First report of the Chinese sleeper Perccottus glenii Dybowski, 1877 in the Drava River, Croatia

Chinese sleeper ( Perccottus glenii ) is one of the most successful invasive fish species, spreading rapidly to colonize a wide range of ecosystem types, from lentic oxbows to large rivers and freshwater lakes. Previously, only a single individual of P. glenii has been recorded from Croatian waters, in the Sava River catchment. This study reports the presence of additional record and the first population of P. glenii in the Croatian part of the Drava River. During a field survey, eight individuals (n = 8) of P. glenii were collected from a shallow and overgrown freshwater oxbow in the Drava River flood zone, within a NATURA 2000 protected site. Since several individuals were within the mature size class (i.e., larger than 55 mm standard length), the results emphasize the importance of early confirmation of the presence of this invasive species, and the need for comprehensive biological surveillance of the Drava River and similar floodplain habitats, especially downstream of this collection site. Two hypotheses (north and south) explaining the arrival of Chinese sleeper into the Croatian part of Drava River are proposed. Semi-closed oxbows, such as the present one, could represent a suitable ecosystem for detailed in situ experiments on P. glenii ecology and migration patterns.


Introduction
Non-native species, especially invasive ones, pose a serious threat to biodiversity and can cause multiple adverse impacts on the newly inhabited ecosystems.The number of introduced non-native fish worldwide has rapidly increased in recent years, with the introduction rate doubling in the last 30 years (Gozlan et al. 2010;Cucherousset and Olden The Chinese sleeper, Perccottus glenii Dybowski, 1877 (Gobiiformes, Odontobutidae) is a medium sized (max.total length ~ 25 cm; Baensch and Riehl 1991) freshwater invasive fish widely distributed throughout western Eurasia.It has been spreading westwards since the early 20 th century (Elovenko 1981;Reshetnikov 2003Reshetnikov , 2004)).The native distribution range of P. glenii extends from the inland Far East of Russia (Berg 1949), northeast China (Zhu 1995) and North Korea (Mori 1936), including the Amur River drainage and surrounding adjoining tributaries (Reshetnikov 2010).However, over the past 50 years, this highly invasive fish species has proliferated throughout Europe, and is presently documented in numerous European countries (Reshetnikov and Ficetola 2011;Reshetnikov 2010Reshetnikov , 2013;;Nehring and Steinhof 2015;Kvach et al. 2016).The westernmost record of P. glenii in freshwater ecosystems in Europe to date is in the Upper Danube (Bavaria), Germany (Reshetnikov and Schliewen 2013;Nehring and Steinhof 2015), raising major scientific concerns over further western proliferation.Reshetnikov (2004Reshetnikov ( , 2013) ) discussed the penetration patterns and historical centers of distribution of P. glenii outside of its natural range, emphasizing that localities from the Far East of Russia to more western regions should be considered the historical centers of distribution during the introduction period , and potential secondary invasive sources to several countries in Central Europe.Specifically, around the 1960s, P. glenii was introduced to western Ukraine (Lviv region) during cyprinid fish stocking.This was likely unintentional, but from there, it has spread widely westwards via well-established freshwater networks (Reshetnikov 2013).Likewise, Grabowska et al. (2020) suggested that, according to a population genetic structure analysis performed on the cyt b genetic marker, European populations of P. glenii consist of at least three distinct haplogroups (i.e., Baltic, East-European and Carpathian), which could indicate separate introduction events from different parts of the native range and subsequent multidirectional pathways to Central Europe.Other authors (Dmitriev 1971;Reshetnikov 2013;Grabowska et al. 2020;Kvach et al. 2021) have proposed two hypothetical ecological invasion pathways of freshwaters by P. glenii, either via an inland freshwater aquatic network (primary source), or from oligohaline waters such as coastal brackish ecosystems (secondary source), as suggested for the south-eastern Baltic Gulf of Finland.Reshetnikov (2003Reshetnikov ( , 2013) ) further emphasized that anthropogenic activities, such as fish farming, fish stocking, and sport angling, play a prominent role in the introduction of P. glenii into isolated and geographically distant waterbodies.
Throughout its range, P. glenii occupies natural and artificial stagnate and lentic eutrophic backwaters (oxbows, floodplains, swamps and ponds) rich in aquatic vegetation with a silty substrate (Nikolskii 1956;Grabowska et al. 2011;Reshetnikov 2013;Rechulicz et al. 2015).Perccottus glenii is rarely caught from a main riverine channel or fast-flowing water currents, although it can temporarily inhabit such habitats, possibly during the invasion period via active or passive dispersal migration (Reshetnikov 2013).It is an opportunistic predatory fish, feeding voraciously on a wide variety of vertebrates (fish, tadpoles and newts) and small benthic or planktonic invertebrates (larvae of Insecta, Oligochaeta or Mollusca), meaning that it competes with the native fish species for the same food resources (Koščo 2003;Reshetnikov 2001Reshetnikov , 2003)).During the reproduction period, males are highly territorial and exhibit aggressive behavior towards male intruders, while females are attracted to the spawning ground (i.e.nest) by a suite of behavioral and acoustic cues (Horvatić et al. 2019).Evidently, P. glenii is capable of drastically affecting the trophic structures of freshwaters, perhaps leading to a decrease in abundance or even local extinction of certain native species.Due to its ecological characteristics and invasive nature, particularly its broad ecological tolerance, opportunistic generalist feeding strategy and aggressive behavior, P. glenii is considered the perfect conqueror (Ćaleta et al. 2010).
Reshetnikov ( 2013) discussed the possible migration routes of invasive P. glenii into European freshwaters, highlighting West Ukraine as the West-European center of distribution and a crucial spreading "hotspot" from which P. glenii migrated into many different European watersheds.The author suggested that between 2005 and 2013 P. glenii likely invaded the Dniester, Vistula, Dnieper, Bug, and Danube catchments from the West Ukraine, using rivers as natural passages to rapidly proliferate downstream from riverheads to river mouths, but also to slowly migrate upstream into the adjoining tributaries (Reshetnikov 2013).Other studies have highlighted two important natural corridors providing a potential gateway from colonized areas in Eastern and Central Europe into Western Europe: 1) northern corridor -waters of Poland and Germany, characterized by having many artificial inter-river connecting channels, and 2) southern corridor -Danube Basin including Croatia and Slovenia, and the watersheds of northern Italy and southern France (Leuven et al. 2009;Reshetnikov and Ficetola 2011).Based on bioclimatic analysis, the mountainous regions of Europe (Dinaric, Alpine and Carpathian Mountain ranges) constitute a barrier to the spread of P. glenii (Reshetnikov and Ficetola 2011).Furthermore, Reshetnikov (2013) highlighted the Transcarpathian region (in West Ukraine) as a source of downstream colonization of the Danube Basin by P. glenii since this region is considered a natural spring area of streams and rivers of the Danube Basin.From there, the expansion of P. glenii to the middle Tisza happened via three rivers: Uzh, Latorica and the upper Tisza (Reshetnikov 2013).
Chronologically, the first record of P. glenii in Balkan waters (Danube Basin) was from the Tisza River (Hungary) in 1997, after a southward expansion of its initial range in the West-Ukrainian subrange (Harka 1998;Reshetnikov 2013).In 2001, it was reported as present in Romania (Nalbant et al. 2004) and Serbia (Gergely and Tucakov 2003), in Bulgaria in 2005 ( Jurajda et al. 2006), and in the Sava River, Croatia in 2008 (Ćaleta et al. 2010).By 2010, P. glenii had already acclimatized throughout the Tisza River and proliferated further southward through the Danube Basin, likely facilitated by flooding events, to colonize the adjoining tributaries, such as the Sava River in Croatia (Ćaleta et al. 2010;Reshetnikov 2013).Until 2010, P. glenii also established stable populations in Lake Balaton, which is artificially connected with the Danube River (Eros et al. 2008;Harka et al. 2008;Antal et al. 2009).
Perccottus glenii is considered the most widespread (Reshetnikov 2010;Reshetnikov and Ficetola 2011) and highly successful invasive fish species in European freshwaters (Copp et al. 2005).It has been included on the European Union Concern List of invasive alien species (European Commission 2016), and was assessed as having a high risk of invasiveness for Croatian waters (Piria et al. 2016).
Since the first record of P. glenii in Croatia, when a single adult was caught by angler from a well-vegetated fishpond channel (Ćaleta et al. 2010), extensive ichthyological field surveys have been carried out in Croatian artificial (reservoirs, channels or irrigations waterways) and natural (lakes, ponds, rivers) freshwaters, but without positive confirmation of the presence of P. glenii.This study reports a new record of its presence, the first record in the Drava River and the first recorded population of P. glenii in Croatia (Figure 1).Individual body measurements (morphometric measurements and meristic characteristics) are reported here for the purposes of future comparison with possible neighbouring or other European populations.Possible migration routes of P. glenii to Croatian waters are discussed in light of the present finding.

Materials and methods
A shallow Drava River backwater (oxbow lake) (45°42′3,45″N; 18°26′37,17″E), situated near the City of Belišće, and with a total area of 2400 m 2 (0.24 ha) (Figures 2, 3) was sampled twice during field studies, on 30 July 2020 and 01 April 2021.The sampling was conducted by boat using electrofishing equipment with direct continuous current (Hans Grassl model EL64 IIGI, 7.5 kW) in combination with hand nets for sample collection.Individuals of P. glenii were caught by a hand-held anode from the oxbow, near the shore at a depth of around 0.5 meters.The oxbow, measuring 35 × 80 m, is enclosed on the north side by a river embankment and it is situated within the Drava flooding zone (approximately 500 m from the main river channel).The aquatic and riparian vegetation (predominately, common reed, Phragmites australis and yellow iris, Iris pseudacorus) is well-developed and dense, and the substrate is mud-silt (Figure 3).Physicochemical characteristics of the water were measured with a two-channeled HQ40D Portable Meter (Hach®, Colorado, USA).We measured water temperature (°C), oxygen saturation (in %), dissolved oxygen (mg O 2 /L), conductivity ( µS/cm), pH , total dissolved solids (TDS; mg/L), available from the portable HQ40D to characterize the waterbody from which P. glenii was sampled.The water transparency was measured by using Secchi disc.The sampling site and fish specimens were photographed with a Canon camera.All specimens were euthanized with an overdose of MS-222 and were preserved in absolute solution of ethyl alcohol for further laboratory analyses.The sampling site is part of the European ecological network NATURA 2000 (site code: HR2001308 Lower course of the Drava River) and also lies within the Mura-Drava Regional Park protected area.Species determination was based on morphological descriptions and taxonomic keys (Miller 2003;Kottelat and Freyhof 2007).The standard length (SL, mm), total length (TL, mm) and weight (W, g) of each fish was measured in the field using a ruler (1 mm precision) and digital scale (0.1 g precision), respectively.In the laboratory, 30 morphometric measurements were recorded for each fish using digital callipers CD-15APX with precision of 0.01 mm (Mitutoyo, Japan).Meristic counts (eight traits) were taken from the left lateral side of the body, using a magnifying glass (Zeiss Stemi 2000-C).Morphometric and meristic measurements generally followed the description by Hubbs and Lagler (1947) and Nikolić et al. (2021).In order to group individuals according to the maturation size (immature < 55 mm SL; mature > 55 mm SL), we followed Miller (2003).One individual was deposited in the collection of the Rijeka Natural History Museum (Croatia) under catalogue number PMR VP4988.

Results
During the field surveys, eight individuals of P. glenii (n = 8, four in 2020 and four in 2021) were collected from the Drava River oxbow (Figure 1), about 60 kilometers upstream of the confluence of the Drava and Danube Rivers (Figure 2).This is the first finding of P. glenii from the Croatian part of the Drava River, since it was previously recorded in a channel of the Sava River (near Slavonski Brod), about 70 km southeast from the current site.There are no artificial connections between these sites and their corresponding watersheds, hence, they can be assumed to represent two independent P. glenii occurrences in Croatia.
On the collection site, the physicochemical characteristics of the water were measured in two occasions (July 2020 and April 2021), and the observed field conditions (algal bloom), water parameters (fluctuating water temperature, low oxygen level and low transparency; Table 1), suggest that sampled well-vegetated and shallow oxbow is a eutrophic waterbody, especially during the summer period when the water level drops significantly (Figure 3).These field surveys recorded a relatively highly diverse fish assemblage at this site: weatherfish Misgurnus fossilis (Linnaeus, 1758) 2. Briefly, the invasive and alien species (C.gibelio¸ P. parva, L. gibbosus, A. melas and P. glenii) make up nearly one-third of the recorded species and dominate the overall fish abundance with roughly 94% (Table 2).On the other hand, protected native fish species (L.delineates, C. carassius and M. fossilis) were very rare in the catch and accounted for barely 2% of the sample (Table 2).In total, native species (including protected taxa) accounted for only around 5% of the sampled individuals from the overall abundance (Table 2).According to Miller (2003), several of the individuals collected here were immature (< 55 mm SL; summer 2020 sample), while others can be considered mature (> 55 mm SL; spring 2021 sample; Table 3).The morphometric and meristic characteristics of P. glenii from the Drava River oxbow are given in Table 4. Generally, for P. glenii, SL values ranged from 38 to 71 mm (mean ± standard deviation = 54.9 ± 12.5), TL varied between 49 and 86 mm (66.5 ± 14.4), and W ranged from 7 to 11 g (8.4 ± 1.5) (Table 4).All individuals had longer predorsal (PRL) than postdorsal (POL) area (37.3 ± 1.9% versus 25.2 ± 0.8%, respectively).The caudal peduncle was relatively thin in all individuals, making up around 13.9% of the SL.On the body, HL accounted for 33.3 ± 1.1% of SL (range 31.0-34.3).Body was higher (MBD: 29.5 ± 2.5%) than wider (BW: 21.6 ± 1.6%), while the lower jaw (LJL: 34.3 ± 3.5%) was longer than the upper jaw (UJL: 31.4 ± 6.3%) (Table 4).D1 was composed from 7.1 ± 0.8 branched rays, while D2, P, V and A consisted of a single unbranched ray and a variable number of branched rays (Table 4).In addition, the number of scales in the lateral line ranged 36.0-40.0(38.3 ± 1.51), while in the transverse and circumpeduncular rows, the number of scales was lower (15.5 ± 1.3 and 9.5 ± 0.7 respectively).

Discussion
The present study reports on the new occurrence of P. glenii from a shallow oxbow situated in the floodplain of the Drava River, and provides new data on the spread of this species in Croatian waters.This finding is the second known occurrence of P. glenii from Croatia (previously, a single individual of P. glenii was caught from the Sava River watershed) and in the scope of this study, it offers morphological details of the investigated population.More importantly, the oxbow is situated within a protected area and NATURA 2000 site, both characterized by the specific and endangered flora and fauna.In 2014, a single specimen of P. glenii was collected from the Keleti-Gyöngyös channel near Szigetvár (Hungary), which belongs to the Drava River watershed and is situated around 20 km aerial distance from the main river course (Takács et al. 2015), and about 62 km northwest from this site.This was the first record of P. glenii from the Drava River watershed, and was believed to have likely originated from a fishpond located upstream of the channel.Accordingly, the present study expands the current knowledge about the potential distribution range of P. glenii in the Drava River watershed.
Habitats similar to this oxbow, such as the freshwater (irrigation) channels, reservoirs, gravel pits and fishponds, have previously been proposed as suitable biotopes for P. glenii acclimatization and population establishment (Reshetnikov 2013).Our field observations suggest that this shallow oxbow, which is rich in aquatic vegetation with well-developed algal bloom, high (and fluctuating) water temperature and low visibility, could present a suitable habitat for the limnophilic P. glenii, and more importantly, could serve as a potential spawning ground for this invasive species.Small and shallow habitats, overgrown with dense aquatic vegetation and covered with ground material or debris are ideal spawning grounds for this litho/phytophilous species.It is noted that in P. glenii, spawning starts when the water temperature exceeds 15 °C, which stimulates females after spawning to start depositing eggs on the lower surface of submerged objects, such as plants, stones, roots, etc. (Miller 2003).Afterwards, the males provide parental care, aggressively guarding the nest and fanning the clutch with their pectoral fins (Bogutskaya and Naseka 2002;Miller 2003).Considering the above, this Drava River could serve as an ideal spawning ground for P. glenii.
Although limited (n = 8), our sample offers the opportunity to compare the morphometric measurements and meristic characters of this Croatian P. glenii population with the nearest neighbouring population from the Danube River channel near Veliko Gradište, Serbia (Nikolić et al. 2021).Specimens in Serbia were larger (both in TL and SL) in comparison with our specimens.In our case, the largest individuals attained a total body length (TL) of 87 mm, while the Serbian specimens achieved a TL of around 140 mm.Body depth (MBD) was greater (in % of SL) in our specimens compared to the Serbian specimens, possibly reflecting the habitat conditions, although our sample size could be too small to reveal an ecological pattern.In Croatia, P. glenii was sampled from a stagnant limnophilic waterbody, while in Serbia, this species inhabits the Danube River channel, where the moderate water current that leads to a more fusiform body form of Serbian specimens.However, additional laboratory and field data are necessary to support this hypothesis.Moreover, the width of the caudal peduncle at the anal fin insertion (CPW) was twice as large in Croatian than Serbian specimens (12% versus 6%, respectively).
Though the Croatian specimens achieved a shorter length, they had a larger caudal peduncle (CPL) compared to the Serbian specimens (41 versus 26 in % of SL).Finally, the snout from Croatian specimens was blunter, while the head was higher at the nape and wider at the posterior margin of preopercle in comparison with Serbian specimens (Table 2).Other morphometric traits (Table 4) were similar between the two populations, in term of % of SL, with only slight differences in the mean and range values.The meristic traits of Croatian specimens, such as number of unbranched and branched rays in the dorsal, pectoral, ventral and anal fins, were similar to Serbian specimens, with minor differences (Nikolić et al. 2021).
According to the sampled fish community, we can hypothesize that this oxbow (connected with the Drava River during floods) is a feeding ground for certain predatory fish (L.aspius and E. lucius), and it serves as a breeding area for other limnophilic and phytophilous taxa (R. rutilus, S. erythrophthalmus, C. carassius, etc.).However, it is important to note that the majority of individuals from our sample were invasive and alien species, such as C. gibelio¸ P. parva, L. gibbosus, A. melas or P. glenii (> 90%), while the native species represented only up to 5% of collected individuals.It is known that alien (and invasive) fish species represent a great ecological stress and significant biological threat to the overall biodiversity, particularly to the native ichthyofauna and protected NATURA 2000 sites, such as this oxbow.It is hypothesized that P. glenii has a detrimental effect of the fish community and other animal groups (herpetofauna, insects, crustaceans, etc.).In most cases, P. glenii occurs in direct competition with native (and often protected) fish species.Several studies have indicated the adverse impacts of P. glenii on native fish fauna, especially C. carassius, L. delineatus, R. amarus or European mudminnow Umbra krameri Walbaum, 1792, mainly through egg and larvae predation or niche competition, resulting in the decline of U. krameri populations (Shlyapkin and Tikhonov 2001;Bogutskaya and Naseka 2002;Reshetnikov 2003;Reshetnikov and Petlina 2007;Koščo et al. 2008;Reshetnikov and Chibilev 2009;Grabowska et al. 2019).Some authors even suggest that the invasion of P. glenii could potentially hasten the extinction rate of the endangered U. krameri, specifically in small and isolated water bodies such as the sampled oxbow (Grabowska et al. 2019).However, in the most extreme scenarios, P. glenii can have an even greater negative impact on nature and related tropic structures, affecting the complete ecosystem and all of its components (fish, zoobenthos, primary producers), as suggested for the Ilgas Nature Reserve ecosystem in Latvia (Kutsokon et al. 2021).There, even though the fish community was never rich in species, P. glenii invaded the shallow marsh waterbodies and completely replaced C. gibelio, leaving behind a monospecific fish community represented by a single invasive fish (Kutsokon et al. 2021).In our situation, this is not yet the case, as the present oxbow (and its fish assemblage) was composed of 16 species, though the majority are alien species, including P. glenii.However, P. glenii was not as abundant (< 1 %) as other invasive species, such as C. gibelio or P. parva.From these observations, it can be assumed that P. glenii likely does not tolerate coexistence with other species, especially those with the similar life history traits (aggressive nest guarding and opportunistic feeding), such as L. gibbosus or A. melas (Kottelat and Freyhof 2007) or that it has not yet managed to establish a stable population.However, having in mind the case from Latvia, special emphasis should be given to the present finding, as the investigated oxbow lies within the NATURA 2000 protected area.In Croatia, and especially within the Mura-Drava Regional Park, populations of C. carassius, L. delineatus and M. fossilis are strictly protected species (Official Gazette of the Republic of Croatia 2016).Within the Mura-Drava protected area, the U. krameri forms several isolated populations in similar stagnant and well-vegetated waterbodies.We hypothesize that at higher densities, P. glenii could pose a serious threat to U. krameri populations in the Drava River watershed.Reshetnikov (2013) emphasized that after the initial introduction and establishment of a population within oxbows, downstream migrations during a flooding event and colonization of floodplain waters are evident throughout the invaded zone.Perccottus glenii is a poor swimmer and therefore inhabits the waterbodies which have weak currents or are overgrown with aquatic macrophytes (Bogutskaya and Naseka 2002).In addition, it is an invasive alien species that poses a significant concern in European Union because of its dramatic influence on the ecosystem, which is so significant that specific measures need to be implemented into the existing protocols with the purpose of its spatial eradication (European Community 2014).
The first sampling session in July 2020, when the P. glenii population was initially recorded from the oxbow, can be considered an early detection of this invasive alien species in the Drava River watershed.The second survey in April 2021 confirmed the presence of P. glenii.According to EU regulations (EU Regulation 1143/2014 on the prevention and management of the introduction and spread of invasive alien species), monitoring and surveillance of newly reported invasive species is imperative, and measures, such as specific target field survey and the inclusion of various stakeholders or sectors, should be taken to conduct rapid eradication to prevent their establishment and spread while the number of specimens is still limited, as in our case.Accordingly, monitoring and surveillance of the Drava River and adjoining floodplain habitats is recommended, especially downstream of the present collection site to confirm the possible presence of the additional populations within vulnerable and protected areas such as the Drava River flooding zone and the Mura-Drava Regional Park.Measures should also be implemented to eradicate all individuals of P. glenii.In addition, the cross-border cooperation is strongly recommended in this case, particularly with the Croatian neighboring countries from the same biogeographical region of the EU where the presence of the P. glenii has already been confirmed (such as Hungary and Serbia), with the mutual goal of the proper management and biomonitoring of this highly invasive fish species.
Recent studies have shown that rivers are common natural corridors for the dispersal of P. glenii (Harka et al. 2003;Hegediš et al. 2007;Reshetnikov and Petlina 2007;Reshetnikov and Chibilev 2009).Dynamics of spatiotemporal migrations of the West-Ukrainian subrange have confirmed this, where rivers such as Dniester, Western Bug, Vistula, Tisza, Siret, Prut and Danube have served as migration corridors and increased the invasive success of P. glenii in floodplain biotopes (Reshetnikov 2013).The exact origin of the P. glenii in the Drava River catchment in Croatia remains debatable.However, based on the previous findings, two hypotheses on the migration route can be proposed (Figure 2): North hypothesis -where individuals migrated southward from the Keleti-Gyöngyös channel (Hungary) and entered the Drava River directly via adjoining water channels and tributaries, or the South hypothesis -where individuals travelled upstream via the Danube River from Serbia and then entered the Drava River at its confluence.In the case of both hypotheses, we believe additional P. glenii findings can be expected downstream of the present record site, possibly within the protected Kopački Rit Nature Park.The South hypothesis could be supported by the first record of P. glenii in the Sava River, situated south of the present sampling site and connected with the Danube River.However, considering that the spread of P. glenii could be rapid from riverheads downstream, but slower when migrating upstream into adjoining waters (Reshetnikov 2013), this may support the North hypothesis, i.e., the present individuals stem from northern Hungarian location (Keleti-Gyöngyös channel); morphological or population genetic analysis could provide further insight.In terms of the size classes of collected individuals, our results are similar to those reported by Skorić et al. (2017), i.e., that the majority of P. glenii individuals collected from the Danube River channel, belonged to 50.0-60.0mm size classes.Here several of the individuals collected were immature (< 55 mm SL; summer 2020 sample), while others can be considered mature (> 55 mm SL; spring 2021 sample) (Miller 2003).
In conclusion, it is reasonable to expect additional P. glenii individuals during future surveys within the Drava River flooding zone.As Nehring and Steinhof (2015) and Skorić et al. (2017) suggested, eradication or biocontrol of P. glenii populations may be possible if appropriate actions and measures are taken after first confirmation and early detection of this highly invasive species in the newly invaded habitat.While sustainable and practical solutions for management of P. glenii are still largely missing, good example of biological control measures in managing (eradication or suppression) the P. glenii population was proposed by Rakauskas et al. (2019).Authors carried out the biocontrol experiments by introducing the native piscivorous fishes (Esox lucius and Perca fluviatilis) into the invaded eutrophic lakes of eastern Lithuania, and the abundance of the P. glenii individuals was noted during the pre-stocking, transitional (stocking) and post-stocking events (Rakauskas et al. 2019).Four years of biocontrol implementation (i.e.repeated stocking of piscivorous taxa) led to a positive effect on the fish community observed in the progressive decline in P. glenii abundances, while the final outcome was the complete disappearance of usually abundant P. glenii from the investigated eutrophic lakes (except for Lake Cirkliškis where the population was strongly suppressed) (Rakauskas et al. 2019).These results suggested the effectiveness of piscivorous fish (such as Esox lucius and Perca fluviatilis) in controlling well-established P. glenii populations (Rakauskas et al. 2019).Therefore, detailed fish monitoring surveys are recommended in the coming years in floodplain areas such as the Drava River zone.

Figure 2 .
Figure 2. Distribution of Perccottus glenii in Croatia and adjoining freshwaters from the neighbouring countries.A) Black circles indicate the closest previous finding site of P. glenii to Drava River, while the red circle shows the present find site from Croatia.Grey shaded area highlights large freshwater drainages where P. glenii has been previously reported.Red broken lines present potential migration routes of P. glenii into the Croatian part of the Drava River, while grey solid lines indicate the country borders.B) The inset indicates the Danube River Basin with the water network (blue lines) and the collection site in Croatia (red square).Abbreviations correspond to the country names.

Figure 3 .
Figure 3. Invasive Perccottus glenii (Odontobutidae) collected from Drava River (Croatia) backwaters in July 2020.A) Sampling site from which four immature individuals of P. glenii were collected by electrofishing.B) Immature specimen of P. glenii (41 mm standard length) sampled immediately from the oxbow.C) Ortho-photo map of the collection site (red dashed square) of P. glenii population in the Drava River flooding zone (dark green).D) Water quality from the sampling site where P. glenii was caught.The green colour indicates the highly eutrophic condition of the shallow and overgrown oxbow.Photo by Sven Horvatić.

Table 1 .
Physicochemical characteristics of the sampled Drava River oxbow collected during 2020 and 2021.

Table 2 .
Fish species captured from Drava River oxbow during the 2020 and 2021 field surveys, with the total number of sampled individuals per species indicated and their percentage from the overall fish abundance highlighted.Plus ( + ) indicates the invasive species, while asterisk (*) indicates strictly protected.

Table 3 .
Capture date, maturation size groups, gender and main body characteristics of P. glenii individuals sampled from Drava River oxbow.