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Article

The Ichthyofauna of the Bednja River, Ichthyological ‘Hot Spot’ in the Danube Basin—Exceptional Diversity under Strong Threats

1
Faculty of Science, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia
2
Faculty of Teacher Education, University of Zagreb, Savska cesta 77, 10000 Zagreb, Croatia
*
Author to whom correspondence should be addressed.
Water 2023, 15(2), 311; https://doi.org/10.3390/w15020311
Received: 24 August 2022 / Revised: 30 December 2022 / Accepted: 5 January 2023 / Published: 11 January 2023

Abstract

:
The Bednja River is the longest river flowing with its full course exclusively through Croatia and one of the largest right tributaries of the Drava River, which belongs to the Danube River basin. Due to the variety of habitats present within the course of the Bednja River, as well as older literature reports, it can be expected that this river harbors a high number of freshwater fish species and would likely benefit from conservation and preservation efforts. We compiled and analyzed the existing literature data on the Bednja River fish communities and performed field investigations at various localities on this river in order to describe the current structure of its freshwater fish fauna and monitor changes induced by human activities. Our results corroborate the presence of a rich and diverse fish community. Moreover, with 36 native species, the Bednja River harbors one of the richest fish communities in Croatia as well as in the Danube basin. Unfortunately, modifications of the native fish community were evident in the form of local extinctions, reductions in the population abundance of several native species and the presence of non-native species. Habitat degradation and fragmentation were identified as the most serious threats provoking negative effects on the native fish populations, followed by predatory and competitive effects of invasive species.

1. Introduction

Croatian ichthyofauna, with around 140 described species, is considered particularly rich and diverse. Only few areas in Europe harbor fish communities as rich as those found in Croatia, both in terms of the total number of species and the proportion of endemic species [1,2,3,4,5,6,7,8,9]. Nevertheless, this richness is mostly connected with rivers and streams of the Dinaric karst and the Adriatic watershed in Croatia, where the isolation of rivers and complex geologic history have promoted speciation and the development of extraordinary ichthyodiversity observed in a high number of endemic species, high levels of genetic diversity, and the presence of specific biodiversity components, such as hybrid biotypes and the development of unique adaptations [1,2,3,4,5,6,7,8,10,11]. On the other hand, ichthyofauna of the Danube River basin in Croatia is usually considered to be more uniform and the level of endemism much lower, because the majority of rivers are longer, with many tributaries, and all interconnected [3,6]. Noteworthy, pressures acting on the rivers belonging to the Danube watershed are significant and fish communities have been highly altered in many locations. The identification of localities within the Danube watershed that harbor high levels of fish diversity and serve as endangered species habitats is important for regional conservation efforts.
The Bednja River has a watershed size of 596 km², and with a length of approximately 133 km, it is the longest river that flows exclusively through Croatia. It is a medium-sized lowland river with a silicate–limestone base. The Bednja River sources from two springs, one near Bednice and the second one near the Trakošćan Lake in the Maceljsko mountains in the Hrvatsko Zagorje region. Downstream, they merge into a single stream, which is obstructed by a dam after approximately 2 km. As a result, water fills the depression around it, becoming the Trakošćan Lake [9]. The Bednja River runs through the cities of Bednja, Lepoglava, Beletinec, Ivanec, Novi Marof, Varaždinske Toplice and Ludbreg and flows into the Drava River near Mali Bukovec [10], so it is a part of the Danube River basin. Its confluence moves upstream and downstream (it is not a static location) due to the Drava riverbed’s frequent fluctuations and big waters, which affect the Bednja’s flow rate [12]. Due to the unique shape and relief of the Bednja River basin, floods in the upper reaches of the stream are of typical torrential character that occur on streams flowing from the slopes of Ivanščica, Ravna Gora and Kalnik, whereas floods in the middle and lower parts of the river basin are classic floods caused by heavy rain and/or melting snow. The lower course of the Bednja River is characterized by a variety of habitats, and along the main course of the river, several oxbows, dead waters and floodplains were present before anthropogenic habitat modifications.
Older literature data [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26] enumerate 35 fish species inhabiting the Bednja River, implying that this river harbors a quarter of Croatian fish species and pointing out the importance of the Bednja River for the conservation of Croatian and European ichthyofauna. Due to the diversity of habitats present in the Bednja River, there is a great diversity of fish species in terms of their taxonomic positions as well as their ecological characteristics (personal observation). Moreover, the Bednja River fish species occupy different trophic levels in ecosystems and have distinct ecological requirements and valences (personal observation). However, not a single comprehensive investigation of the Bednja River ichthyofauna has been conducted so far, nor is any compilation of information in the Bednja River fish communities contained within various older reports. Precisely because the importance of the Bednja River for the conservation of a great number of fish species can be indicated from older reports, we have decided to undertake a more comprehensive investigation of the Bednja River fish communities and to critically evaluate literature data. Besides scientific knowledge on the composition and structure of the Bednja River fish fauna, this investigation also identifies the most problematic threats to fish communities and its changes due to anthropogenically induced threats.
Unfortunately, the Bednja River is exposed to various anthropogenic threats, among which the most problematic ones are habitat degradation through frequent dredging; shoreline armoring and canalizing; disconnecting the backwaters from the main riverbed and mostly drying them off, so they can no longer serve as hatcheries and shelters for juvenile fish; removing the surrounding vegetation, including the root system of shrubs and trees that naturally anchor the coast and protect it from erosion; and finally, the appearance of invasive fish species (personal observation). Currently, regardless of its importance for Croatian and European endangered fish species, the majority of the Bednja River does not enjoy any specific protection. Only the downstream part of the Bednja River (about 1 km) is included in the Natura 2000 network and in the Mura-Drava Regional park.
With this investigation, we aimed to compile all existing literature data on the Bednja River fish community and compare them with the results of the field investigation, in order to describe the current status of the ichthyocenoses of the Bednja River, monitor the changes induced by anthropogenic activities and propose an adequate conservation plan for the stabilization and long-term survival of the native fish community. With this investigation, we will update and compile scientific knowledge on the Bednja River ichthyofauna, but also make a possible design of the concrete conservational measures focusing on achieving and maintaining stable conditions and the viability of the native fish communities.

2. Materials and Methods

Sampling of ichthyofauna was conducted at nine localities on the Bednja River, in the period 2019–2021 (Figure 1). The Bednja River cut its bed into loess and loessoid sediments, depositing alluvial sediments, gravel, sand and clay. The lower course of the Bednja River is characterized by diverse habitats, and along the main course of the river in its natural state, there were backwaters and floodplains. The substrate in floodplains is silty, while in the main stream, it is sandy or silty. Even though the particular focus of the field investigation was on the downstream part and the Ludbreg area, where sampling localities were chosen so that they adequately represent the diversity of habitats, sampling localities in upper parts of the Bednja River were also investigated (Table 1, Figure 1). Thereafter, the fish community was investigated in various stretches of the Bednja River, so our results adequately represent the structure of the Bednja River ichthyocenoses. While the localities of Beletinec (1) and Stažnjevec (2) are characterized by a sandy and stony substrate, as the river flows in its natural course, passing through the city area, the morphology of the river changes. Due to the influence of man and the channelization of watercourses, at the bottom of the locality next to the dam (localities 3, 4, 6, 7 and 8), silt predominates as a substrate, and the riparian vegetation differs from locality to locality; therefore, we find grassy areas, bushes and trees along the river itself. The exception is locality 5 (Bednja under the dam), where gravel predominates in the substrate. As the Bednja River continues its course (locality 9—Mali Bukovec), approaching the confluence with the Drava River, the amount of riparian vegetation is mostly reduced, and the substrate is dominated exclusively by gravel without silt. In addition to the canalization of watercourses, fish biodiversity is also threatened by agricultural and urban land use in the watershed. Moreover, threats acting on different river stretches are various and they are greater in number and intensity particularly in the downstream part of the Bednja River and the Ludbreg area. Thus, the chosen localities are likely not only to enable a sufficient description of the structure of the Bednja River ichthyocenoses, but also to enable us to estimate their impairment caused by anthropogenically induced threats.
In the majority of localities, sampling was conducted by electrofishing, which was the method of choice due to its non-selectivity and lesser impact on fish communities. Electrofishing catches both smaller and larger individuals. In addition, this method causes only transient paralysis of fish muscles, after which individuals recover and can be returned to the watercourse. Sampling was performed using a backpack Hans Grassl EL63II (7 kW) electrofisher so that different microhabitats at each locality were included and to ensure the collection of a representative sample, i.e., that the fish community living in a locality is adequately represented in the sample [27,28]. The length of a transect covered by electrofishing varied between 100 and 800 m, depending on the variety of microhabitats present, and the time spent electrofishing was 1–2 h at each locality. At the Ludbreg Lake site (marked with number 8 on the Figure 1), as an additional sampling method, EU standard nets were also used. The nets were kept in the water overnight (for 12 h), and in the morning, they were collected and fish were taken out and identified. This method is more appropriate for collecting representative fish samples in lakes. Altogether, three EU standard nets were cast. Each net was 30 m long and 1.5 m in height and comprised 12 net panels 2.5 m long. Eye sizes of the net panels were as follows: 43.0, 19.5, 6.25, 10.0, 55.0, 8.0, 12.5, 24.0, 15.5, 5.0, 35.0 and 19.0 mm. All individuals were determined in the field. Sampling in all localities was conducted during the warmer part of the year (April to September), and all localities were visited once for the purposes of this investigation.
In addition to data on the qualitative composition of the Bednja ichthyocenoses and the presence of certain species of fish, based on field research, the basic diversity indices of fish communities were determined: species richness (S), Shannon Diversity Index (H) and A diversity index (A, alpha) or Fisher’s Alpha diversity index. The variety of species present in an ecological community, landscape or region is referred to as its species richness, and the diversity of species in a community can be measured using the Shannon Diversity Index (H increases as the diversity of species in a particular community increases). The probability that two randomly chosen individuals from a sample will be of the same species is measured by Simpson’s Index (D). The Reciprocal Simpson Index (1/D) is also often used and was calculated in this investigation, because its values usually grow with higher biodiversity, so it is more illustrative in describing the diversity of a certain locality than Simpson’s Index. The link between the number of species and the number of individuals in those species is mathematically described by Fisher’s Alpha Diversity Index (A). The computer program BiodiversityPro32 was used to calculate the diversity indices [29].
Besides the field investigation and subsequent analyses, this research included a review of available literature data on the biocenoses of the Bednja River and the Ludbreg area. All available literature data [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26] were first critically evaluated by expert ichthyologists, authors of this paper, and then summarized in order to present the historical structure and state of the Bednja River fish community. We included all literature available to us that reports fish findings in the Bednja River and we extracted information on the presence, localities and abundances of fish species. The respective authors, who are all experts in ichthyology, some with more than 20 years of experience studying Croatian fishes, critically evaluated all the extracted data and excluded those judged as not realistic (wrong determinations of species or mistakes in reporting localities). All other reports were compiled in the overview of the Bednja River fish species. However, it is important to mention that no comprehensive investigation of the Bednja River ichthyofauna has been conducted so far. By studying the literature reports and performing the field research, data were obtained describing the qualitative and quantitative composition of the ichthyofauna of the Bednja River. An expert opinion was given on the composition of the community, i.e., an opinion on which species are absent and which should naturally be present in the research area, and which observed species are non-native. Furthermore, we also checked the listing of all species on the important European and Croatian Directives and lists (Council Directive 92/43/EEC, IUCN Red List, Red Book of Freshwater Fish of Croatia; Bylaw on the strictly protected species in Croatia), because this information is also important for their adequate conservation.

3. Results

A total of 33 species of fish were identified during the field survey (Table 2), but based on this research, as well as available literature data, we can conclude that a total of 42 fish species have been recorded in the Bednja River. Of that number, six species are non-native species, first recorded in the 21st century. One non-native species (Ctenopharyngodon idella) was not recorded in this investigation nor in the more recent literature reports and probably does not inhabit the Bednja River anymore. Unfortunately, in comparison with previous research, another non-native species was observed in this investigation, Ameiurus melas.
Therefore, as many as 36 species can be considered native to the Bednja River, and the presence of 28 native species has been confirmed by this investigation. An interesting result of this research is the finding of Romanogobio kesslerii, which has not been recorded in the Bednja River so far, although it is distributed in other parts of the Danube River basin. Moreover, it is endemic to the Danube basin, strictly protected in Croatia, and also included in the Annex II of the Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora (EU Habitats Directive). Regardless of the high number of species recorded during the field investigation, eight species native to the Bednja River were not recorded. Out of those eight species, three (Barbus balcanicus, Phoxinus lumaireul and Salmo labrax) were found in the past five years in tributaries of the Bednja River, so they are probably still present in the Bednja River catchment. However, as many as five native species (Gymnocephalus cernua, Hucho hucho, Lota lota, Misgurnus fossilis and Leuciscus idus), making up 14% of the Bednja River native ichthyocenosis, were not recorded during the past five years or more, neither in the Bednja River nor in its tributaries, and we can consider them locally extinct. To make the problem of the endangerment of the Bednja River fish community even greater, several species are probably also on the verge of extinction from the Bednja River, since in this investigation, but also in investigations occurring in the past five years, they were found with extremely small numbers of individuals or not at all (e.g., Carassius carassius, Cyprinus carpio, Eudontomyzon vladykovi, Romanogobio kesslerii and Tinca tinca) (Table 2).
All fish species recorded in the Bednja River in this investigation and in the literature (including non-native and extinct species), together with data on their endangerment, as well as their inclusion in the Annexes of the Directive on the Protection of Natural Habitats and Wild Fauna and Flora (Council Directive 92/43/EEC), are presented in Table 3. Six fish species native to the Bednja River are strictly protected in Croatia. Unfortunately, one of the strictly protected species, Misgunus fossilis, has already disappeared from the Bednja River, and three more (Carassius carassius, Eudontomyzon vladykovi and Romanogobio kesslerii) might soon expect the same unfortunate destiny if serious conservational measures are not taken urgently. Even though the majority of species found in the Bednja River are considered least concerned (LC) at the European level (EU IUCN category), the endangerment of most of them at the local level is much higher. Cyprinus carpio is considered vulnerable (VU) in Europe and Hucho hucho is endangered (EN). Twelve species natively inhabiting the Bednja River are included in the Annexes of the EU Habitats Directive (B. balcanicus, B. barbus, Cobitis elongatoides, E. vladykovi, H. hucho, L. aspius, M. fossilis, R. amarus, R. kesslerii, R. vladykovi, R. virgo and S. balcanica), implying that their protection is of particular importance for the EU.
All the obtained results reveal exceptionally high diversity of the Bednja River fish community, but also the presence of species whose conservation is of particular interest for the EU. The richness and diversity of the Bednja River ichthyocenoses are corroborated also by the diversity indices calculated (Table 4). Expectedly, diversity is higher in the downstream parts of the Bednja River (Ludbeg area and Mali Bukovec) than in the upstream localities (Beletinec and Stažnjevec), but also in natural habitats than in anthropogenically made or severely altered habitats. The number of native species found at a certain locality is 6–15, whereas the values of the Shannon index are 0.411–0.892. In six localities (out of nine localities included in the investigation), the Shannon index has values higher than 0.700. In localities where non-native species were present, even though the total species number (species richness) is higher than when only native species are considered, values of Shannon, Reciprocal Simpson and Alpha indices are not considerably higher and sometimes are even lower, which is a consequence of changes in fish species’ relative abundances, the disappearance of native species, reductions in native species abundances and/or numerical dominance of non-native fish species.

4. Discussion

4.1. The Importance of the Bednja River for Ichthyodiversity

The results obtained during the field investigation, as well as the literature data survey, clearly show that the ichthyofauna of the Bednja River, although partly already altered due to anthropogenic influences (some native species have disappeared, while several non-native species are present), is rich and diverse. Based on the literature survey, as well as the conducted field investigation, we can conclude that 36 fish species are native to the Bednja River, which is an exceptional richness not only for Croatia, but on the European level, as well. This result is particularly important from a conservational perspective. In particular, the Bednja River is a natural habitat for a quarter of Croatian freshwater fish species. On the other hand, it does not have any specific protection. Since serious negative effects of anthropogenic activities on several fish species can be noticed, yielding local extinctions of five species already and several more being on the verge of extinction, effective conservational measures must be undertaken urgently [2,3,31]. Those measures should primarily focus on habitat restoration and reestablishment of the river continuum, but also on the invasive species removal and native species reintroduction and augmentation. Noteworthy, not only is the number of species number harbored within various habitats of the Bednja River high, but the Bednja River is also a natural habitat for 12 species included in the EU Habitat Directive Annexes, and their conservation is priority at the EU level.
It is likely that the observed diversity of ichthyofauna in the Bednja River is primarily a consequence of the great diversity of habitats found in the Bednja River. Habitats in the Bednja River vary greatly, from upstream parts of fast flow, rocky and gravelly bottom, low water temperatures and high oxygen concentrations, through the central parts where the flow becomes slower and fluctuations of physico-chemical factors are more pronounced, all the way downstream, where Bednja becomes a typical lowland river that creates backwaters and floodplains. Unfortunately, this diversity has been greatly reduced due to significant anthropogenic modifications of hydromorphological conditions and changes in the Bednja riverbed (personal observation) [32].
The 36 mentioned fish species native to the Bednja River are not equally distributed throughout its length. Fish communities in the upstream parts comprise fewer species that are adapted to conditions of low temperatures, high oxygen concentrations and fast flow. Barbatula barbatula, Eudontomyzon vladykovi and Sabanejewia balcanica were found only in the upstream localities. On the other hand, habitats in the downstream part of the Bednja River and in the Ludbreg area are not suitable for supporting resident populations of Hucho hucho, Barbus balcanicus and Eudontomyzon vladykovi, but are important for these species because they represent an important migration route to the upstream parts of the Bednja River, where there are appropriate hatcheries for these species (personal observation).
When the spawning path becomes obstructed or unsuitable for the passage of these species, their reproductive cycle cannot be completed and their reproductive potential decreases (they have fewer or no offspring), which leads to population collapse and even local extinctions. This scenario has already happened for Hucho hucho, the largest salmonid species in Croatia (personal observation). Barriers on watercourses (not only on the Bednja River, but throughout the entire Danube watershed in Europe) prevented its migration to appropriate hatcheries in the upstream parts of large watercourses and in their smaller tributaries [33]. Based on the literature data, we can conclude that upstream parts of the Bednja River comprised important spawning grounds for this species and we can expect that their reintroduction to the Bednja River would be beneficial for the viability of the whole species (Table 2 and Table 4).
Besides great habitat diversity, we can also expect the geologic history of this area to be responsible for the observed ichthyodiversity. Future investigations are required to reveal the evolutionary history of the ichthyofauna of the Bednja River and additionally explain causes of the observed richness.
The obtained results reveal that the diversity of ichthyocenoses is greatest in habitats that are less altered (for example, Bednja in Ludbreg), and reduced in artificial and significantly altered habitats (Ludbreg canal, Lake Ludbreg). This result confirms that the extinction and degradation of habitats lead to the extinction of native species and disrupt the structure and function of aquatic ecosystems. Differences in diversity between fish communities in individual localities are quite obvious (the lowest Shannon index, recorded in Lake Ludbreg, is 0.488, while the largest, recorded in Bednja in Ludbreg, is 0.892) and they reflect the structure of the communities which are more diverse in downstream river parts and in localities with a greater number of microhabitats. An additional reason for the low diversity in the lake is probably restocking. If we look at the total sample, i.e., the total ichthyocenosis of the Ludbreg area, these results also prove its diversity, because 25 fish species were caught in just two field days, and the Shannon Diversity Index for the total sample is 0.789. Although the number of species is higher when non-native species are included, the diversity is generally somewhat smaller. The reason for this lies in the characteristics of non-native species, i.e., in their high reproductive potential. In places where alien species establish stable populations, they quickly become dominant, which is why diversity indices take on poorer values.

4.2. Changes in the Composition of the Ichthyofauna of the Bednja River

Comparing older literature data on the ichthyofauna of the Bednja River [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26] with more recent data on the species recorded there, changes in the composition of the ichthyofauna of this river become apparent. We can conclude that those changes are manifested in the extinction of five native species (Gymnocephalus cernua, Hucho hucho, Lota lota, Misgurnus fossilis and Leuciscus idus), severe reductions in the population abundance and distribution range within the Bednja River for at least seven species (Carassius carassius, Cyprinus carpio, Eudontomyzon vladykovi, Leuciscus aspius, Leuciscus leuciscus, Romanogobio kesslerii and Sabanejewia balcanica) and the appearance of invasive species (Ameiurus melas, Carassius gibelio, Lepomis gibbosus, Neogobius fluviatilis and Pseudorasbora parva). It is likely that the most important cause of the Bednja River fish fauna exchange is degradation and fragmentation of habitats, coupled with introductions of several non-native species [34]. Due to changes in the shores and bed of the Bednja River and changes in hydromorphological conditions, the habitat has become suboptimal or completely unsuitable for native species, especially those with specific ecological requirements. In addition, barriers have prevented the migration of migratory species, which, in addition to the already-mentioned reduction in reproductive success, has other negative consequences, including a lack of gene flow between populations, reduced genetic diversity and effective population size, and depression due to inbreeding [35,36,37]. All these negative effects of habitat degradation and fragmentation reduce the viability of native species populations, which is why their survival is questionable. Considering the species that disappeared from the Bednja River, as well as the observed situation in the watercourse, we believe that fishing is not a significant threat to the ichthyofauna of the Bednja River, provided that only the native community is supported, without the introduction of non-native species.
Due to habitat degradation, species adapted to specific habitat conditions have disappeared from the Bednja River. Misgurnus fossilis and Lota lota, species that live in areas of slow flow, particularly swampy habitats [1], cannot be found in the Bednja River anymore, precisely because of the disappearance of such habitats, whereas Carassius carassius and Cyprinus carpio are almost extinct for the same reason. All mentioned species tolerate fluctuations in environmental factors well and can survive in water where the oxygen concentration is reduced and the temperature elevated. However, they are closely connected with slow-flowing parts of watercourses with muddy bottoms and lush vegetation. We can assume that the canalization of larger parts of the Bednja River and the disappearance of adequate habitats led to their local extinction. Carassius carassius, besides being closely attached to a certain habitat type, is also more sensitive to fluctuations of the physico-chemical conditions [9]. Similar habitats are also suitable for Gymnocephalus cernua, another species locally extinct from the Bednja River in recent years [1]. For migratory species (Hucho hucho, Salmo labrax, Barbus balcanicus, Eudontomyzon vladykovi), a particular problem is posed by the disruption of the river continuum, which prevents those species from reaching their optimal spawning sites, reducing their reproductive success as a consequence.
On the other hand, places of altered habitats and damaged fish communities are ideal for the appearance of non-native species [38,39]. We have no data on the introduction of non-native species into the Bednja River; some (Neogobius fluviatilis, Pseudoasbora parva) probably colonized the Bednja River themselves, due to their expansion into the Danube watershed (as a consequence of deterioration of habitat conditions for native species and other impacts that disrupt the stability of native communities), and others (Ameiurus melas, Carassius gibelio, Lepomis gibbosus) were probably introduced intentionally, with the aim of sport or recreational fishing. In any case, they create an additional negative impact on the native ichthyofauna through predation (primarily of eggs and juveniles) and competition.
Although the changes in the presence of certain species are likely consequences of anthropogenic influences on the Bednja River, these influences probably affected the species that are still present. We can expect that in them, too, there has been a decrease in population density, genetic diversity, distribution ranges, disruption of population structure and life cycle. Revitalization of natural habitats and restoration of river connectivity will certainly have a positive impact on the entire native fish community of the Bednja River, while reintroduction of locally extinct species will enable the restoration of the structure and function of the entire ecosystem.

4.3. Invasive Species in the Bednja River

Six non-native species have been recorded in the Bednja River so far: Ameiurus melas, Carassius gibelio, Cteopharyngodon idella, Lepomis gibbosus, Neogobius fluviatilis and Pseudorasbora parva, with A. melas being observed for the first time in this study and C. idella disappearing without establishing stable populations. Given their predatory and/or competitive effect, all of these species can also be considered invasive because they affect either the populations of native species or their habitats in one way or another. Even though the effects of all non-native species present in the investigation area are various, probably all damage the populations of native species, and we can consider them a threat to their survival. The situation is often even more complex and worse for native species because invasive species usually establish stable populations and show negative effects when the habitat is changed and the natural balance in ecosystems is already disturbed. Since the negative effects of multiple threats occur simultaneously, they usually work together even more strongly on native communities and lead to their extinction even faster (a phenomenon known as the vortex of extinction) [40,41].
Of the non-native species recorded in the Bednja River, we can consider P. parva and L. gibbosus especially threatening, i.e., we can expect that their influences on the native ichthyofauna are the most significant. It is important to note that these two species, together with Ameiurus melas, are included in the List of Invasive Species of Special Concern in the European Union (Invasive alien species of Union concern; https://ec.europa.eu/environment/nature/invasivealien/list/index_en.htm, accessed on 16 August 2022), so all activities aimed at their removal will be welcome, not only to recover the native Bednja fish communities, but also to remove invasive species of special importance. Pseudorasbora parva feeds on the eggs of other fish and thus reduces their reproduction rate, which leads to a decrease in the density of their populations. In addition, it is a carrier of diseases that increase the mortality of native populations [42,43]. Lepomis gibbosus is a competitor to many native species and reduces their available resources [44,45]. Neogobius fluviatilis is considered one of the most successful invasive species in freshwater ecosystems because it spreads and establishes stable populations extremely rapidly, while its effects on native species have not been well studied yet [46,47]. Carassius gibelio is a widespread species in European watercourses and its dense and stable populations are present in many watercourses. Furthermore, it is a strong competitor to many species, especially C. carassius [1,48]. Ameiurus melas has been introduced intentionally at certain locations for fishing purposes. From the fishpond systems into which it is introduced, it often reaches both streams and lakes [49,50]. Since some of the non-native species were recorded in this investigation for the first time, we can conclude that the fish community of the Bednja River continues to change at the expense of the native species and that specific conservation activities aimed at restoring habitats and native populations, as well as removing and preventing the occurrence of alien species, are both necessary and urgent.

4.4. Conclusions

The ichthyofauna of Bednja River is extremely rich and diverse—as many as 36 species of fish live in it, which is a quarter of the total number of fish native to Croatia (Table 3). Such wealth is primarily due to the richness and diversity of habitats present in this river, but also the geological past of this area, with which the evolutionary history of fish is closely linked [5,7,9]. From the Bednja spring (more precisely from its two springs) to its confluence with the Drava River, there is a whole spectrum of different habitats, from upstream parts of fast flow and rocky/gravel bottom, over a slower middle course to downstream parts where the Bednja River has typical lowland river features of slow water flow, with parts where water does not flow, the bottom covered with fine sediment, higher fluctuations of environmental factors and the presence of backwaters, and wetlands. Unfortunately, the change in habitat is the main reason that the ichthyofauna of the Bednja River has changed and been damaged. Some native species have already disappeared from the Bednja River, while several alien species live in it today.
Among the 36 species considered native to Bednja, as many as 12 are included in the Annexes to the EU Habitats Directive (Table 3), which means that their protection is considered particularly important at European Union level. These species are Eudontomyzon danfordi, Rhodeus amarus, Cobitis elongate, Misgurnus fossilis, Sabanejewia balcanica, Barbus balanicus, Barbus barbus, Romanogobio kesslerii, Romanogobio vladykovi, Rutilus virgo, Hucho hucho and Leuciscus aspius. Most species of the Bednja River are not considered particularly endangered globally (most species are listed in the IUCN Red List as least endangered species), although their endangerment at the local level can be significantly higher. Five native species to the Bednja River are strictly protected in Croatia: Danube lamprey, weatherfish, Balkan spined loach, crucian carp and Kessler’s gudgeon.
As priority species for the establishment and preservation of the good status of their populations, we suggest the species listed in the Annexes to the Habitats Directive, because their conservation is also the responsibility of EU member states. Activities that we can expect to contribute to achieving and maintaining the good status of priority species include habitat restoration so that it is suitable for these species, re-establishing river continuity and removing alien species and preventing their emergence. These activities (which we only list here in advance, and which have yet to be defined) could improve the state of fish species whose conservation is of particular interest to the EU, but also restore the ecosystem of the downstream Bednja, which is also a contribution to the European Strategy on biodiversity. The removal and control of alien species, in addition to contributing to the favorable status of priority species, is also in line with the objectives of the EU Biodiversity Strategy.

Author Contributions

I.B.: Conceptualization, resources, investigation, writing—original draft; S.P.: conceptualization, investigation, writing—original draft; L.O.: investigation, writing—original draft; Z.M.: investigation, writing—review and editing; P.M.: investigation, writing—review and editing; D.Z.: investigation, writing—review and editing; M.Ć.: investigation, writing—review and editing; L.I.: investigation, writing—review and editing; L.N.: investigation, writing—review and editing; N.R.: investigation, writing—review and editing; S.H.: investigation, writing—review and editing; R.K.: investigation, writing—review and editing; G.T.: investigation, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

The investigation was financed by the City of Ludbreg and Croatian Waters agency.

Institutional Review Board Statement

This study was conducted in accordance and with permit of Ministry of Agriculture (approval code: 525-13/00545-18-2, approval date: 11.4.2018.; approval code: 641-01/18-02/1038, approval date: 8.3.2019.; approval code: 602-01/20-01/1, approval date: 11.3.2020.; approval code: 640-01/21-01/6, approval date: 22.1.2021.; approval code: 525-13/0732-22-2, approval date: 3.2.2022).

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to the City of Ludbreg personnel, particularly to Petra Međimurec and Major Dubravko Bilić, for their strong support of this investigation and continuous efforts in the Bednja River protection.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Investigation area with marked sampling localities. 1—Beletinec, 2—Stažnjevec, 3—Bednja—accumulation above the dam, 4—channel along the accumulation, 5—Bednja under the dam, 6—accumulation above the dam in Ludbreg, 7—Bednja in Ludbreg, 8—Ludbreg Lake, 9—Mali Bukovec. Red square on a smaller map denotes geographic position of the investigation area inside Croatia.
Figure 1. Investigation area with marked sampling localities. 1—Beletinec, 2—Stažnjevec, 3—Bednja—accumulation above the dam, 4—channel along the accumulation, 5—Bednja under the dam, 6—accumulation above the dam in Ludbreg, 7—Bednja in Ludbreg, 8—Ludbreg Lake, 9—Mali Bukovec. Red square on a smaller map denotes geographic position of the investigation area inside Croatia.
Water 15 00311 g001
Table 1. Sampling localities in which field research of the Bednja ichthyofauna was conducted in 2019 (localities 1 and 2) and 2021 (localities 3–9). Locality numbers in the table correspond to the localities depicted in Figure 1.
Table 1. Sampling localities in which field research of the Bednja ichthyofauna was conducted in 2019 (localities 1 and 2) and 2021 (localities 3–9). Locality numbers in the table correspond to the localities depicted in Figure 1.
Locality NumberLongitudeLatitudeLocality Name
14851815118966.75Beletinec
24744805122412Stažnjevec
3507830.45122559Bednja—accumulation above the dam
4507916.25122654Channel along the accumulation
55081225122660Bednja under the dam
6509132.85123079Accumulation above the dam in Ludbreg
7509122.85122911Bednja in Ludbreg
85088085122780Ludbreg Lake
9518315.95127936.12Mali Bukovec
Table 2. Results of the field investigation—number of individuals of certain species caught in a particular locality of the Bednja River from 2019 to 2021. The names of non-native species are marked in superscript numbers.
Table 2. Results of the field investigation—number of individuals of certain species caught in a particular locality of the Bednja River from 2019 to 2021. The names of non-native species are marked in superscript numbers.
Species NameLocality 1Locality 2Locality 4Locality 6Locality 8Locality 7Locality 3Locality 5Locality 9
Abramis brama 12 18472
Alburnoides bipunctatus 13140 25 9531
Alburnus alburnus 1133614 3244018
Ameiurus melas1 4
Barbatula barbatula6
Barbus barbus1 135 6 2617
Blicca bjoerkna 2 2115 1
Carassius carassius 1
Carassius gibelio2 20 139
Chondrostoma nasus 22 2031718
Cobitis elongatoides 238 720336
Cyprinus carpio 2
Eudontomyzon vladykovi1
Esox lucius 3 1227
Gobio obtusirostris 274 1
Lepomis gibbosus3 122 1835
Leuciscus aspius 2 1
Leuciscus leuciscus 3
Neogobius fluviatilis4 1
Perca fluviatilis 2 75
Pseudorasbora parva5 11
Rhodeus amarus4130127 129
Romanogobio kesslerii 1
Romanogobio vladykovi 11417 15 48
Rutilus rutilus 16018147161154115
Rutilus virgo 1354 534434
Sabanejewia balcanica4
Sander lucioperca 2 24
Scardinius erythrophthalmus 69 26
Silurus glanis 1 1
Squalius cephalus7123136 48 6141
Tinca tinca 1
Vimba vimba 53 32 17
Table 3. Presence and status of Bednja river fish species. *—species has not been recorded in the Bednja River nor in its tributaries for at least the past five years and can be considered extinct in Bednja. **—species has not been recorded in the Bednja River lately, but was found in its tributaries, so it is still present in the Bednja River catchment. IUCN categories: DD—data-deficient, LC—least concern, NT—near threatened, EN—endangered. The table is based on the literature survey and field investigation.
Table 3. Presence and status of Bednja river fish species. *—species has not been recorded in the Bednja River nor in its tributaries for at least the past five years and can be considered extinct in Bednja. **—species has not been recorded in the Bednja River lately, but was found in its tributaries, so it is still present in the Bednja River catchment. IUCN categories: DD—data-deficient, LC—least concern, NT—near threatened, EN—endangered. The table is based on the literature survey and field investigation.
Species NameNative/ForeignAnnexes of the Habitat DirectiveEU Threatened Category (IUCN)National Threatened Category [30]Strictly Protected Species in Croatia
Abramis brama (Linnaeus, 1758)Native LC/
Alburnoides bipunctatus (Bloch, 1782)Native LCLC
Alburnus alburnus (Linnaeus, 1758)Native LC/
Ameiurus melas Rafinesque, 1820Non-native
Barbatula barbatula (Linnaeus, 1758)Native LC/
Barbus balcanicus Kotlík, Tsigenopoulos, Ráb & Berrebi, 2002NativeIILCVU
Barbus barbus (Linnaeus, 1758)NativeVLC/
Blicca argyroleuca Heckel, 1843Native LC/
Carassius gibelio (Bloch, 1782)Non-native
Carassius carassius (Linnaeus, 1758)Native LCVUyes
Chondrostoma nasus (Linnaeus, 1758)Native LC/
Cobitis elongatoides Băcescu & Mayer, 1969NativeIILC/
Ctenopharyngodon idella Valenciennes, 1844 *Non-native
Cyprinus carpio Linnaeus, 1758Native VUEN
Eudontomyzon vladykovi Oliva & Zanandrea, 1959NativeIILCNTyes
Esox lucius Linnaeus, 1758Native LC/
Gobio obtusirostris Valenciennes, 1842Native LCLC
Gymnocephalus cernua (Linnaeus, 1758) *Native LC/
Hucho hucho (Linnaeus, 1758) *NativeII, VENEN
Lepomis gibbosus (Linnaeus, 1758)Non-native
Leuciscus aspius (Linnaeus, 1758)NativeII, VLCVU
Leuciscus idus (Linnaeus, 1758) *Native LCVU
Leuciscus leuciscus (Linnaeus, 1758)Native LC/
Lota lota (Linnaeus, 1758) *Native LCVU
Misgurnus fossilis (Linnaeus, 1758) *NativeIILCVUyes
Neogobius fluviatilis (Pallas, 1814)Non-native
Perca fluviatilis Linnaeus, 1758Native LC/
Phoxinus lumaireul Schinz, 1840 **Native LC/
Pseudorasbora parva (Temminck & Schlegel, 1846)Non-native
Rhodeus amarus (Bloch, 1782)NativeIILC/
Romanogobio kesslerii (Dybowski, 1862)NativeIILCNTyes
Romanogobio vladykovi (Fang, 1943)NativeIILCDDyes
Rutilus rutilus (Linnaeus, 1758)Native LC/
Rutilus virgo (Heckel, 1852)NativeII, VLCNT
Sabanejewia balcanica (Karaman, 1922)NativeIILCVUyes
Salmo labrax Pallas, 1814 **Native //
Sander lucioperca (Linnaeus, 1758)Native LC/
Scardinius erythrophthalmus (Linnaeus, 1758)Native LC/
Silurus glanis Linnaeus, 1758Native LC/
Squalius cephalus (Linnaeus, 1758)Native LC/
Tinca tinca (Linnaeus, 1758)Native LC/
Vimba vimba (Linnaeus, 1758)Native LCVU
Table 4. Diversity indices calculated on the basis of sampled native fish species at individual localities of the Bednja River and for the entire sample.
Table 4. Diversity indices calculated on the basis of sampled native fish species at individual localities of the Bednja River and for the entire sample.
Species Richness, SShannon Index, H Reciprocal Simpson Index, 1/DAlpha Index, A
Indices based on the native species
Beletinec60.411 1.4891.463
Stažnjevec90.621 2.6011.892
Channel along the accumulation in Ludbreg area150.767 6.1922.461
Accumulation above the dam in Ludbreg70.870 5.1671.948
Ludbreg Lake70.488 2.0921.358
Bednja in Lubreg140.892 9.2043.083
Channel Lubreg140.642 3.1293.364
Bednja under the dam140.857 8.2822.779
Mali Bukovec140.818 7.2043.655
TOTAL SAMPLE280.755 9.8344.336
Indices based on the entire samples, also including non-native species
Beletinec60.411 1.4891.463
Stažnjevec100.603 2.6252.159
Channel along the accumulation in Ludbreg area180.726 6.2363.060
Accumulation above the dam in Ludbreg90.830 5.6182.716
Ludbreg Lake90.552 2.5041.812
Bednja in Lubreg140.892 9.2043.083
Channel Lubreg180.702 3.9813.841
Bednja under the dam160.844 8.7233.261
Mali Bukovec150.8123 7.6093.970
TOTAL SAMPLE330.743 10.3715.234
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Buj, I.; Pleše, S.; Onorato, L.; Marčić, Z.; Mustafić, P.; Zanella, D.; Ćaleta, M.; Ivić, L.; Novoselec, L.; Renić, N.; Horvatić, S.; Karlović, R.; Tvrdinić, G. The Ichthyofauna of the Bednja River, Ichthyological ‘Hot Spot’ in the Danube Basin—Exceptional Diversity under Strong Threats. Water 2023, 15, 311. https://doi.org/10.3390/w15020311

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Buj I, Pleše S, Onorato L, Marčić Z, Mustafić P, Zanella D, Ćaleta M, Ivić L, Novoselec L, Renić N, Horvatić S, Karlović R, Tvrdinić G. The Ichthyofauna of the Bednja River, Ichthyological ‘Hot Spot’ in the Danube Basin—Exceptional Diversity under Strong Threats. Water. 2023; 15(2):311. https://doi.org/10.3390/w15020311

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Buj, Ivana, Sara Pleše, Lucija Onorato, Zoran Marčić, Perica Mustafić, Davor Zanella, Marko Ćaleta, Lucija Ivić, Lucija Novoselec, Nikola Renić, Sven Horvatić, Roman Karlović, and Goran Tvrdinić. 2023. "The Ichthyofauna of the Bednja River, Ichthyological ‘Hot Spot’ in the Danube Basin—Exceptional Diversity under Strong Threats" Water 15, no. 2: 311. https://doi.org/10.3390/w15020311

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