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The recent expansion of the round goby (Neogobius melanostomus) in tributaries of the lower Danube (Bulgaria) prompted assessing potential negative effects on local fish fauna such as competition for food resources. Diet overlap between the round goby and native fishes was analyzed using samples of fishes and macroinvertebrates collected monthly for a one-year period. Significant dietary overlap between N. melanostomus and Barbus petenyi, Perca fluviatilis, Gobio gobio, Vimba vimba, and Neogobius fluviatilis was observed. Dietary overlap in the Iskar River was found to be considerable mainly in the spring months. Conversely, overlap in the Vit River was observed during summer and late fall. The main reason for the overlap noted was the preference for Chironomidae and Hydropsychidae larvae and Gammaridae. The results suggest that there is potential for competition between the round goby and native fish species inhabiting the tributaries of the Danube. Potentially harmful effects on rare, protected fish species in the area are discussed.

Aquaculture production in Bulgaria has increased over the last decade, reaching 16 442 tonnes in 2019. Fish production has doubled in comparison with 2007, while that of mussels has increased tenfold. The Bulgarian contribution to EU aquaculture production has been increasing significantly in both volume and value over the years, making up 1.15% of the volume and 1.0% of the value of EU production in 2019. Freshwater aquaculture accounts for 78% of total production. Common carp dominates with about 29.4% (4836 t), followed by rainbow trout with 29.2% (4820 t) in 2019. The cultivation of sturgeon species and caviar production are among the most dynamically developing aquaculture segments. Mariculture in the Black Sea has increased in recent years, with the production of Mediterranean mussel reaching 2932 tonnes in 2019. An average of 405 farms operated during the period 2010-2019. Four regions (Plovdiv, Stara Zagora, Burgas and Montana) account for 50% of the total fish production. Pond aquaculture is the dominant technology used in Bulgaria, and it serves as the basis for numerous other activities, including management of fish stocks in various water bodies mainly for recreational fishing. Approximately 35 net-cage farms currently operate in bigger dams. Recirculating fish farms output made up only 0.15% of the total amount of aquaculture for the period 2010-2019. The aquaculture sector exhibited difficulties in recovering from the financial crisis of 2007-2008, manifested by a slow growth for the period 2010-2014. From 2015 to 2019 there has been a significant growth, manifested in a sharp increase of total revenue and profitability, especially among the larger enterprises in the sector, as well as an increase in the number of employees, and the labour productivity. As a result, in 2019 the registered total revenue per enterprise and total revenue per employee were more than double the respective figures for 2010. The profits of larger enterprises increased more than three times on average, but smaller entities, micro-enterprises with less than 5 employees, operated at the border line between profit and loss. The COVID-19 crisis could have lasting consequences. Despite EUR 1.2 million direct payments in the sector in 2020, there has been a significant drop in the export of aquaculture products. Consumption of fish and other aquaculture products remains low compared to those in the other EU countries. [ABSTRACT FROM AUTHOR]

The family Gobiidae is the largest teleost family, consisting of more than 2,000 species in more than 200 genera. They have adapted to a wide range of habitats, inhabiting mostly marine and brackish waters. Recently invasive Ponto-Caspian gobies have caused significant changes to the structure of fish assemblage throughout the European waters. The aim of the study was to verify the possibility of identifying three species of the genus Gobiidae based on the analysis of the mitochondrial region of the cytochrome oxidase subunit 1 (COI) gene. 339 samples originated from seven countries and 17 different sites were investigated. Qualitative and quantitative assessment of the extracted DNA was conducted by measuring light absorbance and electrophoretic separation. Approximately 650-bp (base pairs) fragments were amplified from the 5' region of the mitochondrial COI with the PCR (polymerase chain reaction) reaction. Sequences of 652 bp were obtained and compared with reference sequences in the National Center for Biotechnology Information (NCBI). Based on the sequence analysis, we determined that in the analysed geographical regions, the goby preliminary identified morphologically as the round goby was represented by three species: round goby (Neogobius melanostomus), tubenose goby (Proterorhinus marmoratus), and monkey goby (Neogobius fluviatilis). The latter two are newly reported for their new habitats. Genetic characterisation of such populations offers the opportunity to assess their genetic stability, which provides agencies managing the aquatic environment in EU countries with a tool for revising the legislation, including regulations relevant for updating the status of native and invasive species in European waters. [ABSTRACT FROM AUTHOR]

In the present study, we provide data on the first established, self-sustaining population of non-native brook trout (Salvelinus fontinalis Mitchill, 1814), family Salmonidae, in Bulgaria. The brook trout was detected in upland section of the Palakariya River (Iskar basin) at an altitude between 1350 and 1500 m a.s.l. Distribution, abundance and size structure of S. fontinalis were studied in the period 2019-2021. The coexistence of individuals of different sizes (from 4.1 cm to 24.6 cm); no restocking activities in the last 10 years and the suitable environmental habitat features support the contention of a self-reproducing population of S. fontinalis in the Palakariya River. [ABSTRACT FROM AUTHOR]

Reproductive traits are among the most important biological characteristics that facilitate or impede the establishment of non-native fish into new environments. In several tributaries of the Lower River Danube (Bulgaria), the round goby (Neogobius melanostomus Pallas, 1814) is a recent invader. Specimens from the edge of the invasion front were collected monthly (March 2017 to May 2018) from each river. The pioneer individuals displayed relatively small body sizes. First maturation of females occurs at 49 mm total length. Based on the dynamics of oocyte size and GSI, it was established that spawning begins in early spring, at a relatively low water temperature (6-7 °C). The breeding season extended from March to June. Average absolute fecundity was 162 ± 62 oocytes, while average relative fecundity was 94 oocytes/g ± 24. Female fecundity is linearly dependent on the length and weight. Both of the known male alternative reproductive tactics were observed. Body condition factor was lowest during April for both sexes. The relationship between reproductive traits and invasive potential in N. melanostomus is discussed. [ABSTRACT FROM AUTHOR]

All sturgeon species currently living in the Danube River are either critically endangered or at the edge of extinction. Sturgeons are mainly benthic feeders, with a diet of molluscs, crustaceans, and small fishes. During the last two decades, we have seen a significant change in the species composition of benthic invertebrates in the Danube River as a result of anthropogenic impacts – hydrological and habitat modifications, introduction of alien and invasive species. The aim of this study was to investigate the food composition and the importance of different prey taxa, and to describe the feeding strategies of young sturgeons along the Bulgarian section of the Danube River. From June to August 2019, the stomach contents of 61 fish specimens from the species Acipenser stellatus, A. ruthenus, A. gueldenstaedtii were collected using gastric lavage. It was found that Obesogammarus obesus (Amphipoda) prevails in the diet of young of the year (YOY) A. stellatus and A. gueldenstaedtii, while Corbicula fluminea (Bivalvia) was the main food item of A. ruthenus. Chironomidae larvae also had high occurrences in the diet of all sturgeon species. For the YOY specimens up to 35 cm total length, nearly 94% of prey items belonged to O. obesus and diet overlap was observed. Improved knowledge about the diet of the young sturgeons could lead to the determination of the species' major feeding habitats in the Danube River, providing crucial information for their protection and restoration. [ABSTRACT FROM AUTHOR]

Length-weight relationships (LWRs), Fulton (K) and relative (Krel) condition factors for Acipenser ruthenus Linnaeus, 1758, Acipenser gueldenstaedtii Brandt & Ratzeburg, 1833 and Acipenser stellatus Pallas, 1771 from the Danube River were estimated. Fish were captured by bottom drifting trammel nets in the period June - August, 2015-2019. A total of 790 specimens were measured and analyzed. More than 80% of the specimens were young of the year (YOY) fish with total length (TL) up to 35 cm. The values of the parameter b of the weight - length relationship for fish with lengths up to 35 cm ranged from 2.433 (A. ruthenus) to 2.859 (A. gueldenstaedtii). For specimens with TL > 35 cm the values of parameter b ranged from 3.227 (A. stellatus) to 3.668 (A. ruthenus). Relative condition factor (Krel) ranged from 1.003 ± 0.020 (A. gueldenstaedtii) to 1.144 ± 0.018 for A. ruthenus. Fulton's condition factor (K) ranged from 0.300 ± 0.003 for A. stellatus to 0.514 ± 0.011 for A. gueldenstaedtii. [ABSTRACT FROM AUTHOR]

In the 1960-1970s, three coregonid fishes (Coregonus albula, Coregonus maraenoides and Coregonus peled) were imported from the former Soviet Union and introduced to some reservoirs in Bulgaria. Irregular import, hatching in local fish-farms and stocking continued until the 1990s. The aim of our study was to examine the occurrence of the alien coregonid species in one of the stocked reservoirs, Iskar Reservoir, and to identify the species recorded based on their morphology. A total of 54 coregonid specimens were collected at five stations in the period from October to March in 2015 and 2016. According to the studied morphometric and meristic data, all the coregonid specimens collected were identified as the Peipsi whitefish, Coregonus maraenoides. The morphology of C. maraenoides from Iskar Reservoir revealed some differences from the native population (Peipus Lake, Estonia and Russia), being most likely a result of adaptation of this species to the local environmental conditions or related to the brood stock management in the fish farms. Our results showed that C. maraenoides has successfully established in the reservoir. The existence of self-sustaining population has been supported by the following: a long period from the last stocking of C. maraenoides to the reservoir up to our study (>15 years); presence of specimens of different size classes in the catches; and presence of sexually mature female and male specimens. [ABSTRACT FROM AUTHOR]

Background. Peipsi whitefish, Coregonus maraenoides Polyakov, 1874, is an endemic species of Lake Peipsi (Chudskoe) in Estonia and Russia. In the mid-1970s, it was introduced into the biggest Bulgarian artificial body of water--the Iskar Reservoir (Danube River basin). A recent survey confirmed the existence of a self-sustaining population of the Peipsi whitefish in the dam lake. Establishing a suitable method for determining Peipsi whitefish age would allow fishery managers and biologists to extract valuable information on various population parameters. Therefore, the aim of the presently reported study was to test the suitability of scales, pectoral fin rays, and otoliths for the most reliable age determining of Peipsi whitefish. Material and methods. Age estimates were obtained from transverse sections of sagittal otoliths and pectoral fins, and scales from 54 Peipsi whitefish, collected between October and March 2016--2017. Two readers estimated ages from all three structures independently. The precision and bias of age estimates between readers and among structures were compared using age bias plots, coefficient of variation (CV), percent agreement (PA), and level of readability. Mean consensus ages from two readers for each structure were compared. Results. Mean consensus age estimates obtained by analysing the scales (2.0 years) were significantly lower than those obtained by analysing the fin rays (2.6) and otoliths (2.7). Between-reader percent agreement was lower and the coefficient of variation was higher for otoliths (PA = 22.2%; CV = 27.6) compared with scales (PA = 46.6%; CV = 10.02) and pectoral fin rays (PA = 67.4%; CV = 10.12). Comparison of age estimates from the different structures revealed the highest PA and the lowest CV values between otoliths and pectoral fin rays (PA = 62.6%; CV = 10.03%), while the lowest PA and highest CV (PA = 35%, CV = 25.03) were observed between age estimates from fin rays and scales (CV, P > 0. 005). The otolith and scale ages agreed for 46.7% and CV was 21.5. The scales were considered by both readers with the highest level of readability (88.9%) than the pectoral fin rays (70.4%) and otoliths (50%). Conclusion. The use of fin rays is recommended for the age estimation of C. maraenoides from the Iskar Reservoir but further work is needed to validate the accuracy of ageing methodology for small, under one-year old Peipsi whitefish. [ABSTRACT FROM AUTHOR]

The investigated fish pass is a part of a low-head hydrostatic pressure machine (HPM) located on an existing weir in the upper Iskar River. Monitoring of the hydraulic features of the fish pass was performed under different hydraulic conditions. Diversity and abundance of the local fish fauna was investigated with the aim to verify target fish species. A major limiting factor for using of the fish pass during flood periods was high water velocity (both inside the fish pass and at the fish pass downstream entrance), while during the low water discharge periods it was the insufficient water depth in some pools and the significant differences between the water levels in adjacent pools. The difference between the water levels at the entrance and exit of the fish pass reduces the chances for access of small-sized fish species. Some measures for improving fish pass functionality were developed. Several re-construction solutions were proposed in an attempt to help solving the identified problems. [ABSTRACT FROM AUTHOR]

The largemouth bass Micropterus salmoides (Lacépède, 1802), a North American fish from the family Centrarchidae, is reported for the first time from Bulgaria. In April-June 2018, adult largemouth bass (2+ years) were captured in two localities in the Struma River Basin (south-west Bulgaria). In May 2018, individuals from several schools of recently hatched M. salmoides were collected. The presence of multiple-age classes suggests that M. salmoides survives and reproduces naturally in Bulgarian freshwaters. The increasing number of reports of captured specimens along the Struma River suggests the species may be widespread. The largemouth bass was probably imported and dispersed in Bulgarian freshwater bodies by anglers without a prior risk assessment or authorization by the responsible institutions. In accordance with nature conservation efforts, to prevent further dispersal of the largemouth bass in Bulgaria effective management policies should urgently be developed and implemented by the appropriate authorities. [ABSTRACT FROM AUTHOR]

The round goby Neogobius melanostomus is one of the most successful Ponto-Caspian fish invaders, established in a variety of aquatic ecosystems throughout Europe and North America. Diet and feeding strategies of N. melanostomus from three recently invaded tributaries of the Lower Danube River (Bulgaria) were investigated. Research design is based on monthly examination of the round goby diet and macroinvertebrate density. The results show that the round goby consume a wide range of different organisms – 76 taxa in total (identified to family, genus or species level). The most common prey taxa in all three rivers were Chironomidae (Orthocladius/Cricotopus spp., Tanytarsini g. sp.), Trichoptera larvae (Psychomyiidae g. sp., Hydropsyche spp.) and Ephemeroptera nymphs (Baetis spp.). Only in one of the rivers, a predominant feeding on crustaceans (Gammarus spp.) was observed. The taxonomical composition of consumed invertebrates suggests that the round goby prefers the ecological group of "sprawling/walking" insect larvae associated to coarse substratum. Feeding activity was observed throughout the whole year, even during low-temperature months. Three seasonal and four size-depended shifts in the species' diet were determined. Generalization feeding tactics predominated in all three rivers. Specialization was rare and specific for each tributary, where a few individuals consumed invertebrates from fine substratum and/or submerged vegetation. Possible relationships between the spread of N. melanostomus and the recent invasion of the Asian clam (Corbicula fluminea) in the Bulgarian Danube tributaries were discussed. Generalisation and feeding plasticity contribute to the round goby's establishment success in the Danube tributaries and might facilitate its further upstream spread. [ABSTRACT FROM AUTHOR]

The threat posed by invasive non-native species worldwide requires a global approach to identify which introduced species are likely to pose an elevated risk of impact to native species and ecosystems. To inform policy, stakeholders and management decisions on global threats to aquatic ecosystems, 195 assessors representing 120 risk assessment areas across all six inhabited continents screened 819 non-native species from 15 groups of aquatic organisms (freshwater, brackish, marine plants and animals) using the Aquatic Species Invasiveness Screening Kit. This multi-lingual decision-support tool for the risk screening of aquatic organisms provides assessors with risk scores for a species under current and future climate change conditions that, following a statistically based calibration, permits the accurate classification of species into high-, medium- and low-risk categories under current and predicted climate conditions. The 1730 screenings undertaken encompassed wide geographical areas (regions, political entities, parts thereof, water bodies, river basins, lake drainage basins, and marine regions), which permitted thresholds to be identified for almost all aquatic organismal groups screened as well as for tropical, temperate and continental climate classes, and for tropical and temperate marine ecoregions. In total, 33 species were identified as posing a 'very high risk' of being or becoming invasive, and the scores of several of these species under current climate increased under future climate conditions, primarily due to their wide thermal tolerances. The risk thresholds determined for taxonomic groups and climate zones provide a basis against which area-specific or climate-based calibrated thresholds may be interpreted. In turn, the risk rankings help decision-makers identify which species require an immediate 'rapid' management action (e.g. eradication, control) to avoid or mitigate adverse impacts, which require a full risk assessment, and which are to be restricted or banned with regard to importation and/or sale as ornamental or aquarium/fishery enhancement., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)