R E P O R T. Stock Assessment of Turbot (Psetta maxima) by Swept Area Method in the Bulgarian Black Sea Area during the autumn winter 2007

Transcription

1 R E P O R T Stock Assessment of Turbot (Psetta maxima) by Swept Area Method in the Bulgarian Black Sea Area during the autumn winter 2007 BULGARIAN ACADEMY OF SCIENSES INSTITUTE OF OCEANOLOGY VARNA 2008

2 The present study was carried out by a team of experts from the Institute of Oceanology BAS and the Institute of Fisheries Resources, Varna, during the period in execution of a contract between the National Agency of fisheries and Aquaculture Sofia, MAF and the Institute of Oceanology BAS, Varna. List of authors: BULGARIAN ACADEMY OF SCIENCES INSTITUTE OF OCEANOLOGY VARNA Assoc.researcher Marina Panayotova, PhD Assoc.researcher Valentina Todorova, PhD Assoc. prof. Tsenka Konsulova, PhD NATUONAL CENTER OF AGRICULTURAL SCIENCES INSTITUTE OF FISHERIES RESOURCES VARNA Assoc.researcher Violin Raykov

3 Contents 1 Introduction Study objective and tasks Fishing vessel and fishing gears Material and methods Sampling design Onboard sample processing Laboratory analyses Statistical analyses Results Biomass of turbot stocks in the Bulgarian Black Sea area in autumn - winter Distribution of turbot in the Bulgarian Black Sea in autumn - winter Population parameters of turbot in the Bulgarian Black Sea in autumn - winter, Length frequency Sex ratio Age composition and growth Fulton s condition factor Natural mortality coefficient (М) Diet composition Species composition of by-catch Conclusions and recommendations References... 49

4 1 Introduction Among species that inhabit the Black Sea, turbot (Psetta maxima) is the most valuable commercially. It is a target of intensive exploitation since 50s and the changes in its stocks are highly dependent on the fishing pressure, as well as on the alterations in the Black Sea environmental state during the time. For the sustainable exploitation and management of the Black Sea turbot stocks requires annual investigations and assessments by direct and analytical methods. Since 2006 the investigations on turbot stocks had been re-opened after 15-years interraption, which allow accomplishment of recent assessments by direct method swept area. The annually gathering of data during next years, concerning turbot stocks and population parameters will allow application of set of analytical methods different modifications of VPA, which will increase the precision of stock determination necessary for the proposal and introduction of measures for sustainable or close to sustainable utilization of this biological resource. 2 Study objective and tasks The major goal of the present study is to estimate the exploited biomass of turbot stocks by the swept area method in the Bulgarian Black Sea during autumn - winter, The results attained will be compared to previous research results in order to outline the trends in turbot stock temporal changes, as well as its spatial distribution. The tasks during the study encompass the collection of biological samples for determination of length frequency, weight structure, age structure and sex ratio of turbot population and identification of the species composition and distribution of fishes in by-catch. 3

5 3 Fishing vessel and fishing gears The dimensions of the bottom trawl (Pict. 1and 2) employed are as follows: Length of the head rope 26 m; Vertical spread 3 m; Eye of the net 10х10 cm; Effective part of wing spread m. At both regions lugs were carried out only during daylight with single haul duration between min. at trawl speed 1.8 knots. Picture1. Bottom trawl of FV Elis employed in the study. 4

6 Picture 2. Catch in trawl codend of FV Elis trawl 4 Material and Methods In the Bulgarian Black Sea region the survey was carried out during the period in accord with the swept area method that uses stratified sampling, the area covered enclosed between the parallels 43 о 40 N - 42 о 05 N and the meridians 27 о 50 Е - 28 о 50 Е. At each of the 42 previously selected swept fields a single haul with duration between min. was completed, the results were interpolated over the rest of the area. Bad meteorological conditions and rough sea led to survey repeatedly interruption, after which lugs were resumed successfully at

7 4.1 Sampling design To establish the exploited turbot stock in front of the Bulgarian Black Sea coast a standard methodology for stratified sampling was employed (Gulland, 1966; Sparre, Venema, 1998; Sabatella, Franquesa, 2004). To address the research objectives the region was divided in three strata according to depth stratum 1 (35 50 m), stratum 2 (50 75 m) and stratum 3 ( m). The study area was partitioned into 128 equal in size not overlying fields, situated at depth m, of which 70 in the northern region and 58 in the southern region. In the northern region, for the aims of the study, additional forth stratum was introduced, which covered depths between 15 and 35 m because is this area the bottom structure allows bottom trawling in shallow waters as distinguished from southern region. At 42 of the fields chosen at random (22 in the northern region and 22 in the southern region) sampling by means of bottom trawling was carried out (Figure 2). The seabed area covered during a single haul represents a basic measurement unit, which is very small compared to the total study area, nevertheless deemed representative since turbots do not aggregate in dense assemblages (Martino, Karapetkova, 1957). Each field is a rectangle with sides 5' Lat 5' Long and area around km 2 (measured by application of GIS), large enough for a standard lug extent in meridian direction to fit within the field boundaries. The fields are grouped in larger sectors so called strata, which geographic and depth boundaries are selected according to the density distribution of the species under study. At each of the fields only one haul with duration between min. at speed 1.8 knots was carried out. The arrangement of the swept fields is represented on the map of Figure 2. As a result of the trawling survey a biomass index was calculated. 6

8 Figure 2. Research area and plan of the sampling fields. 7

9 4.2 Onboard sample processing The data recorded and samples collected at each haul include: Depth, measured by the vessel s echosounder; GPS coordinates of start/end haul points; Haul duration; Abundance of turbots caught; Weight of total turbot catch; Absolute and standard length, individual weight of turbots. Individuals with size less than the minimal allowable length (45 cm) according to the FAA were released back to the sea. Otolyths were collected for age determination; Gender was identified; Stomachs were collected from part of the individuals for food determination; The species composition of the by-catch was identified. 4.3 Laboratory analyses The samples collected onboard were processed in the laboratory for determination of age and food composition. The age was established in otolyths under binocular microscope. The food spectrum was determined by separation of the stomach contents into taxonomic groups identified to the lowest possible level. 4.4 Statistical analyses Swept area method The swept area method was used for estimation of turbot exploited biomass. 8

10 According to this method, the trawl sweeps a well defined path, the area of which is the length of the path multiplied by the width of the trawl, called the "swept area" or the "effective path swept". The swept area, a, can be estimated from equation 1: (1) a = D*hr*X2 D = V*t where: а - swept area, V - velocity of the trawl over the ground when trawling, X2 is that fraction of the head-rope length hr, which is equal to the width of the path swept by the trawl, the "wing spread", t - is the time spent trawling, D- distance covered. For the estimation of turbot biomass, the catch per unit of area (CPUA) is used equation 2: Cw / t Cw 2 (2) = кг / км a / t a where: C w/t catch in weight per unit of area, a/t the area swept per hour. The biomass of the investigated species for each stratum is obtained from equation 3: (3) B = ( C ) A w / a * where: C w / a - the mean catch per unit area of all hauls, А the total size of the area under investigation in stratum. The variance of biomass estimate for each stratum is (equation 4): 1 n 1 n 1 n [ Ca] 2 (4) VAR( B) = A * * * Ca() i i= 1 The total area of survey region, equal to the sum of all strata areas, becomes: А = А1+А2+А3 The mean catch for the entire survey area is obtained from equation 5: (5) Ca( A) Ca1 * A1 + Ca2* A2 + Ca3* A3 = A where: Ca1- catch per unit area of stratum 1 and etc., А1 area of stratum 1 and etc., А total area of survey region. 2 9

11 The total biomass in the survey area is estimated by - equation (6): (6) ( A) A B = Ca * where: Ca( A) - mean catch for the entire survey area, А total area of survey region. Estimation of Maximum Sustainable Yield (MSY) The Gulland's formula for virgin stocks is used equation 7: (7) MSY = 0.5*M*Bv where: M coefficient of natural mortality; Bv virgin stock biomass. Estimation of Total Allowable Catch (ТАС) The Ricker s (1975) method for estimation of catch rate per unit recruitment is used: (8) ( ) t= t Y F B e Gt Zt =. t. t= t Gt Z r t λ 1 where: Gt - is the the weight-specific growth coefficient by are groups ( ) G ln W / W t = t+ 1 t ТАС = Y(%) for F 0.1 / B according to Prodanov, Kolarov (1983) The Beverton, Holt (1957) method (9) Y / R F*exp[ M * ( T T )] * W = c r 1 3S 3S S * + Z Z + k Z + 3k Z + 3k where: S = exp[-k*(tc-t 0 )]; W, к, t0 - parameters from VBGF; Tc age of entering in the exploitation phase; T r age of maturation; F fishing mortality coefficient; M natural mortality coefficient; Z = F + M total mortality coefficient

12 After series of transformations, Beverton, Holt (1966) worked out a modified form of the equation, but the yield-per recruit have been estimated as relative (Y /R). Relative yield-per-recruit model (10) Y / R = E * U M / k 3U 3U U 1 + (1 + m) (1 + 2m) (1 + 3m) 2 3 where: U = 1-(L c /L ) m = (1-E)/(M/k) = k/z E = F/Z exploitation coefficient. Age and growth For the estimation of turbot growth rate, the von Bertalanffy growth function (1938) is used, (according to Sparre, Venema, 1998): { [ ]} (8) L L exp k ( t t ) t = 1 0 { [ ]} (9) W W exp k ( t t ) t = 1 0 n where: L t, W t are the length or weight of the fish at age t years; L, W - asymptotic length or weight, k curvature parameter, t o - the initial condition parameter. The length weight relationship is obtained by the following equation: (10) n W t = ql t where: q condition factor, constant in length-weight relationship; n constant in length-weight relationship. Fulton s condition factor (Ricker, 1975) W (11) K = * L where: W weight in kg.; L length in cm. 11

13 Coefficient of natural mortality (M) Pauly's empirical formula (1979, 1980) is applied: o (12) log M *log L * log k *logT C = (13) log M * log W * log k * logt C = where: L,, W and к parameters in von Bertalanffy growth function; Т о C - average annual temperature of water, ambient the investigated species. Food composition To estimate the importance of each food item among the forage, an Index of Relative Importance IRI (Pinkas et al., 1971) was calculated as follows equation 14: (14) IRI = (C N + C W )*F where: C N is the percentage of food item i in total number; C W - is percentage of food item i in total weight; F is percentage of occurrence frequency in the food item i. To estimate the importance of each food item among the stomach contents, IRI expressed on a percent basis (Cortes, 1997) was also calculated: o (15) % IRI i 100* IRI = i n IRI i i where: n is the total number of food categories considered at a given taxonomic level. 12

14 5 Results 5.1 Biomass of turbot stocks in the Bulgarian Black Sea area in autumn - winter, The trawling survey for turbot stock assessment in the Bulgarian Black Sea area carried out during autumn - winter (November December) 2007 comprised 42 hauls with total duration of 68 hours, during which 364 turbot specimens were caught with total weight of kg. The average CPUE in the Northern area amounted to kg per hour, while in the Southern area it was kg per hour respectively. The average individual weight of turbot specimens caught in the Northern and the Southern regions was kg and kg respectively. The abundance of turbot caught varied from 0 to 56 specimens per haul. In order to calculate the exploited stock by the swept area method, the Bulgarian Black Sea area was partitioned into 128 fields, which according to their depth belonged to one of the three strata: stratum 1 (35-50 m) 29 fields; stratum 2 (50-75 m) 44 fields and stratum 3 ( m) 40 fields. In the northern region during the current study an additional stratum 4 (15-35 m) was introduced, which encompassed 15 fields. The lack of forth stratum in the southern region is due to peculiarities bottom structure as well as sharp increase of depth in areas outside the bays. The total area, in which the turbot exploited stock, is deemed to be distributed, amounts to 8010 km 2. Turbot biomass estimation in swept fields requires calculation of the catch per unit effort (CPUE) and the catch per unit area (CPUA) in accord with the methods given (Chapter V.). The results for CPUE (kg/hour), CPUA (kg/km 2 ) and biomass in swept fields are given in Table 1. 13

15 Table 1. Catch per unit effort (CPUE), catch per unit area (CPUA) and biomass of turbot by sampling fields in the Bulgarian Black Sea area in autumn - winter, Region Stratum Field North South CPUE kg/h CPUA kg/km 2 Biomass (in tons) 1 F (35 50 m) G G H I L M (50 75 m) L К J F H I I ( m) L (15 35 m) 1 (35 50 m) 2 (50 75 m) 3 ( m) M E H G E D C D C B C C C D E E D D D E G F G F F F F

16 The mean values of catches per unit area by strata, the respective standard deviations and standard errors, Student s distribution and confidence intervals of the mean (CI, 95%) are given in Table 2. Region stratum Hauls Table 2. Statistics of CPUA by strata. Ca kg/km 2 Std.deviation (s) Std. error n s / Student distributio n t n-1 CI of Ca North ( , ) (72.33, ) (3.10, ) (76.00, ) South (79.46, ) (347.10, ) (41.65, ) Figure 2 represents the spatial distribution of CPUE in the Bulgarian Black Sea area in autumn - winter, It is made evident by the figure, that in Northern region highest catches per unit effort were achieved at depths between 50 and 70 m in front of Krapets and Shabla and also at depths between 20 and 45 m along the straight line, connecting Kavarna - Varna. The maximum value of catch per hour trawling kg/hour in Northern region was attained in front of Shabla. High values of CPUE with rates between and kg/hour were registered in the area in front of Krapets Shabla, Varna and Shkorpilovtsy - Byala. During the present study, for the first time were accomplished larger numbers of hauls at depths between 20 и 35 m in the in the shallow area of Northern region. The obtained results show high catches per hour trawling and respectively high turbot biomasses per unit area. This fact determines the major importance of 12-nautucal miles zone and especially shallow areas as turbot spawning and nursery grounds. In these areas there is a possibility for more effective control during active exploitation of resource. 15

17 Figure 2. Distribution of turbot CPUE in the Bulgarian Black Sea during autumn - winter,

18 Comparative results with autumn winter study in 2006 suggest the fact, that in the area in front of Krapets Shabla, the turbot find optimum conditions for living. Unfortunately, the bad hydro-meteorological conditions didn t allow us to make hauls in the offshore area at depths over 80 m in this region. The relatively constant catches with values between 1 5 kg/hour were observed in areas, situated on the line, connecting Shkorpilovtsy cape Kaliakra at depths from 40 to 80 m. This fact suggests that the turbot has steady distribution in this area, but has low abundance and biomass. Catches per unit effort in the Northern region range between 0 and kg/hour. The average catch per one hour trawling in the Northern area amounts to kg/hour and the average individual weight of caught turbot specimens was kg. In the Southern region, the catches per unit effort varied from 0 to kg/hour, as the average catch of turbot for one hour of trawling was kg. The average individual weight of caught turbot was estimated at kg. In comparison with Northern region, it is evident that the catches per unit effort in both regions were almost with same values, but in difference with Northern region, the average individual weight of turbot in Southern area was significantly higher with kg. The maximum CPUE with value of kg/hour was achieved in front of Burgas Bay at depths between 70 and 80 m. Catches in the order of 5 15 kg/hour were realized on the line, connecting cape Maslen nos and Ahtopol at depths between 40 and 80 m. The calculated CPUA for the entire Bulgarian Black Sea area is presented on Figure 3. The obtained results for catch per unit area show, that in the Northern region, the value of CPUA was higher compared to the Southern region. The CPUA rates attained maximum value of kg/km 2 in the Northern region, and the average catch for the area was kg/km 2. In the Southern region, the maximum CPUA value was lower kg/km 2 compared to the Northern region, but the average catch for unit area in the Southern region was higher kg/km 2. Figure 3 shows that the average CPUA was higher at depths between 50 and 75 m for both regions in comparison with other depth zones. With increasing of depth, the values of CPUA start to decrease. Relatively high catches per square kilometer were registered at depths between 15 17

19 35 m in the Northern region, with average value of kg/km 2, respectively. The maximum values in both regions were situated at depths between 50 and 75 m. The values of average CPUAs in the Northern region by strata were as follows: Stratum 1 (35 50 m) kg/km 2 ; Stratum 2 (50 75 m) kg/km 2 ; Stratum 3 ( m) kg/km 2 ; Stratum 4 (15 35 m) kg/km 2. In the Southern region the maximum of CPUA was achieved in front of Burgas Bay at depths between m. The average CPUAs in the Southern region by strata were as follows: Stratum 1 (35 50 m) 153 kg/km 2 ; Stratum 2 (50 75 m) kg/km 2 ; Stratum 3 ( m) kg/km 2. The obtained results for the Northern region show that turbot stock was concentrated at depths between m, as in the areas with depth and over 75 m, the turbot catch for unit area was relatively equal. A comparison of the current results with turbot stock assessment made in spring 2007 is evident, that the current established average catches per unit area by strata varied in wide range from 50 to 474 kg/km 2, while during the spring season they were changed between 127 and 292 kg/km 2. The cause for such distribution probably was due to hydrometeorological conditions, because during the whole study there were rough sea and strong winds and trawling were repeatedly ceased due to strong and continued storms. All this probably was leaded to migration and redistribution of food resources of turbot and respectively to its stocks. But, despite of differences in hydrological conditions, the depths between 50 and 75 m were characterized with high catches per unit area over 300 kg/km 2 in both investigated seasons spring and autumn winter. The analyses of the two parameters catch per unit effort (CPUE) and catch per unit area (CPUA) reveal that both during spring and autumnwinter the most suitable depth for commercial fishing of turbot is in the range of 50 and 75 m. 18

20 Figure 3. Distribution of turbot CPUA in the Bulgarian Black Sea during autumn - winter,

21 Based on the results from the survey carried out in autumn - winter 2007, the biomass of turbot exploited stock in the Bulgarian Black Sea was estimated Table 3. Table 3. Biomass of turbot exploited stock in the Bulgarian Black Sea, autumn - winter 2007 Region Stratum Fields autumn - winter North South Area km 2 Ca kg/km 2 Biomass (tons) Total Total GRAND TOTAL According to the calculations made the biomass of turbot in autumn - winter 2007 in the Northern region of the Bulgarian Black Sea amounted to t and in the Southern region it was t. The total current exploited biomass was estimated at t. In the Northern region, the total turbot biomass amounted to t, while the pointed biomass was at t during the spring survey in The difference in calculated turbot biomasses during spring and autumn winter seasons is acceptable and is probably due to accuracy of the method and variable meteorological conditions during the field study waves and currents, which affect the trawl fishing efficiency. The hauls made in Stratum 4 during spring and autumn winter 2004, covering shallow waters between m show, that in this area have been concentrated considerable part of turbot stock, since the estimated biomass of turbot in this stratum amounted approximately at t during the spring and 327 t in autumn - winter. The proximity of Stratum 4 to the coast and its belonging to the 12 nautical miles zone makes the process of yields control during the spawning season and against 20

22 prohibited gears much easier. Simultaneously with this, the availability of both juvenile and adult specimens makes the zone a suitable donor and biologically important area for the conservation and distribution of turbot to greater depth. In the Southern region the exploited biomass of turbot during autumn - winter 2007 was estimated at t as compared to t during spring The difference also can be contributed to the accuracy of method, number of fields, recruitment, etc. The present study estimated commensurable turbot biomasses in both Northern and Southern regions. The studies on the distribution and size structure of turbot population in the Bulgarian Black Sea during autumn winter 2007 revealed mixed distribution of different ages in all investigated depths, as totally % from all caught individuals had absolute length under 45 cm and therefore those should not be a commercial target. For the calculation of the exploited biomass undersized specimens were also included. Using the additional biological information for size frequency of turbot catches in the two regions we reduced the calculated biomass to for the Northern region and to t for the Southern region, since the undersized specimens represented respectively % and 9.92 % from the biomass of catches and juveniles should not be commercially exploited. The obtained results for biomass of standard individuals (with absolute length over 45 cm) show, that in both studied regions the present estimated biomasses are higher compared to the spring 2007 assessments, respectively t for the Northern region and t for the Southern region, as the growth of biomass in the Southern region is approximately 300 t. Maximum sustainable yield (MSY) by regions, according to the method of Gulland (1970), was calculated for both exploitation biomasses the added biomass of undersized and standard individuals and the biomass of only the standard individuals. The results obtained are given as follows: North MSY = 0.5*0.25* = тона MSY* = 0.5*0.25* = тона undersized and standard standard 21

23 South MSY = 0.5*0.25* = тона MSY* = 0.5*0.25* = тона undersized and standard standard In other words MSY should not exceed t in the Northern region, and t in the Southern region of the Bulgarian Black Sea. The method of Ricker (1975) was employed for calculation of the catch per unit recruitment of the exploitation stock of turbot (2 12 years). Table 4 represents the Ricker s method results, which reveal the influence of fishing mortality on the initial and mean turbot biomass, as well as on the catch quantity in case that the recruitment abundance (1 year old individuals) is constantly equal to 100 billions fishes. Table 4. Initial (В t ) and mean ( B) biomass and turbot yield (Y). F B B Y Y/Bt (%) Y/ B It is evident from Table 4 that according to F 0.1 criteria the optimal value of the fishing mortality coefficient is achieved at F opt = 0.3. For this value of F, the catches constitute % of the initial biomass. According to the estimated value of the exploited biomass of turbot calculated by the swept area method, the sustainable yield in the Northern region should not exceed t, if only the biomass of the standard specimens is 22

24 considered or t, in the case of mixed stock, and for the Southern region the sustainable yield should be t or t, respectively. The value of MSY estimated by the above approach is too high and could lead to decline in the abundance and biomass of turbot stock, as well as to affect negatively the age-size structure of turbot population. For this reason we recommend a two-fold reduction of the fishing mortality coefficient to a value of F opt = In this case the catches constitute % of the initial biomass. Thus for the Northern region the sustainable catch represents or t for the standard size or mixed population respectively. For the Southern region the catches should not exceed or t for the standard size or mixed population respectively. In conclusion, the total allowable catch of turbot in 2008 calculated according to Ricker s method and F 0.1 should not exceed t for the Northern region and t for the Southern region. The model yield-per-recruit by Beverton, Holt (1957) was employed, which incorporates the following suggestions: the stock of the species under study is in a balanced state, or the model describes the stock state on the condition that the fishing effort is constant for long enough period of time so that all survived fishes are exposed to its influence after they have entered the stock; the recruitment is constant; the fishes from each cohort have hatched in the same day; the fishing and natural mortality are constant since the moment of entering in exploitation phase; there is full mixing within the stock; the length-weight relationship is a third power function. The model allows the calculation of Y/R for different values of the initial parameters (F and t c ) and estimates the effect of those values on the relationship yield-per-recruit of the investigated species. Moreover, the two parameters F and t c can be managed by humans since the fishing mortality is proportional to the fishing effort, and t c is a function of gear selectivity. The model of Beverton, Holt (1957) was employed for turbot stock. 23

25 In order to calculate yield-per-recruit relationship for 2007 the following values of the parameters were used: W =8.650 kg, k=0.136, M=0.25, t 0 = , t r = 4, estimated from biological data during autumn - winter The results for the relationship Y/R at three different exploitation levels (t с =2, 3 and 4 years) are represented graphically on Figure Y/R Y/R tc=2 Y/R tc=3 Y/R tc= F Figure 4. Yield-per-recruit relationship of turbot for 2007 in three different levels for entering in exploitation phase. Accordingly to the obtained results, presented on Figure 4 is obvious, that the curves yield-per-recruit have a clear maximum at F = 0.2 when entering the exploited biomass at the age of 2 and 3 and F = 0.3 when entering the exploitation phase at age 4 years. Therefore, for achieving maximum sustainable yields (MSY) when exploiting 3- and 4-years old specimens (as suggested by the study results), the fishing mortality coefficient should not exceed 0.2. The value of the fishing mortality coefficient could be raised to 0.3, if only the age groups elder than 4 years are exploited, however we deem that the latter level of exploitation 24

26 is too high and represents a threat to the stock, therefore it should be decreased. For estimation of the yield-per-recruit in relative units the equation of Beverton, Holt (1966) and the parameters of von Bertalanffy function were used. As a result the values of the exploitation coefficient were calculated according to the criteria: Е 0.1 (level of exploitation at which the boundary increase of the relative yield-per-recruit constitutes 1/10 from its value at Е=0), E max (level of exploitation producing maximum yield) and E 0.5 (value of Е, under which the stock will be reduced to 50 % compared to the biomass in unexploited state). The exploitation coefficient values for turbot in 2007 were calculated. The values of the criteria are given in Table 5 and Figure 5. Table 5. Values of exploitation coefficient (Е) for turbot in Species P a r a m e t e r s M/k L c /L E 0.1 E 0.5 E max turbot According to the method of Beverton, Holt (1966), the level of exploitation (E=F/Z) for turbot could be selected dependent on fisheries demands, as for the established ratio L c /L, the values of Е should not exceed

27 Figure 5. Relative yield-per-recruit for turbot in The method of simulations was also applied (Beverton, Holt) in three different scenarios, but in assumption that the recruitment is constant, equal to one. Scenario 1 Yield/FishB0 vs.fishing mortality Y/FishB0 7.30E E E E E E E E E E E E E E E E E E E E E E E Fishing mortality Figure 6. Equilibrium Yield vs. fishing mortality At constant recruitment (R=R 0 ), the optimum level of fishing mortality F is 0.2 (Fig.6). At this level yield as a fraction from fishable biomass at unexploited state has maximum level. After that value, the yield steeply 26

28 decrease to F=1.5.The model assumes that after this value of F, slight increase of yield per recruit occur, as the levels of fishing mortality are close to those corresponding to absence of fishing on the species. From Fig. 7 is evident, that ratio between stock spawning biomass (SSB) and stock spawning biomass of unexploited stock (SSB0) decrease too with raise of fishing mortality rate. The identical tendency is observed in the ratio between fishable biomass (Fish B) and fishable biomass in unexploited state (FishBo) with increase of fishing mortality fig. 8. SSB/SSB0 vs Fishing m ortality SSB/SSB E E E E E E-03.65E E Fishing m ortality Figure 7. Stock spawning biomass against fishing mortality. FishB/FishB0 vs Fishing mortality Fish/FishB E E E E E E-02.68E-02.35E Fishing mortality Figure 8. Fishable biomass against fishing mortality. 27

29 With increase of fishing mortality coefficient, the total stock biomass of turbot also decreases Fig.9. TotalB/ToatalB0 vs Fishing mortality TotalB/TotalB E E E E E E E E Fishing mortality Scenario 2 Figure 9. Total biomass against fishing mortality. Y/R Fraction of Unexploited biomass B E-02 YperR/B E E E E E E E E E Fishing mortality Figure 10. Yield-per-Recruit as fraction of unexploited biomass The level of F opt in this case is equal to 0.2 (average) Fig. 10. The corresponding value of Y/R at the level of fishing mortality is higher, and then falls down. After F=1, Yield-per-Recruit vs. F has plateau form according to average levels, after simulations. 28

30 SSBper recruit/ssb0 1.2 SSBper Recruit/SSB E E E E E E E E Fishing mortality FishBperR/FishB0 vs F 1.2 FishB-per Recruit/FishB E E E E E Fishing mortality 1.2 TotalB per Recruit/TotalB0 vs.fishing mortality 1 1 ToatlB/TotalB E E E E E E Fishing mortality Figure 11. Variations in spawning, fishable and total biomass against fishing mortality. 29

31 From Fig.11 is evident, that ratios of stock spawning biomass (SSB) per recruitment/stock spawning biomass of unexploited stock (SSB0), fishable biomass (Fish B) per recruitment/fishable biomass in unexploited state (FishBo) and total biomass per recruitment/total biomass in unexploited state also decrease with raise of fishing mortality. Scenario 3 Yield per Recruit (kg) Absolute Biomass vs. F 250 Y/R Absolute Biomass Fishing mortality Figure 12. Yield-per-Recruit Absolute biomass against fishing mortality. The Yield per Recruit has optimal value at F opt =0.18, respectively t. The maximum value of Yield per recruit has at F max = 0.36, respectively t Fig. 12. The difference between corresponding per recruit absolute levels of yield at optimum and maximum levels of fishing mortality is very small. This fact clearly speaks that at the F max the level of Yield per Recruit will increase, but after F=0.4 decreases. Unsustainable, high levels of fishing mortality in this case could lead to stock degradation. In this case, the ratios between stock spawning biomass (SSB) per recruit, fishable biomass (Fish B) per recruit and total biomass per recruit also decrease with raise of fishing mortality Fig

32 SSB per recruit (kg) vs.fishing mortality SSBper Recruit E E E Fishing mortality FishB per Recruit (kg) vs.fishing mortality FishB/Recruit Fishing mortality Total Biomass per Recruit (kg) vs.fishing mortality TotalB/Recruit Fishing mortality Figure 13. Variation in spawning, fishable and total biomass against fishing mortality. 31

33 The calculated values for the exploitation biomass of turbot in the Northern and Southern regions, their respective values for maximum sustainable yield and the quotas proposed are given in Table 6. Table 6. Biomass, maximum sustainable yield and quota for turbot. Region Biomass (t) Gulland Ricker, F 0.1 MSY Beverton, Holt North Mixed biomass Biomass (mature ind.) South Mixed biomass Biomass (mature ind.) F max, F opt Quota (t) In accordance with swept area method surveys, carried out in 2007 and based on performed analysis on data, concerning turbot population state, we deem that the state of the population is relatively stable and the share of immature individuals decreased in comparison with spring survey in 2007 and studies in The relatively stable state of stock biomass allows turbot catches to be set at t in 2008 in front of Bulgarian Black Sea area. 5.2 Distribution of turbot in the Bulgarian Black Sea area in autumn - winter, 2007 The trawling survey in front of the Bulgarian Black Sea coast was carried out during the period , such as the study area was limited by the parallels 43 о 40 N and 42 о 05 N and the meridians 27 о 50 Е and 28 о 50 Е. The area studied covered depths between 20 and 90 m. 32

34 The 42 bottom hauls were accomplished 21 of them in the Northern region and 21 in the Southern region, respectively. During the study, the 364 turbot specimens were caught, 96 of them with size smaller than the minimum allowable length according to the FAA (2005). These turbots were released back to the sea. The total weight of the catch was kg, and the average weight of one turbot specimen was kg. The observations on studied area between depths 20 and 90 m show, that turbot forms mixed shoals during autumn winter with juvenile and adult specimens occurring together at all depths. One and two years old individuals with total lengths about cm, were observed only at small depths up to 35 m. The abundance of caught specimens in different swept fields varied between 0 and 56. The lack of caught individuals in some fields was due to bad meteorological conditions and strong bottom currents, as well as catching of large objects from bottom anchors and petrol cans, which impeded normal trawl performance. Turbot occurred at all studied depths between 20 and 90 m in shelf area with maximum density registered in front of Krapets Shabla, Kavarna and cape Galata at depths from 20 to 70 m. 5.3 Population parameters of turbot in the Bulgarian Black Sea area in autumn - winter, 2007 The population parameters include the length frequency, age structure and sex ratio of turbot, as well as the growth rate, mortality, etc Length frequency The length frequency of turbot population in the Bulgarian Black Sea area in autumn - winter 2007 includes size groups that range from 14.5 to 71.5 cm. The percentage share of specimens from each size group in the total abundance is represented on Figure

35 25 20 L, cm % Share (%) Figure 14. Length frequency of turbot population in the Bulgarian Black Sea area. As evident from Figure 14, the highest frequency had the size groups from 44.5 to 56.5 cm, while specimens with lengths over 62.5 cm were only few. Specimens with length below the minimum allowable by the FAA (2005) constitute % of the total abundance of caught turbots, which suggests that fishing by bottom trawling catch over 30% from immature individuals, decreasing the turbot stocks. On the Figure 15 is presented the size structure of catches during all studies, carried out in 2006 and The figure shows, that the maximum of caught individuals was shifted to the senior size classes during the time. This is sign for relatively recovery of size structure of turbot population along Bulgarian coast. 34

36 Spring 2006 Autumn - Winter 2006 Spring 2007 Autumn - winter Share (%) Length (cm) Figure 15. Length frequency of turbot along the Bulgarian coast during the period The size structure of turbot catches in the study fields is given on Figure 16. The size structure was analysed in accord with the regulations determined by the FAA (2005) individuals with absolute length bellow 45 cm are treated as undersized and those longer than 45 cm as standard. Diameter of circles reflects the total abundance of turbot caught in the respective field. According to the data, presented on Figures 14, 15 and 16 is obvious that during the present study, the abundance of undersized individuals of turbot is the lowest in comparison with preceding studies. The higher abundance of small turbots was examined in the Northern area in front of Shabla, Kavarna and Varna at depths between m. 35

37 Figure 16. Size structure (according to FAA, 2005) and abundance of turbot in survey fields, autumn - winter Circle diameter corresponds to total abundance. 36

38 In the Southern region, the undersized individuals have higher abundance in the area in front of Burgas Bay at depths between 50 and 75 m. The figure demonstrates tendency of decreasing number of undersized fish from north to south direction. In the Northern region the abundance ratio undersized:standard specimens is % and %. In the Southern area the ratio is respectively % and %, such as the lower share of small specimens was observed in comparison with spring study The Figure 16 shows, that higher abundance of turbot catches was achieved in the Northern area at depths between 20 and 75 m, such as trend of abundance decrease was manifested to the south Sex ratio The sex ratio of turbot catches in the Bulgarian Black Sea area in autumn - winter 2007 was characterised by following ratio between female and male individuals - respectively 49.45% and %, such as for 9.34 % of the individuals the gender was not possible to be determined, due to their immaturity and release back to the sea Figure 17. Unidentifyed 9.34% Sex ratio M 41.21% F 49.45% Figure 17. Sex ratio of turbot. 37

39 The distribution of the two genders by size groups represented on Figure 18 shows that for the sizes cm gender is difficult to be identified, due to their release to the sea. In size classes from 41.5 to 47.5 cm, the male individuals predominate over the female. From size class 50.5 cm up to 56.5 cm, the share of female individuals increase and the specimens longer than 59.5 cm are exclusively female. The results suggest that females grow to larger length and respectively weight than males. Length class (cm) Sex ratio Unidentifyed Male Female Share (%) Figure 18. Length frequency of turbot by gender Age composition and growth The age composition of turbot catches in the Bulgarian Black Sea area in autumn - winter 2007 encompassed 1+, 2+, 3+, 4+, 5+, 6+ and 7+ -years old individuals. The age structure was dominated by individuals years old which altogether represented % of the turbot 38

40 abundance caught during the study Figure 19. The share of immature individuals of age at 1+ and 2 + years old is around % and individuals of age 5+ and elder accounts for % of the total abundance. Individuals senior than 7+ years, did not occur in the catches Share (%) Age (years) Figure 19. Age structure of turbot. To calculate turbot growth rate during autumn - winter 2007 the data for the average length and weight by age groups for each separate gender were used. The values of the parameters in von Bertalanffy growth function, which describe the linear and weight growth, are given in Table 7. Table 7. Values of parameters in VBGF for both genders. Parameters M F L (cm) k t q n W (kg) k t

41 The growth rate was calculated separately for each gender, resulting in asymptotic length and weight of females higher than those of males. For both genders, the length-weight relationship is negatively alometric, i.e. the weight is lower than the corresponding to the given length. The linear and weight growth of both genders in autumn - winter 2007 at age years old is according to the relationship represented on Figure 20 and Figure 21. Length growth Length (cm) Age (years) M F Figure 20. Length growth of turbot by ages. 9 Tissue growth 8 7 M F 6 Weight (kg) Age (years) Figure 21. Weight growth of turbot by ages. 40

42 According to the results illustrated on the above figures (Figures 20 and 21), it is evident that the individuals from both genders grow with similar rate in length and weight until the age of 4 years, and subsequently the females have significantly higher growth rate Fulton s condition factor The Fulton s condition factor was calculated, used as a measure of the physiological state of the fish. Figures 22 and 23 represent the changes in Fulton s condition factor with age and with length respectively for the two genders during the autumn winter, Fulton's Condition Factor (К) M F 5 К Age (years) Figure 22. Relationship between Fulton s condition factor and age of turbot. It is evident from the Figure 22 that Fulton s condition factor (К) for male and female turbot decreases with age. For male individuals, the values of Fulton s condition factor are lower in comparison with female turbots. 41

43 Fulton's Condition Factor (К) M F 4 К Length (cm) Figure 23. Relationship between Fulton s condition factor and length of turbot. Fulton s condition factor decreases with growth of absolute length for both genders Natural mortality coefficient (М) The natural mortality coefficient of turbot is calculated by the Pauly s empirical equation (1980), which describes the natural mortality as a function of k, L, W and the ambient temperature (T). The temperature measured during the study was 6.1 о С. The results obtained for the natural mortality coefficient of the two genders are listed as follows: From the parameters in the equation describing the length growth Male М = Female М = Total for both genders М =

44 From the parameters in the equation describing the weight growth Male М = Female М = Total for both genders М = Diet composition The diet composition was determined from stomachs fixed in 4 % formalin. The food components were divided into taxonomic groups molluscs, crustaceans and fish and identified to the lowest possible taxonomic level. For each component the share in the total abundance and biomass was determined and the occurrence frequency was calculated. The most important diet component was determined according to the index of relative importance (IRI). The obtained results for the investigated area are given in Table 8. Table 8. Diet composition of turbot along the Bulgarian Black Sea coast during the autumn winter, Species C N C W F IRI %IRI Mollusks Mytilus galloprovincialis Crustaceans Upogebia pusilla Polybius navigator Polybius vernalis Crustacea semi-digested Fishes Merlangius merlangus N.melanostomus Gobiidae Mullus barbatus Sprattus sprattus Pisces semi-digested The diet composition of turbot from Bulgarian Black Sea coast during autumn - winter 2007, was determined from 86 stomachs, out of which % were empty. During the autumn winter study, the shallow waters were covered, which enable investigations on turbot diet in these depths. Owing to this, we determine that in depths m, turbot feeds mainly on crustaceans. 43

45 The number of identified species in turbot stomachs is, as follows: mollusks 1, crustaceans 3 and fishes 4. For the part of family Gobiidae representatives, the species identity was impossible to be determined due to high digestion stage. The observations show that during autumn winter 2007, the dominant food component are fishes, mainly whiting and gobies Table 8, which frequency of occurrence were % and %, respectively. According to the obtained results Table 8, the share of different food components by number and biomass was presented on Figures 24 and 25. Mollusks 5.22% Crustaceans 20% Fishes 74.78% Figure 24. Turbot food items percentage in total number during the autumn winter, Mollusks 7.17% Crustaceans 6.41% Fishes 86.43% Figure 25. Turbot food items percentage in total weight during the autumn winter,

46 From Table 8 and Figures 24 and 25 is obvious, that during autumn winter season turbot feeded mainly on fish, which constitutes % of food items abundance and % of food biomass. The second important food components were the crustaceans, which constitute 20 % of food abundance and 6.41 % of food biomass. The mollusks were the second important food component, according to the biomass, with share of 7.17 %, respectively. According to the obtained results (Table 8), the most important food component as turbot food are whiting IRI = and gobies (IRI = ). Representatives of mollusks and crustaceans have approximately the same significance. From three taxonomic groups mollusks, crustaceans and fishes, which were identified in turbot stomachs, the fishes are of dominant importance with % of turbot food composition. Correspondingly crustaceans have share of 2.21 % and mollusks of 0.62 % of turbot food composition. Therefore, during autumn - winter, the turbot diet ration includes mainly fish, such as mollusks and crustaceans were observed in stomachs of turbots, which inhabit shallow areas. In the stomachs of many inspected fishes parasites were encountered, mainly nematodes Species composition of by-catch Demersal fishes which were caught in trawl nets as a by-catch of turbot along the Bulgarian Black Sea coast are listed as follows: Squalus acanthias Linnaeus, 1758 Piked dogfish Raja clavata Linnaeus, 1758 Thornback ray Dasyatis pastinaca (Linnaeus, 1758) Common stingray Merlangius merlangus (Linnaeus, 1758) Black Sea whiting Hippocampus guttulatus Cuvier, 1829 Long-snouted seahorse Scorpaena porcus Linnaeus, 1758 Black scorpionfish Mullus barbatus Linnaeus, 1758 Red mullet Neogobius melanostomus (Pallas, 1814) - Round goby 45

47 Picture 3. Common stingray Picture 4. Red mullets 46

48 Picture 5. Round gobies 6 Conclusions and recommendations In agreement with the results of the survey carried out in autumn winter, 2007 for turbot stock assessment by the swept area method the following conclusions and recommendation were made: The exploitation biomass of turbot in the Northern Bulgarian Black Sea region was estimated at tons for the entire population and at tons for the standard size part of the population. The exploitation biomass of turbot in the Southern Bulgarian Black Sea region was estimated at tons for the entire population and at tons for the standard size part of the population. Turbot stocks in the Northern and Southern regions have commensurable sizes. The turbot has a patchy distribution on the Bulgarian shelf with most abundant assemblages recorded in front of profiles of 47