slupsk furrow and the Bornholm deep at a depth of 40-80 m in the period from the second half of February to the end of May. After spawning, the feeding migrations are directed to the shallow coastal areas, southward to the German and Pol- ish coasts (to the west till Ruegen island and to the east to the areas of Rozewie) as well as northward to the Swedish coast. During late autumn and early winter the spawning migration to the main spawning grounds sets on. One part of the mature stock feeding in the Arkona Sea migrates to the Bornholm basin to spawn. A genetic study (Florin and Höglund, in press) on Baltic flounder from 12 different places ranging from Åland in the northern Baltic Sea to the Danish west coast sup- ports the notion of genetically differentiated flounder stocks. However, the results indicate that rather than a high number of small stocks, three major different groups of flounder could be identified: (1) Skagerrakk/Kattegat (Subdivisions 20-21), (2) Southwest Baltic Sea (Subdivisions 22-25 and the deeper areas of 26 and 28), and (3) Eastern Baltic Sea (Subdivisions 27, 29-32 and the coastal areas of 26 and 28) (for more details, see Florin and Höglund, in press or Gårdmark et al., 2006).

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In 2010 a workshop on flounder assessment in the Baltic (WKFLABA, ICES, 2011) was held in Oeregrund, Sweden. Reviewing and discussing the above cited literature, the workshop resulted in the following flounder stock identifications: Based on egg buoyancy, WKFLABA proposes two spawning groups of flounder in the Baltic: Shallow water spawners with the eggs developing in contact to the bottom and deep water spawners with eggs developing free floating in the water column. Additional information from tagging, genetics, fecundity studies suggest 5 stocks of deep water spawners in the Baltic. Three are in line with that identified for dab and plaice, i.e SD 22, SD 23 and SD 24 + SD 25. Additionally, there are two separate stocks, one in the Gdansk (SD 26) and one in the eastern Gotland basin (mainly in SD28E but also stretching into SD 26 and 29). Approximate locations of the deep water spawning stocks are shown in Figure 4.1. These stocks contribute by about 80- 90% to the Baltic landings. For the shallow water spawning flounder, WKFLABA suggests 6 stocks (Fig 4.2.): 1. one along the Swedish coast in SD 27, 2. one around the Gotland Island (SD 27 and SD 28 West) and 3. one in Irbe Strait, including western part of Gulf of Riga and up to Hiiumaa island in Estonia (SD 28 East), 4. one in SD29and SD 30 5. two in SD 32: a. one along the Finnish coast (SD 32 North) b. one along the Estonian coast. (SD 32 South) There is some uncertainty on shallow water spawning flounder occurring in SD 25 (Otterlind 1967) and SD 26 (Florin & Höglund 2008) due to the salinity there is in fa- vour of deep water spawning. Age reading of flounder still is not consistent between countries. For example, the average length of 6 year old flounder (mean age in landings) in one and the same Subdivision varies from 23 to 32 cm in 2009. This may illustrate the differences in age reading and expertise level of age readers. According to the flounder age reading workshop (WKARFLO,), WKFLABA confirmed to use only improved ageing meth- odology (sliced or broken and burned as an alternative). WKFLABA found indications that staging of maturity could be a potential problem in further stock assessment. Due to different maturity staging results there are prob- lems with determination of spawning stock size. WKFLABA recommended to WGBFAS: 1 ) not to perform classical age based stock assessments for flatfish stocks in Baltic Sea in WGBFAS 2011 2 ) to use biological populations in future assessment if possible.

A.2 Fishery On average (1973-2002, misreporting years 1994-1996 excluded) 50% of the total flounder landings of the Baltic Sea (13 000 t in the same period) were taken from ICES Subdivisions 24 and 25. About 45% of the landings from the stock come from the ICES WGBFAS REPORT 2011 577

Subdivision 25. In the Subdivisions 24 and 25, Poland, Denmark and Germany are the main fishing nations. There is no Danish flounder directed fishery. The species is by-catch in the cod- fishery. The major season for flounder by-catch is the winter where some fishing boats may catch up to two tons per day, depending on depth and area. Most flounder are caught in the area east and southeast of Bornholm (SD 25). There is a high vari- ability between years. The amount of the flounder catch discarded depends on price for and size of the flounder. In the most recent years the price declined and therefore the amount of flounder discarded increased. German flounder landings are also mainly by-catch in the cod-directed fishery. In ICES Subdivision 24 there is a German trawl fishery directed to flounder, in particu- lar in the 3rd and in the 4th quarters. This fishery contributes a maximum of about 35 % to the total German flounder landings. In SD 24 about 85% of the landings are taken in the trawl fishery. In SD 25 virtually all German flounder landings are taken by trawl. The German flounder landings depends largely on the market situation (price and demand for flounder). In 2007, some periods of good prices for flounder were reported by the fishermen. Therefore the variation in the landing cannot be considered an indicator for the stock size. Trawl and gillnet fisheries directed to flounder exist also in Poland. Polish flounder catches increase when cod resources decrease. A share of about 60% of the Polish landings is caught by the directed flounder fishery in the Polish EEZ (SD 26 in- cluded). The fishing season spans the months January and February and June to November. IBSFC Fishery Rules concerning the flounder stock in ICES Subdivisions 24 and 25: Under the Rule 8 it is prohibited to target a fishery, to collect, land, sell, process or store flounder having, when fresh, a total length less than 25 cm in Subdivi- sions 22-25. Under the Rule 10.1 it is prohibited to use trawls, Danish seines or gillnets having a mesh opening (measured when wet) smaller than that specified for fisheries listed below: flatfish in Subdivisions 22-27 fished with gillnets, normal trawls and Danish seines – 120 mm. Additional national rules concerning flounder are in force: Till 2007, it was not allowed to land female flounders, caught in the German 12 Nm zone from 1st February to 30th April. In Poland a closed season from 15 February to 15 May is in force.

A.3 Ecosystem aspects Based on measurements of the salinity necessary for spermatozoon activation and egg buoyancy, flounder of the Baltic Sea may be separated into different populations: the pelagic spawning flounder consisting of three stocks one respectively in the Oere- sund, the Arkona- and Bornholm basin and the Gdansk- and Gotland basin and the demersal spawning flounder with one stock (Nissling et al. 2000, 2002). For the flounder stock in SD 24 and 25 the reproductive volume is defined by >=12.0 psu and >= 2 ml O2 /l. Therefore the recruitment success can fluctuate dependent on hydrological condition on the spawning grounds.

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B Data

B.1 Commercial catch Catch at age (CANUM) and mean weight at age in the catch (WECA) data were available on yearly basis from Poland, Sweden and Germany. According to the statements of WKAFAB (Gardmark et al., 2007) the commercial catch data were not used for an assessment due to discarding and age reading prob- lems. In 2007, instead of commercial catch data, the survey data from the German BITS SD 24 quarter 1 were used for an exploratory assessment (SURBA). For 2008 ACOM has decided that no exploratory assessment is to be made because of the data deficiencies. In the past, the age determination was done from whole otoliths. That age determination procedure has been stated to be uncertain (WKAFAB, ICES 2007b, WKARFLO, ICES 2007a). A second Workshop for Age Reading of Baltic Flounder (WKARFLO) was held at the end of May in 2008. The Terms of Reference of this workshop comprised also the development of a strategy to establish a reference collection of Baltic flounder otoliths and for the slicing of historical otoliths. Until having a time series of sliced otoliths, the length based SURBA will be the only method to explore the stock state. The a g e based SURBA may remain an alternative to catch based assessments, at least as long as reliable discard estimates are not available. B.2 Biological For the biological input data to the SURBA run in 2007 see WGBFAS report 2007, ICES, 2007c. B.3 Surveys In the period 1978-2000 Germany carried out a stratified fixed station bottom trawl survey in Subdivisions 24 and 25 in the 1st quarter as well as in 22 and 24 in the 4th quarter. These surveys were planned for recruitment investigations of cod. Flounder data were sampled regularly and stock indices could be estimated. The station grids and a description of the herring bottom trawl (HG20/25) used are presented by Schulz and Vaske (1988). In 1991, R/V “Eisbär” was replaced by R/V “Solea” and in 1993 the positions of the stations in SD 25 were changed. A special young fish survey on flounder has been carried out in the Oderbank area (SD 24) since 1978. This survey proved not suitable to estimate the recruits for the total stock in SD 24 and 25. It is supposed that the Oderbank stock is a local one (Westernhagen, 1970). Since 2001 the Baltic International Trawl Survey (BITS) has been carried out using a new (stratified random) design and a new standard gear (TV3). In this context Ger- many terminated the survey in SD 25 and continued with the survey only in SD 22 and 24. The TV3#520 has about the same catchability for demersal species as HG20/25 and thus a conversion factor close to 1. ICES WGBFAS REPORT 2011 579

The survey data from the German BITS SD 24 quarter 1 formed the basis for an exploratory SURBA assessment in 2007 (ICES, 2007c).

B.4 Commercial CPUE None

B.5. Other relevant data None

C Historical Stock Development

No analytical assessment on commercial catch data is possible until remarkable im- provement in basic data can be demonstrated. An exploratory SURBA on the female age equivalents in the number at length per hour data of the German BITS SD24 quarter 1 from 1992 to 2007 was made in 2007 (ICES, 2007c). In 2010 the trial stock assessment was performed during the WKFLABA (ICES, 2011) workshop. First, the XSA was applied and age structured data were extended to cover most recent years (2006-2009). The age reading was based on whole otoliths. The preliminary analysis of the input data showed similarly poor age data consistency as in former assessments for both commercial and survey data. The XSA diagnostic showed low correlation between survey and XSA estimates of stock size. The diagnostic was somewhat better when shrinkage parameter was released. In addi- tion, the difference model (to large extent age independent) was fitted, showing bio- mass trends moderately similar to XSA estimates with released shrinkage. The first tests of age data consistency, when aging is performed using thin sectioning, are promising, but more aging with this method and data analysis must be performed, until stronger conclusions on usefulness of such aging for assessment may be reached.

D Short-Term Projection

Model used: Software used: Initial stock size: Maturity: F and M before spawning: Weight at age in the stock: Weight at age in the catch: Exploitation pattern: Intermediate year assumptions: Stock recruitment model used: Procedures used for splitting projected catches:

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E. Medium-Term Projections

Model used: Software used: Initial stock size: Natural mortality: Maturity: F and M before spawning: Weight at age in the stock: Weight at age in the catch: Exploitation pattern: Intermediate year assumptions:

Stock recruitment model used:

Uncertainty models used:

1. Initial stock size: 2. Natural mortality: 3. Maturity: 4. F and M before spawning: 5. Weight at age in the stock: 6. Weight at age in the catch: 7. Exploitation pattern: 8. Intermediate year assumptions: 9. Stock recruitment model used:

F Long-Term Projections

Model used: Software used: Maturity: F and M before spawning: Weight at age in the stock: Weight at age in the catch: Exploitation pattern: Procedures used for splitting projected catches: ICES WGBFAS REPORT 2011 581

G Biological Reference Points

H Other Issues

I References

Aro, E. 1989. A review of fish migration patterns in the Baltic. Rapp.P.-v. Reun. Cons.int. Ex- plor. Mer.190: 72-96.

Florin, A.-B. & Höglund, J. Population structure in flounder (Platichthys flesus) in the Baltic Sea. In press.

Gårdmark, A., Florin, A-B. & Ångström, C. 2006. Stock structure and status of the Baltic Sea flounder (Platichtys flesus)– a summery of studies at the Swedish Board of Fisheries. Working Document (No. 1) presented at WGBFAS 2006.

Gårdmark, A. and Florin, A.-B. 2007. Flounder (Plaichtys flesus) growth and length distributions – an input to length based assessments. Working Document to the ICES Baltic Fisheries Assessment Working Group (WGBFAS), 17 – 26 April 2007, Copenhagen, Denmark.

Gårdmark, A., Florin, A.-B., Modin, J., Martinsson, J., Ångström, C., Ustups, D., Ådjers, K., Heimbrand, Y., Berth, U. 2007. Report of the Workshop on Alternative Assessment Strate- gies for Flounder (Platichtys flesus) in the Baltic Sea (WKAFAB) - an intersessional workshop supporting the ICES Baltic Fisheries Assessment Working Group (WGBFAS). 2 – 4 October 2006, Öregrund, Sweden. 29 pp.

Needle, C. L. 2004. Absolute abundance estimates and other developments in SURBA. Work- ing Document to the ICES Working Group on Methods of Fish Stock Assessment, 11-18 February 2004, Lisbon, Portugal. 25 pp.

Nissling, A., Westin, L., and Hjerne, O. 2000. Spawning succes in relation to salinity of three flatfish species, Dab (Pleuronectes limanda), Plaice (Pleuronectes platessa) and Flounder (Pleuronectes flesus) in the brackish water Baltic Sea. ICES C.M. 2000/R:05. 28 pp.

Nissling, A., Westin, L., and Hjerne, O. 2002. Reproduction succes in relation to salinity for three flatfish species in the Baltic Sea. ICES J. Mar Sc. 59(1): 93-108.

Schulz, N. and Vaske, B. 1988. Methodik, Ergebnisse und statistische Bewertung der Grundtrawlsurveys in der Mecklenburger Bucht, der Arkonasee und nördlichen Bornholmsee in den Jahren 1978-1985 sowie einige Bemerkungen zu den Jahrgangsstärken des Dorsches (Gadus morhua L.) and des Herings (Clupea harengus L.). Fischerei- Forschung 26 (3): 53-67.

Westernhagen, H. von. 1970. Erbrütung der Eier von Dorsch (Gadus morhua), Flunder(Pleuronetes flesus) und Scholle (Pleuronectes platessa) unter kombinierten Temperatur- und Salzgehaltsbedingungen. Helgoländer Wissenschaftliche Meeresuntersuchungen 21 (1+2): 21-102.

ICES 2001. Report of the Baltic Fisheries Assessment Working Group. Gdynia, 18 –27 April 2001, ICES CM 2001/ACFM:18. 546 pp.

ICES 2003. Report of the Baltic Fisheries Assessment Working Group. ICES Headquarters, 7 – 16 April 2002, ICES CM 2003/ACFM:21. 542 pp.

ICES. 2006a. Report of the Baltic Fisheries Assessment Working Group (WGBFAS), 18 – 27 April 2006, Rostock, Germany. ICES CM 2006?ACFM:24. 672 pp.

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ICES. 2007a. Report of the Workshop on Age Reading of Flounder (WKARFLO), 20 – 23 March 2007, Öregrund, Sweden.

ICES. 2007b. Report of the Workshop on Alternative Assessment Strategies for Baltic Flounder (WKAFAB), 2 – 4 October 2006, Öregrund, Sweden.

ICES 2007c. Report of the Baltic Fisheries Assessment Working Group. ICES Headquarters, 17– 26 April 2007 (ICES CM 2007/ACFM:15).

ICES 2007d. Report of the Baltic International Fish Survey Working Group (WGBIFS), 26–30 March 2007, Rostock, Germany.

ICES 2008. Report of the Baltic Fisheries Assessment Working Group. ICES Headquarters, 8-17 April 2008 (ICES CM 2007/ACOM:06).

ICES. 2008a. Report of the Workshop on Age Reading of Flounder (WKARFLO), 26 – 29 May 2008, Rostock, Germany (ICES CM 2008/ACOM:38).

ICES 2009. Report of the Baltic Fisheries Assessment Working Group. ICES Headquarters, 22- 28 April 2009 (ICES CM 2009/ACOM:07).

ICES 2010. Report of the Baltic Fisheries Assessment Working Group. ICES Headquarters, 15- 22 April 2010 (ICES CM 2010/ACOM:10).

ICES. 2011. Report of the Workshop on Flatfish in the Baltic (WKFLABA), 8 – 11 November 2010, Öregrund, Sweden. (ICES CM 2010/ACOM:68) ICES 2011. Report of the Baltic Fisheries Assessment Working Group. ICES Headquarters, 12- 19 April 2011 (ICES CM 2011/ACOM:10).