CM 2000&‘02

On the flounder yield and spawning stock medium-term

forecasts in Estonian waters

Temro Drevs

Estonian Marine Institute, Viljandi 18 B, 11216, ,

Fax +372 6281 563, e-mail [email protected]

Abstract - The possibilities to model the changes in flounder stock and its exploitation in Estonian waters were assessed. It was found that Estonian flounder yield is strongly correlated with inflows of saline water from the North Sea to the

Baltic Sea. In the (SD 32) the yield increased 2 years after the strong and 3 years after moderate inflow, probably because of changes in migration pattern or locomotion activity. Increase of spawning stock biomass and better spawning conditions result in the yield maximum &7 years after the inflow. In subdivision 29 the fluctuations of the yield have been smaller than in the SD 32, but the effect of the inflows is clearly expressed. The yield in SD 29 and 28 has attained maximum 54 years after the inflow. According to the tagging data, the migrations of Bounder do not usually exceed 50-60 miles, but the tagging experiments were performed before the last stagnation period, 19781993. The spawning stock biomass, estimated by

Virtual Population analysis (VPA) and Separable VPA in Estonian waters of the SD

32, also depends on the inflows. Fishing mortality has increased after the inflow.

Flounder/ salinity/stock prediction/ Virtual Population analysis/Separable VPA 2

On the basis of tagging Vitinsh [17] divides Baltic flounder into three populations,

Western-Finnish, eastem-, and Gdansk populations. The first of them inhabits from the Island (North-West Estonia) to 26’ eastern longitude (figurel).

Inside this population continues redistribution of fishes takes place. It is possible that the flounder from the northern coast belongs too to the same population.

Only 2-10 % of recaptured flounder, which was tagged near the Island Naissaar (near

Tallinn), was caught outside of ;ke Estonian waters of the Gulf of Finland [14,17].

According to the data of Mike&u [ll], Shchukina [14], and Vitinsh [17] the northern and southern coastal flounder stocks of the Gulf of Finland are mixing in very small extent (O-2%). 8 % of recaptured flounder, tagged in SD 29-2 near North-

West coast of the Island Hiiumaa, were recaptured in Estonian waters of the Gulf of

Finland [15]. From flounder, tagged near the mouth of the Gulf of Finland near

Finnish coast, 21% of recoveries were made in SD 29 [3]. Of the flounder recaptured in the Gulf of Finland, 71 % were found in Finnish coastal and 29 % in Estonian coastal areas [3]. The fishes that were tagged near the Island (SD 29-4) were recaptured mainly in SD 29,2&5, and 32-1 [l I]. Flounder’s migrations in most cases do not exceed 50-60 miles [ 171. The populations, which spawn in different depths, mix after the spawning [ 14, 161. Tagging experiments allow deciding that Estonian coastal flounder stock in the Gulf of Finland in most cases can be estimated by VPA.

On the other hand, the effect of periodical hydrographical and hydro chemical changes on the population parameters may be significant, too, especially during the last stagnation period in the The purpose of current work was to investigate the possibilities of forecasting the changes in flounder yield, spawning stock biomass, and fishing mortality, using hydrographical conditions.

2. MATERIALS AND METHODS

Data from literature, HELCOM, and Estonian monitoring data were used for investigation of the relationship between hydrographical factors and flounder stock indices in Estonian waters. Salinity, concentration of 02 in different depths, total phosphorus, and nitrogen in 1979-1998 were analyzed in relationship of the flounder yield, population number, biomass, and fishing mortality in 1985-1999. For the reference points for hydrographical data the stations Jl (57?9N - 20’03’E) in the

Gotland Deep and F3 (59’5l.N - 24’5WE) in the Gulf of Finland were used. The relationship between salinity in different depths in Gotland Deep and Gulf of Finland, and Estonian Sounder yield was calculated. The Sounder spawning stock number, biomass, and fishing mortality in Estonian waters of the Gulf of Finland were estimated by Virtual Population Analysis (VPA) [12] and Separable VPA [9]. For the natural mortality the value M = 0.2 was used for the age groups 3 - 8+ years [l]. In

1985-1990 remarkable part of flounder was caught by trawling. The exploitation pattern changed in 1991. Since 1991 the catch by trawling has been very small. The

Separable VPA is assuming constant exploitation pattern. Therefore the Separable

VPA was done for the period 1985-1990 and 1991-1999. The F values, calculated by the Separable VPA for the last year (1999) and last age group (8+), were used in the

VPA. 4

Age was determined by the author in 1991-1999. From the years 1985-1990 the unpublished data of S. M&es and P. Komissarov were used.

3. RESULTS

The Estonian flounder yield in SD 32, 29, and 28 has decreased rapidly during the last stagnation period and attained minimum in 1995 (figure2).

The flounder yield in the Gulf of Finland is correlated with salinity, especially in deeper layers. The relationship between the salinity in station F3 (Gulf of Finland between Tallinn and ) in the depth of 60 m and the Estonian yield in the Gulf of Finland in 1979-1998 is presented in figure 3. The salinity at station Jl (Gotland

Deep) at the depth of 200 m and yield 2 years later in the Gulf of Finland is strongly correlated (figure 4). Also a strong correlation has been found between salinity in the

Gotland Deep in the,depth of 240 m and Estonian flounder catches in the Gulf of

Finland 3 years later (figure 5). The yield also increases a little 3-4 years after increase of salinity in the Gulf of Finland (figure 6). It also means, that the yield attains maximum 6-7 years after the strong or moderate inflow into the Baltic. The inflow must be strong enough to affect the salinity in the Gulf of Finland. On the contrary, the yield in the Gulf of Finland continues to decrease. Using salinity data in deeper layers in the Gotland Deep [5], the yield in the SD 32 can be predicted for 6-7 years (figure 7). 8 years after the inflow the yield begins to decrease rapidly. The

CPUE (catch per unit of effort) for traps and nets in the Gulf of Finland increased some months after the increase of the salinity in deeper layers of the Gulf of Finland in 1996 [6]. The yield variability is higher in the Gulf of Finland and less expressed in SD 29. In SD 29 the yield usually increases 3 years after the inflow and 5-6 years after the strong or moderate intlow from the North Sea to the Baltic Sea attains maximum value (figure 8,9). The weak inflow in 1985 probably stopped the decrease of the yield until the year 1988 in SD 29 (tigure2). The yield in SD 28 can be also predicted for 5 years, using salinity in the Gotland Deep.

At the time of inflow oxygen content decreases in deeper layers in the Gulf of

Finland (HELCOM and Estonian monitoring data). Oxygen content in deeper layers

(70 m) is not correlated with the yield in the Gulf of Finland. The R-squared value of linear regression between salinity in the depth of 70 m at the station F3 and the

Estonian flounder yield in the Gulf of Finland in 1979-1998 was 0.0005. In the

Gotland Deep 02 and H2S [4] have significant effect on the spawning conditions of the deepflounder.

Flounder catches in numbers can be also predicted by salinity and used for estimations of spawning stock biomass by the Separable VPA. These estimations may be not very exact because of changes of the mean weight in age groups and also differences in age determinations.

The relationship between salinity at station Jl (Gotland Deep) and spawning stock biomass (SSB) in Estonian waters of the Gulf of Finland is presented in figure 10.

After the inflow the SSB increases. The hydrographic conditions and fishing have changed the total SSB as well as the biomass of the different age groups in Estonian waters of the Gulf of Finland (figure1 1). Minimum fishing mortality in the Gulf of

Finland corresponds to the minimum salinity in 1995 (tigurel2).

A weaker correlation exists between total phosphorus and the yield in the Gulf of

Finland. The concentration of the total nitrogen is weakly correlated with the yield in the Gulf of Finland. 6

The last available data on the salinity in the depth of 200 m at station Jl (Gotland

Deep) 12.13 PSU in 1997 allows to predict the yield in the Gulf of Finland (SD 32) in the year 2003 near 140 t and in SD 29 also 140 t. SSB in SD 32 could be predicted near 1000 t.

4. DISCtJSSION

The reason of the increase of the catches immediately after the increase of the salinity in the Gulf of Finland could be changes in migration. The tagging data do not show remarkable migrations of flounder between ICES subdivisions in Estonian waters. The changes in salinity and 02 before the year 1993 in the Baltic Sea were bigger than in 1945-1949, 1961, 1962,1967, 1974,~and 1975-1977 when the tagging experiments were carried out, and they maybe cannot describe the changes during the last stagnation period. Since the moderate inflow in 1993, the maximum catch in the

Gulf of Finland was in 1999, 6 years after the inflow. According to the current

Estonian monitoring data, salinity did not increase in 1999 in the Gulf of Finland, but the year class 1996 has been more numerous and in 1999 it attained partly the minimum legal size for fishing (table I). The relatively more numerous year class

1996 appeared in 1998 and was still strong in 1999. This year class was not numerous in 1997. It seems that this year class could partly migrate into the Gulf of Finland. The year classes 1997 and 1998 in the Gulf of Finland are, too, relatively more numerous.

Grauman [SJ found that in years of inflow the abundance of pelagic deepflounder eggs increases. Increases also the area, where the eggs are found. The effect of the inflows on the bankflounder’s reproduction is unknown. The latter spawns in the depth of 4-22 m [l 11. In Estonian waters in SD 29 and SD 32 the both populations 7 spawn, and after spawning they mix. Tagging experiments showed that after spawning in the depth of 70-80 m in the Gulf of Finland, the flounder moved to the shallow waters [ 141.

Increase of the fishing mortality in 1985-1990, was also an important mason of the decrease the flounder stock besides the hydrographical changes. After 1985 the biomass of older age groups has decreased rapidly (figure 11). The estimation exactness is not connected with the changes in gears. In 1985-1990 remarkable part of flounder in the Gulf of Finland was caught by trawling in the depths up to 100 m.

From 1991 mainly coastal catch.has been present. From experimental trawling~ in

I994 in the depths of 46-84 m [ 10 ] among 43 1 aged ff ounders the most numerous was age group 4 (37 %), 3-year-old fish composed 30 % [A. The tish older than 8 years was absent. In 1985-1987 the age groups 5-6 were most numerous and age group 9+ was more common.

The 02 in deeper layers in the Gulf of Finland has not disappeared and H$S has been absent. This could partly explain absence of correlation between oxygen saturation and the yield. It is also known that in benthos communities for 25 last years remarkable changes have not been found [2]. Therefore the common opinion that decrease of concentration of Or in deep layers in the Gulf of Finland could force the flounder to more shallow water, and therefore increase the catches, is not strongly supported.

Inflow of more saline water that changes also 02 content and other conditions can increase the locomotion activity of flounder. The probability of catching a fish, especially in passive gears, depends on swimming speed [133. Result can be increase of catches by passive gears. 8

The VF’A in 19851999 shows a rapid decmase of the spawning stock biomass after

1985 and increase ai& the inflow in 1993.

5. CONCLUSION

The changes in yield of Baltic flounder can be predicted 67 years in Estonian

waters of the Gulf of Finland and S-6 years in Estonian waters of the SD 28 and 29,

using hydrographical data, especially the salinity in deeper layers in the Baltic Sea.

The estimation of spawning stock number and biomass depends on method, which is

used. It seems to be clear that spawning stock number and biomass increases after the

inflow, too. Increase of pelagic flounder eggs abundance in years of inflow was

known before. The spawning stock biomass increases probably already in the year of

the increase of the salinity in Estonian waters (2-3 years after the inflow into the

Baltic Sea) as a~result of migration and also 34 years later as a result of better

spawning conditions after’ the increase of the salinity. Fishing mortality in Estonian

waters of the Gulf of Finland increased after the inflow and was minimal before the

Increase of the salinity in 1995.

1 thank Dr. U. Lips for providing hydrographical and hydro chemical data. 9

REFERENCES

[I] Anon., Report of the Baltic Fisheries Assessment Working Group, Part 1 of 2,

ICES Head quarters ( 1997) 230-23 1. ~

[2] Anon., Eesti-Soome vaheliste valguskaablitrasside valiku keskkonnamGjude

hindamine (keskkonnaekspertiis), Eesti Mereinstituut nr. 15/99 (1999) 13-14.

[3] Aro E. and Sj6blom V., The migration of flounder in the Northern Baltic Sea,

ICES C.M. J: 26:1(1983) l-10.

[4] Berg&em .S., Matthaus., W., Meteorology, Hydrology and Hydrography. Third

Periodic Assessment of the State of the Marine Environment of the Baltic Sea, 1989-

1993; Backround document. Balt. Sea Environ Proc. No. 64 B, (1996) 16-18.

[5] Dahlin H., Fonselius S., Sjiiberg B., The changes of hydrographic conditions in the Baltic Proper due to 1993 major inflow to the Baltic Sea, ICES C.M. C: 58

(1993) 13 p.

[6] Drevs T., Relationship between salinity and Baltic flounder (Plu~ichfhys flesus

(L.)) migration in Estonian waters, ICES C. M. ‘EE: 02, Theme Session

Environmental Factors (1997) 13 p.

[7] Drevs T., Kadakas V., Lang T., and Mellergaard S., Geographical variation in the age/length relationship in Baltic flounder (Plurichthys flesus,), ICES Journal of

Marine Science 56 (1999) 134-137.

[S] Grauman G. B., Spatial distribution of flounder eggs and larvae in the Baltic Sea, in: Kairov E.A., Leonova A.P., Lishev M.N., Malikova M.L., Polyakov M.P., Rimsh

E.Ya, Smimova S.V., Rybokhozyaistvennye issledovaniya (BaltNIIRH) 16 (1981) pp.28-38. (In Russian). 10

[9] Kizner 2.1.; Vasilyev D.A., Instantaneous separable VPA (IS/PA), ICES Journal

ofMarineScience54(1997)39%411.

[lo] Lang T. and Mellergaard S., The BMB/ICES Sea-going Workshop u Fish

Diseases and Parasites in the Baltic Sea” - introduction and conclusions, ICES

Journal of Marine Science 56 (1999) 129-133.

[I I] Mikelsaar N., Kambala vostochnoj cbasti Baltijskogo morja. Avtoreferat

dissertatsij na soiskanie uchenoj stepeni kandidata biologicheskih nauk (1958) 18 p.

(In Russian).

[I21 Mohn R.K. and Cook R.:‘.Introduction to Sequential Population Analysis,

ScientificCouncil Studies Number 17, Workbook, Dartmouth, 1993,110 p.

1131 Rudstam L.G., Magmwon J.J., and Tonn W.M., Size selectivity of passive fishing

gear: a correction for encounter probability applied to gill nets, Can J. Fish. Aquat.

Sci.,41,8 (1984) 1252-1255.

[14] Shchukina I. Soome lahe lestast, Abiks Kalurile,l (1969) 7-8.

[ 151 Shchukina I., Pitanie i migratsij rechnoj kambaly (Pleuronectes jlesus tra$zurus

Duncker) v rajone ostrova Hiiumaa, in: Veldre LR., Lishev M.N., Malikova E.M.,

Polyakov M.P., Pozhogina P.M., Shlimovich B.I., Trudy BaltNllRXa Ministerstva rybnogo xozyajsh&SSR,IV, 1970, pp. 361-378.(In Russian).

[16] Solemdal P., Transfer of Baltic flattlsh to a marine environment and the long term effects on reproduction, OIKOS Supplementum 15 (1973) 268-276.

[ 171 Vitinsh M, Some regularities of Flounder (Platichrhysflesus L.) distribution and migrations in the Eastern and North-Eastern Baltic, Rybohozyajstvennye issledovaniya SSSR i GDR v bassejne Baltijskogo morya 14, I (1976) 3948. (In

Russian). Table I. Age. composition of floder in the Gulf of Finland (%) in 1991-1999.

43% 1991 1992 1993 1994 1995 1996 1997 1998 1999 Y- 1 1.1 0.9 1.4 0.2 1.8 0.4 9.2 9.8

2 8.2 9.6 23.0 28.4 5.4 15.3 22.8 42.9 20.2

3 44.6 35.6 23.0 27.3 33.4 28.3 27.9 30.7 39.2

4 36.9 36.5 25.7 21.0 40.7 30.7 23.0 9.1 19.9

5 8.5 13.0 17.5 14.7 15.6 15.4 13.0 3.9 6.3

6 1.3 2.0 6.0 4.4 3.3 7.0 8.5 2.5 2.4

7 0.3 1.0 2.4 2.3 0.9 1.3 3.2 0.8 1.7

8 0.2 0.7 0.9 0.3 0.5 0.1 1.1 0.5 0.3

9 0.2 0.3 0.2 0.1 0.1 0.1 0.06

10 0.1 0.2 0.1 0.2 , I II I I t

Figure 1. The study area. 1400

1200

1000

= 800 '21 2 800

400

200

0

Year

Figure 2. Estonian flounder yield in SD 28,29, and 32 in 19744999. 1

y= 8E-05e2.w9x g 800 R* = 0.4013 $ 800

6.5 7 7.5 8 Salinity (PSU)

Figure 3. Relationship between salinity at the station F3 (Gulf of Finland) in the depth

of 60 m and Estonian flounder yield in the Gulf of Finland in 19794998. IO 11 12 13 Salinity in the depth of 200 m in 1980-1997 (PSU)

Figure 4. Relationship between the salinity at the station Jl (Gotland Deep) in the

depth of 200 m and Estonian flounder yield in the Gulf of Finland lagged 2 years.

I s 1400 , 1200 0 y = 4E-09e2~037’x 1000 R2 = 0.8325 800 l 800 400 200 .- 0 7 ,o 11 11.5 12 12.5 13 .a, > Salinity at station Jl in the depth of 240 m in 1981-1994 Figure 5. Relationship between the salinity at the station Jl (Gotland Deep) and

Estonian flounder yield in the Gulf of Finland lagged 3 years. 1200- . l l 1000 - 000 y = o.ooo3e’~‘u67x

l l t, -I 6.5 7 7.5 6 Salinity at station F3 in the depth of 60 m (PSU)

Figure 6. Salinity at the station F3 (Gulf of Finland) in 1979-1996 and Estonian flounder yield in 1982-1999 in the Gulf of Finland.

1400 l 17ll.-Jo - l Se 1000 -

600 --

11 11.5 12 12.5 13 13.5 Salinity in the Gotland Deep below 225 m in 1971-1993 (PSU) Figure 7. Relationship between the salinity in the Gotland Deep [5] and the flounder yield in the Gulf of Finland lagged 6 years. & * 700- 5x EI .-- I v = 1 E-05e’.353C Q) 600 l D .-c ’ R2 = 0.8786 /

11 11.5 12 12.5 13 13.5 ;alinity in the Gotland Deep in the depth below 225m (PSU)

Figure 8. Relationship between the salinity in the Gotland Deep in 1973-1993 151 and the Estonian flounder yield in the SD 29 lagged 3 years (in 1976-1996).

700 -

600 -- l

y = o.o004e’.- 500 * E R2=0.5219 g 400 l / v) g 300 " + 200

100

0 11 11.5 12 12.5 13 13.5 Salinity in the Gotland Deep below 225 m (PSU)

Figure 9. Salinity in the Gotland Deep in 1971-1993 [5] and Estonian flounder yield in the SD 29 lagged 6 years (in 1977-1999). Figure 10. Salinity at the station Jl (Gotland Deep) in the depth of 200 m and flounder spawning stock biomass (SSB) in Estonian waters of the Gulf of Finland.

600 -4-3 years 500 +4 years + 5 years s 400 +6 years + 7 years f/J% 300 + 8+ years 200

1984 1989 1994 1999 2004 Year

Figure 11. Flounder spawning stock biomass (SSB) by different age groups in the

Gulf of Finland in 1985-1999, estimated by Separable VPA and VPA. 1.6

1.4

1.2 G 3 ' g 0.8 e 3 0.6

0.2 0 1 I I I YT 1984 1986 1988 1990 1992 1994 1996 1998 2000 Year

Figure 12. Fishing mortality of flounder in Estonian waters of the Gulf of Finland by

age groups in 1985-1999.