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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 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. [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]

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]

We experimentally tested the potential use of artificial substrate - plastic bottles for egg deposition of non-native pumpkinseed sunfish, Lepomis gibbosus L. in a former sand-pit lake in western Bulgaria. The lake is situated close to an urban area, and is characterised by high pumpkinseed abundance. The study was carried out for a period of 105 days during the active pumpkinseed breeding season (from May to August). Five sites along the littoral area of the lake, with different habitat characteristics were selected. Each site was divided into two sections - an experimental and a control section. The plastic bottles (cut into two) were fixed to the lake bottom in the area of the experimental sections. Pumpkinseeds actively used the artificial substrate for nesting and the average use of each bottle was 2.3 times. Simultaneously, on the natural substrate of the experimental sections a total of 372 nests were built. The results showed that the choice of artificial substrate was neither associated with the type of bottom substrate, nor the remaining environmental traits that were studied. However, the use of artificial substrate has made a significant contribution to the total number of nests only on sites with "unsuitable" habitat parameters. These results show that waste products can seriously facilitate pumpkinseed's abundance. [ABSTRACT FROM AUTHOR]

Highlights: [•] Impact of a small-scale hydropower device on downstream moving fish was assessed. [•] Blade strike was related to fish length, swimming speed, and blade rotational speed. [•] Strike increased and decreased for trout and eel when behaviour was considered. [•] A pinch point created by the hydropower device caused damage to euthanised fish. [•] A field study supported observations from flume experiments and blade strike models. [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]

Fish established outside their native range often express various changes in external morphology, which may result in significant variation between individuals inhabiting different regions. In the current study we aim to explore differences in morphology among several populations of the round goby (Neogobius melanostomus), an invasive fish in Europe and North America. A total of 753 round gobies were collected from several invasive and native populations, including marine, brackish, freshwater, lake and river ecosystems belonging to the Black Sea, Baltic Sea and Great Lakes watersheds. We analysed 35 metric and meristic characters using ANOVA, discriminant analysis and allometric coefficients. Our results indicate significant differences between most of the populations in characters such as inter orbital distance, eye diameter, head width, second dorsal fin length and depth. River dwelling round gobies had elongated ventral fins compared to those from standing water bodies—a possible adaptation for upstream dispersal in flowing habitats. Most of the morphologic characters expressed positive allometry with significant differences in allometric coefficients between populations. Probably due to their recent invasive history, some of the head characters and allometric models grouped together round gobies from the Great Lakes watershed with those from a newly established population in the Danube tributaries. Similarities between geographically distant round goby populations might reflect common adaptations in external morphology during the course of invasion. [ABSTRACT FROM AUTHOR]