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The current state of the turbot, Scophthalmus maximus
(Linnaeus, 1758), population in the northwestern part of the
Black Sea
Bohdan Hulak, Yevhen Leonchyk, Volodia Maximov, George Tiganov, Vladislav
Shlyakhov, Mikhail Pyatnitsky
Received – 02 January 2021/Accepted – 01 September 2021. Published online: 30 September 2021; ©Inland Fisheries Institute in Olsztyn, Poland
Citation: Hulak B., Leonchyk Y., Maximov V., Tiganov G., Shlyakhov V., Pyatnitsky M. (2021). The current state of the turbot,
Scophthalmus maximus (Linnaeus, 1758), population in the northwestern part of the Black Sea. Fisheries Aquatic & Life 29: 164-175.
Abstract. Turbot is one of the most valuable fish species in
the Black Sea commercial fishery. The serious, dramatic
depletion of the turbot stock and catches that began in the
1980s was caused by overfishing and poor ecological
conditions. The state of the turbot stock began to improve in
the northwestern part of the Black Sea in 2016. Landings were
at their 30-year maximum in this part of the sea. Catches per
unit effort (CPUEs) have been at a stable, high level there for
the last few years. Average turbot weight and length have also
been increasing. The Stock Synthesis (SS3) framework was
used in the stock assessment. According to SS3 analysis,
fishing mortality (F) reached the minimum level of 0.29 in
2018. The cumulative SPR (Spawner Potential Rate) index
was 0.27, which approximately equaled SPRMSY = 0.25. Thus,
currently the turbot stock is mostly moderately exploited at
a level close to the management target in the northwestern
part of the Black Sea. However, the entire Black Sea
population has not fully recovered yet.
Keywords: stock assessment, Black Sea, turbot, Stock
Synthesis, overfishing
Introduction
The Black Sea turbot, Scophthalmus maximus L., is
a bottom-dwelling predator. It prefers sandy and
silty-sandy bottoms. It does not make long migra-
tions along the marine shelf zone and is character-
ized by local movement associated only with feeding
and reproduction. Long-term observations during
trawl surveys showed that the distribution of turbot
along the offshore area is uneven and depends
largely on the width of the shelf zone (Nadolinskiy et
al. 2018). Its maximum abundance and distribution
density are confined to areas with a broad shelf. Ac-
cording to tagging and morphometric data, Black Sea
turbot forms several local populations, or
subpopulations (Popova 1954, Karapetkova 1964,
1980, Nadolinskiy et al. 2018). The geographical fea-
tures of the broadest shelf zone in the northwestern
part of the Black Sea and minimal turbot migration
FISHERIES & AQUATIC LIFE (2021) 29: 164 - 175
Archives of Polish Fisheries
DOI 10.2478/aopf-2021-0018
RESEARCH ARTICLE
©Copyright by Stanis³aw Sakowicz Inland Fisheries Institute in Olsztyn.
©2021 Author(s). This is an open access article licensed under the Creative Commons Attribution-NonCommercial-NoDerivs License
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
B. Hulak, Y. Leonchyk [+]
S. E. Odessa Center of Southern Research Institute of Marine
Fisheries and Oceanography (SE Odessa Center YugNIRO), 12
Shevchenko av.,65058, Odessa, Ukraine
E-mail: leonchik@ukr.net
V. Maximov, G. Tiganov
National Institute for Marine Research and Development “Grigore
Antipa” (NIMRD), 300 Mamaia Blvd. 900581 Constanta, Romania
V. Shlyakhov, M. Pyatnitsky
The Azov-Black Sea branch of FSBI VNIRO “AzNIIRKH”, 21b
Beregovaya str., 344002, Rostov on Don, Russia
activity contribute to the isolation of this fish popula-
tion. Based on the morphological features of the shelf
and the continental slope, oceanographers deter-
mined the western border of the northwestern Black
Sea region as Cape Kaliakra and the eastern one as
Cape Khersones (Dubinets et al. 1988). Obviously,
narrowings of the shelf near Cape Kaliakra and on
the section of Cape Khersones – Cape Sarych create
natural barriers that prevent turbot from resettling.
In addition, migrations of demersal fishes are hin-
dered by an area filled with hydrogen sulfide at
depths of more than 120 meters throughout the
Black Sea. Accordingly, the turbot population of the
northwestern part could be considered as a separate
stock unit isolated in the largest shelf area
(Shlyakhov 2014). Turbot tagging and trawl surveys
carried out by YugNIRO in the northwestern part of
the Black Sea in the 1980s confirmed the locality of
a so-called western stock unit of this fish (Efimov et
al. 1989). The presence of this isolated substock in
the Black Sea turbot population is also supported by
results of the most recent genetic studies. The fish in-
habiting the northern part of the Sea have significant
genetic differences from those inhabiting the south-
eastern and southwestern parts of the marine shelf
(Firidin et al. 2020). Thus, it makes sense to assess
the turbot stock in the northwestern part separately
from the entire Black Sea population.
As turbot is one of the most important species in
the Black Sea commercial fishery, the assessment of
its stock and fishery management has traditionally
attracted particular attention at national and regional
levels. In 2008-2017, assessments of the Black Sea
turbot stock were conducted by the Working Group
of Experts of the Scientific Technical and Economic
Committee on Fisheries (EWG STECF) of the Euro-
pean Commission and in 2017–2018 by the Subre-
gional Group for Black Sea Stock Assessment
(SGSABS) under the auspice of the General Fisheries
Commission for the Mediterranean (GFCM). These
two groups consist mainly of experts from research
institutes and universities of Black Sea countries.
The authors of this article also participated in the
work of these research groups. The input data for the
whole stock assessment were formed by combining
production and biological data from various regions
of the Black Sea. According to these estimations, the
entire Black Sea turbot stock was overfished and se-
verely depleted for a long period. In 2014, fishing
mortality of turbot exceeded FMSY (reference point
for fishing mortality when the stock is exploited at
a Maximum Sustainable Yield level) by five times.
The spawning stock was reduced to an extremely low
level of 1,010 tons in the same year, which was much
lower than the minimum allowed level (Blim) of 3,535
tons (STECF 2015).
However, there were differences in the assess-
ments of the stock status by sea areas. An LCA as-
sessment of the western stock in Ukrainian waters in
1997-2013 found F/FMSY = 0.96›1.77 and desig-
nated the stock status as transitional from fully ex-
ploited to overexploited in that period (Shlyakhov
2014). SAM (Nielsen and Berg 2014) assessments of
the STECF and GFCM working groups for the entire
Black Sea stock were much more pessimistic for this
period (Table 1). Marine regions inhabited by turbot,
which are located on the shelf of the southern part of
the Black Sea, did not show positive signals at all.
Indicators of the entire stock state began to im-
prove slightly in 2016. According to the latest results,
a stable, positive tendency for the turbot stock was
noted throughout the Black Sea (GFCM 2019). The
total spawning stock in 2018 increased compared to
previous years. Its level remained quite low but was
almost 1.5 times higher than the reference point Bpa
= 2295 tons. However, the fishing mortality rate was
0.44 in 2018, which was three times more than FMSY
= 0.14. This stock assessment was also conducted
165 Bohdan Hulak et al.
Table 1
State of the turbot stock and commercial population
parameters in 2015-2017
Years
Indexes 2015 2016 2017
F/FMSY 5.38 4.40 3.10
Bcur/Blim 0.29 0.44 0.57
Bcur/Bpa 0.20 0.33 0.41
Bcur – current spawning stock biomass,
Bpa – spawning stock biomass at the level where fishing does
not affect reproduction.
with SAM analysis. Illegal, unreported, and unregu-
lated (IUU) fishing was recorded in these studies.
Therefore, it should be assumed that the entire tur-
bot population was overfished in spite of the recent
positive tendency in its abundance.
Obviously, the issue of differences in turbot
abundance trends in various areas of the Black Sea is
of great importance and requires special studies.
Currently, we have the material that allows us to
identify distinctive features of the abundance dynam-
ics of this species in the most important area of its
habitat and fishing, which is the northwestern part of
the Black Sea.
Materials and methods
Fish samples were collected from gillnet and trawl
catches. Gillnets with mesh sizes of 180–200 mm
(knot to knot) are the traditional gears for this species
in commercial fishery. All trawl hauls were con-
ducted during special trawl surveys. The total num-
ber of fish examined during the period of 2002-2018
was 5,500 individuals. Each individual was
measured from the beginning of the snout to the end
of the rays of the caudal fin and weighed. Otoliths
were collected from 2,800 individuals for age read-
ing. The sexual maturity of fish was also evaluated.
During 2008-2018, a Romanian demersal fish
survey was conducted regularly for the turbot stock
and other demersal fish assessments using the holis-
tic trawl survey method (surface method) that can be
applied to restricted areas. Vessel speed and horizon-
tal opening of the trawl were taken in consideration
as survey parameters. The characteristics of the bot-
tom trawl were 2/27-34 m; the horizontal trawl
opening was 13 m; the vertical trawl opening was
2 m. Trawling was conducted in spring and fall. The
total number of trawlings done in one year was ap-
proximately 80 (40 in spring and 40 in fall). The dis-
tribution of the hauls is presented in figure 1.
Stations were positioned in order to cover the entire
Romanian Black Sea shelf habitats and depths. The
total time of each haul was 60 min and the trawling
speed was between 1.7-2.2 knots. Catches ranged
from 10 to 580 kg.
Individual average fish parameters (length,
weight, maturity) along with catch at age data were
used for population dynamics and fishing impact
The current state of the turbot, Scophthalmus maximus (Linnaeus, 1758), population in... 166
Figure 1. Distribution of trawling stations in the spring (a) and fall (b) seasons in the Romanian area.
modeling. The results of the trawl surveys and CPUE
data were used for model tuning. The share of IUU
fishing by countries was estimated and included in
the catches according to the STECF method (STECF
2014).
Stock Synthesis (SS3) was applied for the turbot
stock assessment in the northwestern part of the
Black Sea. This package provides a statistical frame-
work for calibrating population dynamics models us-
ing fishery and survey data (Methot and Wetzel
2013). In contrast to virtual population analysis, SS3
uses the forward projection of populations in the sta-
tistical catch-at-age (SCAA) approach and allows for
errors in catch at age matrices. Stock spawning bio-
mass (SSB), recruitment, and fishing mortality were
evaluated with the SS3 model, in which selectivity
was cast as age-specific only.
There is no concept of a single annual Fin SS3
since this rate varies by age, sex, season, and area.
Moreover, it is misleading if there are large differ-
ences in fleets or, even worse, if they operate in differ-
ent areas. In addition, an average Fdepends on the
calculation method chosen. Because of the inferiority
and uncertainty of the Fcriterion within the frame-
work of this model, it was also decided to use the pa-
rameter of the reproductive capacity of the
population provided by SS3. This parameter, the
Spawner Potential Rate (SPR), is a single measure of
the equilibrium effect of fishing. SPR is defined as the
proportion of the unfished reproductive potential left
in the population at any given level of fishing pres-
sure (Goodyear 1993). It can be used for setting the
target and limiting reference points in fisheries.
SPRMSY is the main reference point instead of FMSY.
So, SPR can be calculated as the ratio of the equilib-
rium reproductive yield per recruit that would occur
with the current year’s fishing effort and biological
parameters of fish to the equilibrium reproductive
yield per recruit that would occur with the same bio-
logical parameters without fishing activity. It ranges
between 0 and 1, with a value of 1 representing an
unexploited stock. Therefore, the status of a stock
can be classified into three different groups which are
under- (SPR > 0.4), moderately (0.2 < SPR < 0.4) and
over- (SPR < 0.2) exploited.
Natural mortality at age was fixed at independ-
ently obtained values. The Beverton-Holt relation-
ship was taken for recruitment assessment. The
high flexibility of SS3 allowed covering various data
from commercial and survey catches: landings,
abundance indices, catch-at-age, weight-at-age,
and maturity proportions. This model contains
subcomponents that simulate population dynamics
of stocks and fisheries, derive the expected values
for various observed data, and quantify the magni-
tude of differences between observed and expected
data. A feature such as a multi-fleet approach pro-
vides the spatial and fishing gear differences in this
assessment. The integrated abilities of SS3 for ob-
taining reference points and forecasting allowed
evaluating the current status and future prospects
of the turbot stock in the northwestern part of the
Black Sea. The quality of the assessment was vali-
dated through integrated model tools. The surplus
production model Combi 4 (Babayan et al. 2018)
was also applied in the assessment to compare it
with the results of SS3.
Results and discussion
Fishing statistics and catch composition
Very specific turbot population dynamics were ob-
served in the northwestern part of the Black Sea. At
first, there was an increase in turbot catches in this
region after 2000 (Fig. 2). This was because of im-
provements in recording catches, which were imple-
mented after Ukrainian enterprises began exporting
turbot. After 2012, this positive trend was replaced
by a drop in catches similar to the general recession
in the Black Sea, and the approach of fish to the
coastal zone of the northwestern part of the sea de-
clined significantly. However, the situation in this re-
gion, including the largest shelf zone in the Ukrainian
waters, started to change since 2016. Catches stabi-
lized significantly at their highest levels in the north-
western part of the Black Sea for the last three
decades (Hulak et al. 2019). Simultaneously, there
167 Bohdan Hulak et al.
was a pronounced tendency for an increase in turbot
abundance in 2015-2018. This increase indicated
directly the ongoing fast recovery of the western stock
since the changes were not because of the intensifica-
tion of fishing activities but to increases in catch per
unit effort (CPUE). CPUE was 12 kg/net in 2015, 45
kg/net in 2016, 51.8 kg/net in 2017, and 45.3 kg/net
in 2018 in Ukrainian marine zone. This positive
trend after 2015 was also confirmed by data from
trawl surveys conducted by Romanian scientists
(Maximov et al. 2017, 2018). Landing increases
mainly occurred at the expense of fishing in Ukraine
including the Crimea area. In Romania, landings also
increased, but they were not as significant because of
EU restrictions.
The assessment of turbot stock in the northwest-
ern part of the Black Sea using the CC3 model was
based on Ukrainian, Romanian, and Russian landing
data (Fig. 2) and tuning indices since 2002. Prior to
2002, Turkish vessels were known to fish illegally in
the territorial waters of Ukraine, Romania, and Bul-
garia. Their catches were landed in Turkey and re-
ported in Turkish landing statistics. By 2001,
authorities in Ukraine, Romania, and Bulgaria had
intensified efforts to stop illegal Turkish fishers.
These actions even resulted in a sudden drop in
Turkish landings in 2002 (Fig. 3). Fisheries data col-
lection in the northern part of the sea started to im-
prove from 2002.
Biological analyses have shown that, since 2016,
the number of individuals has increased in catches
The current state of the turbot, Scophthalmus maximus (Linnaeus, 1758), population in... 168
Figure 2. Official landings of turbot in the northwestern part of the Black Sea.
Figure 3. Long-term dynamics of turbot landings in the Black Sea (entire basin), tons (GFCM, 2019).
mainly because of older age groups. There was an in-
crease in age groups 6+ and older (Fig. 4). In addi-
tion, the same trend was observed for the length
groups of turbot. An increase in the number of indi-
viduals was noted starting from 55 cm. The average
weights at age and corresponding length groups also
increased. Obviously, this is a positive trend indicat-
ing that, recently, the turbot stock might have been
exploited sustainably in the northwestern part of the
Black Sea.
Stock assessment results
The vectors of natural mortality and the proportion of
mature individuals were taken from the EWG GFCM
for the entire Black Sea turbot stock assessment
(GFCM 2019) (Table 2). The assessment results were
interpreted as providing information on stock devel-
opment up to January 1, 2019, i.e., not for 2019. The
SSB in the northwestern part of the sea showed
a generally increasing trend after a drop in 2002 with
a peak of 6.4*103tons in 2019 and the virgin SSB
was estimated at 29.3*103tons (Fig. 5, left panel).
169 Bohdan Hulak et al.
Table 2
Natural mortality and mature proportion vectors
Vectors
Age groups
0+ 1+ 2+ 3+ 4+ 5+ 6+ 7+ 8+ 9+ 10+
Natural mortality 0.47 0.34 0.28 0.24 0.22 0.20 0.19 0.18 0.17 0.16 0.16
Maturity 0.0 0.0 0.0 0.43 0.69 1.0 1.0 1.0 1.0 1.0 1.0
Figure 4. Age frequency distribution of turbot in the northwestern part of the Black Sea.
Figure 5. Spawning Stock Biomass, SSB (tons) forecast with 95% asymptotic intervals (left panel) and age-0 recruitment forecast with
95% asymptotic intervals (right panel).
However, SSB will go down after 2019 according to
this forecast. Here, and throughout the forecast, this
was done with the assumption that F=F2016-2018 =
0.366. Recruitment was estimated as the number of
age-0 fish related to the spawning biomass in accor-
dance with Beverton-Holt stock-recruitment rela-
tionship. It reached a maximum of 5.6*106ind. in
2015 and decreased to 0.4*106ind. in 2017 (Fig. 5,
right panel). The SS3 estimated a significant drop in
recruitment in 2016–2018. However, the situation
was forecast to improve from 2019.
Fishing mortality Fwas calculated considering
ages 4 to 8 (Fig. 6, left panel). Fishing mortality
showed a fluctuating trend with local peaks observed
in 2008, 2013, and 2016 with the minimum value of
0.293 in 2018. According to SS3 estimations for the
turbot stock in the northwestern Black Sea, SPRMSY
= 0.25. The SPR index showed a drop to 0.17 in
2016 and reached a maximum of 0.27 in 2018. In
the other years of the observed period, this value was
close to the SPRMSY level (Fig. 6, right panel). Thus,
recently, the turbot stock was mostly moderately ex-
ploited.
According to SPRMSY, the target turbot catch in
the northwestern part of the Black Sea is 836 tons
with an upper limit of 935 tons as the MSY value. Al-
though the actual total landings of turbot (Fig. 2)
were below these recommended values in recent
years, the commercial exploitation of this stock
should not be increased because of IUU catches.
These IUU catches of Black Sea turbot were
comparable with official ones in recent years and
were previously substantially higher (GFCM 2019).
In addition, it should take into account that the total
turbot stock is overexploited throughout the Black
Sea. So, fishing at a level below FMSY would facilitate
the recovery of the Black Sea turbot population.
Quality of the assessment
A retrospective analysis of the results showed that
the applied model is quite stable even though IUU
catches were estimated. The values of Mohn’s
rho-index (the average relative bias of retrospective
estimates) (Mohn 1999) were -0.060 for SSB and
0.083 for Fover the last three years. The age fre-
quency distributions (AFDs) aggregated across time
were reconstructed quite well by the model for both
fishing fleets and surveys. The double normal distri-
bution with defined initial and final levels (type
selex24 in SS3) was applied to fit all AFDs. The
Pearson residuals for the fishing fleets and surveys
were quite low (mostly between -2 and 2) and were
without patterns (Fig. 7 and 8); although, it is often
difficult to pinpoint the cause of residuals that have
substantial patterns. Usually, the two potential rea-
sons for these residuals are unrealistic starting val-
ues and poor model specification. We managed to
avoid these problems in this study. However, the fit-
ting presented some inconsistency for surveys pro-
vided by Romania, where some drops could not be
described well by the model (Fig. 8). The fitting also
The current state of the turbot, Scophthalmus maximus (Linnaeus, 1758), population in... 170
Figure 6. Total fishing mortality Fwith 95% asymptotic intervals (left panel) and Spawner Potential Rate with 95% confidence intervals
(right panel).
171 Bohdan Hulak et al.
Figure 7. Pearson’s residuals for commercial fishery data.
Figure 8. Pearson’s residuals for trawl surveys.
presented some inconsistency for the observed and
expected CPUE indices. Most peaks and drops on the
chart were poorly described by the model (Fig. 9).
The biomass estimation obtained by the surplus pro-
duction model Combi 4 was consistent with SS3 re-
sults (Fig. 10). This model also indicated that turbot
biomass has been increasing in the northwestern part
of the sea since 2016.
Factors determining stock dynamics
To understand the current situation of the turbot
stock, we refer to the long-term landing dynamics of
this species. In the 1980s, catches of all Black Sea
countries dropped significantly for the first time. Ac-
cording to statistics, this was preceded by a sharp
increase in turbot landings. Almost all of these
catches were generated by the Turkish fleet (Fig. 3).
Turkish landing reached a maximum of 5216 tons in
1983. Moreover, small Turkish vessels equipped
with multi-kilometer nets began to deploy their gears
on the wide northern shelf of the Black Sea, where
this fish was much more abundant than in Turkish
waters (Acara 1985). Even the introduction of eco-
nomic zones in the Black Sea did not immediately
stop the Turkish fleet, which frequently fished the
economic zones and even the territorial waters of
neighboring counties (Öztürk 2013). This could not
but have affected the turbot population. Considering
the critical state of the fish stock, the USSR intro-
duced a 10-year ban on turbot fishing in 1981. How-
ever, the main fishery country of the Black Sea,
The current state of the turbot, Scophthalmus maximus (Linnaeus, 1758), population in... 172
Figure 9. Fitting for Ukrainian CPUE series – Crimea (left panel) and mainland (right panel).
Figure 10. Pearson’s correlation test for Spawning Stock Biomass, SSB, estimated with Stock Synthesis, SS3, and Combi.
Turkey, did not support these restrictions. Cases of
poaching continued in Ukrainian waters. Catches be-
gan to fall again. For this reason, EU countries and
Ukraine strengthened the protection of their waters
from penetration by Turkish poachers. Since 2002,
after strengthening control of Ukrainian, Romanian,
and Bulgarian waters, Turkish fishermen were
forced to harvest turbot only in their country’s zone,
and this is when the downward trend in turbot
catches in Turkey started (Fig. 3).
Whereas previously Turkish landing data in-
cluded catches in waters of other countries, the previ-
ous low values of landings in Ukraine and Russia
were largely because of the discrepancy between real
catches and official statistics. The share of IUU
catches in northern part of the Black Sea exceeded
official data by more than five times for a long time.
Only since 2000 has there been an increase in Ukrai-
nian landings associated with an improvement in
catch recording along with the development of the
fish export business. Currently, the number of unreg-
istered catches in the northwestern part of the Black
Sea has diminished, and official data reflect the state
of the population much better.
Obviously, the poor state of the turbot population
in the southern part of the Black Sea is because of its
over-exploitation in the absence of any catch regula-
tions in Turkish waters. The legislation of this coun-
try does not provide for the any catch limit or quota
allocation among fishers. The bottom trawls used for
fishing in countries located in the southern part of the
Black Sea also affect the turbot stock negatively. The
greatest impact on the turbot population is exerted
by the active beam trawl fishing of rapa whelk. Ex-
periments with these fishing gears in Ukraine
showed that the by-catch of turbot juveniles was al-
ways large and could lead to decreases in the fish
population (Hulak et al. 2019). It is known that this
method of fishing has been used in Turkey and Bul-
garia for over twenty years, and the number of vessels
deploying this gear exceeds 500 units. Beam trawls
have never been deployed in northern part of the
Black Sea, and the use of bait hooks and nets with
mesh sizes of 70-170 mm is prohibited in Ukraine to
reduce the by-catch of juvenile fish.
It is obvious that differences in the states of tur-
bot stocks across these marine areas is explained by
the fact that the most intensive methods are used and
fishing regulations are insufficient in the southern
part of the sea on the small shelf zone off the coast of
Turkey. At the same time, on the much larger north-
ern shelf of the Black Sea, where restrictions on the
use of bottom fishing gears are in force, turbot has
had the ability to restore its abundance quickly. The
current differences observed in the dynamics of catch
trends in Turkey and other countries also lead to this
conclusion. In Turkey, catches have continued to de-
cline amid recent increases in total catches in the
Black Sea (Fig. 3).
Conclusions
Black Sea turbot was subjected to high overfishing,
resulting in the collapse of this fishery in the 1980s.
The most serious and excessive fishing pressure, in
terms of volume, has been observed for many years in
the southern part of the sea. Fisheries regulations
were insufficient to preserve the turbot stock and
protect juveniles in this region. This negatively af-
fected the general state of the turbot stock in the
Black Sea. At the same time, a notable increase in
turbot abundance has been observed in the north-
western part of the Black Sea over the past several
years. This separate population, or subpopulation, is
characterized by a different abundance dynamic,
which was formed in this marine area because of the
low migration activity of these fish and certain geo-
graphical conditions such as the presence of
a deep-water zone with a significant volume of water
saturated with hydrogen sulfide in the central part of
the sea.
As mathematical modeling in the SS3 framework
showed, turbot is currently mostly moderately ex-
ploited at a level close to the SPR management target
in the northwestern part of the Black Sea. In spite of
the highest landings for the past 30 years, recently
CPUEs have been at a stable, high level in the north-
western marine area. Moreover, fishing mortality
173 Bohdan Hulak et al.
decreased in 2017 and 2018 and reached its mini-
mum level. The spawning biomass has been increas-
ing since 2016. Average turbot weight and length
have also been increasing since 2016. Thus, the sta-
tus of the turbot population is improving here, and
this is positively affecting stock estimates throughout
the Black Sea.
The improved state of the turbot population, or
subpopulation, in the northwestern part of the Black
Sea is ensured by the presence of the largest shallow
water shelf where turbot can breed and feed on vari-
ous small fishes and invertebrates. Measures aimed
at protecting juveniles from by-catch are also better
implemented here. This supports the large difference
between the abundance of this fish in the northern
and southern parts of the Black Sea.
However, in general, the entire Black Sea turbot
stock has not yet fully recovered. Therefore, fishing
mortality should not be increased; all by-catches of
small size specimens with beam trawls and IUU
catches must be minimized to maintain the viability
of this species in the Black Sea.
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