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Over 2850 km long, the Danube River is certainly the most important river in Europe, flowing through Germany (687 km), Austria (358 km), Slovakia (172 km), Hungary (417 km), Croatia (138 km), Serbia (587 km), Bulgaria (472 km), Romania (1075 km), Moldovia (<1 km) and Ukraine (54 km) in a west-east direction, and representing a “water backbone” for many European countries located in its basin.

The black bullhead is an invasive species that forms dense populations and has a negative impact on the native ichthyofauna. Recent studies indicate that it is one of the most dominant invasive fish species in Serbian waters. Feeding habits based on stomach contents of individuals caught in Lake Sava were analyzed as a first step towards evaluating its possible negative impact on the native fish fauna of the lake. The following indices for diet analysis were applied: vacuity index, frequency of occurrence, numerical abundance and prominence value. Trophic niche breadth and seasonal trophic diversity were calculated using the Shannon's index. The diet spectrum comprised 16 different prey categories from five groups: Mollusca, Crustacea, Insecta, Teleostei and plants. Fish were the main prey in all seasons, followed by aquatic invertebrates. Plant material and terrestrial insects were used as food in relatively small quantities. Predation on fish eggs was also detected. Our research confirmed that this species is an opportunistic generalist. An ontogenetic diet shift was also detected. The lowest value of trophic diversity was found in age 1+ individuals (H=1.39), while the highest was found in age 3+ individuals (H=2.00). The widest niche breadth was recorded in spring 2011 (H=2.12).

Uzorci vode i sedimenta uzeti su tokom leta 2017. godine iz akumulacija Garaši, Vlasina, Perućac, Zaovine, Međuvršje i Savsko jezero. Analiza As, B, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb i Zn izvršena je pomoću optičke emisione spektroskopije sa induktivno spregnutom plazmom. Zabeležene su veće koncentacije elemenata u sedimentu u odnosu na vodu. Koncentracije elemenata nisu prelazile MDK vrednosti propisane za vodu. Koncentracija Cr za Zaovine, Cu za Međuvršje i Ni za Perućac, Zaovine i Međuvršje prelazile su MDK vrednosti propisane za sediment. Sediment svih jezera bio je kontaminiran; najmanja kontaminacija zabeležena je za akumulaciju za vodosnabdevanje (Garaši)., Water and sediment samples were collected during summer of 2017 from reservoirs: Garaši, Vlasina, Perućac, Zaovine, Međuvršje, Sava Lake. Analysis of As, B, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, and Zn were performed using inductively-coupled plasma optical emission spectrometry. Elemental concentrations were higher in sediment compared to water samples. Elemental concentrations did not exceed the MACs prescribed for water. Concentration of Cr for Zaovine, Cu for Međuvršje, and Ni for Perućac, Zaovine and Međuvršje exceeded the MACs prescribed for sediment. Sediments of all reservoirs were contaminated; the least contamination was recorded for a water supply reservoir (Garaši).

Istraživanja su vršena tokom leta 2017. godine na tri akumulacije u Srbiji: Garaši, Perućac i Medjuvršje. Merenja temperature, koncentracije rastvorenog kiseonika, pH, totalno rastvorenih čestica, konduktiviteta i koncentracije hlorofila-a vršena su multiparametarskom sondom (YSI 6600 V2) od površine do 5 m dubine, sa intervalom od 1 m. Dobijeni rezultati pokazali su da se akumulacija Garaši izdvojila kao akumulacija sa najvišim sadržajem rastvorenog kiseonika i najvišom koncentracijom hlorofila a., i vodom koja je alkalna (pH>8)., This study was conducted during the summer of 2017 in three reservoirs in Serbia: Garaši, Perućac, and Medjuvršje. Measurements of temperature, dissolved oxygen concentration, pH, suspended particles, conductivity and concentration of chlorophyll-a were performed with a multiparameter water quality probe (YSI 6600 V2) from water surface to 5 m depth, with an interval of 1 m. The obtained results showed that Garaši reservoir stood out as the reservoir with the highest dissolved oxygen contentration and the highest concentration of chlorophyll a., and with alkaline water (pH> 8).

Water and sediment samples were collected during summer of 2017 from the reservoirs: Garaši, Vlasina, Perućac, Zaovine, Međuvršje, and Sava Lake. Analysis of As, B, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, and Zn were performed using inductively-coupled plasma optical emission spectrometry (ICP-OES). Accumulation of elements in water and sediment samples varied among reservoirs. Elemental concentrations were higher in sediment than in water samples. Furthermore, elemental concentrations did not exceed the MACs prescribed for water. The concentrations of As, Cd, and Pb in sediment of all reservoirs were greater compared to their average concentrations in the continental crust. Concentration of Cr for Zaovine, Cu for Međuvršje, and Ni for Perućac, Zaovine and Međuvršje exceeded the MACs prescribed for sediments. A severe enrichment was recorded for Cd at all studied reservoirs, as well as for As at Garaši and Cr at Zaovine. Sediments of all reservoirs were contaminated; the least contamination was recorded for a water supply reservoir (Garaši).

The Danube River basin represents a hotspot for the European freshwater diversity. The Danube in Serbia is characterized by a significant level of fish diversity, as well as by economically important fish resources. However, the Danube fish fauna is facing a number of adverse anthropogenic impacts, such as unsustainable fishery, habitat loss and fragmentation, water pollution and non-native species invasions. In this chapter, the major adverse impacts on the Danube fish communities in Serbia are presented, and the critical management measures are discussed, such as those related to supportive stocking and other conservation efforts. In order to ensure adequate protection and sustainable management of fish resources in the Danube in Serbia, it will be critical to introduce effective monitoring system, to establish and enforce adequate management measures, and to intensify research efforts on critical issues related to fish management and conservation.

The surface area of Montenegro’s portion of the Black Sea drainage basin is 7075 km2, or 0.9% of the entire basin, and is made up of the rivers Tara, Piva, Lim, Ćehotina and Ibar, and Lake Plav. The major polluters of watercourses are wastewaters and solid waste from inhabited settlements. According to water categorization regulations, the rivers of the Danube Basin in Montenegro are, based on the status quo, classified into four quality classes. Thus, the mid- and lower courses of the larger rivers fall within quality classes II and III. The water quality of the Lim, downstream of Berane, worsens at its exit from Montenegro and falls into quality class III. The Ibar profile downstream of Bać is largely out of a prescribed class. The Ćehotina downstream of Pljevlja is in water quality class III, and occasionally out of a prescribed class. The Tara belongs to the Durmitor National Park, but its water quality is class I only at its most upstream profile; its profiles downstream of Kolašin are from class II to class III. These findings are confirmed by analyses of their aquatic macroinvertebrate communities, i.e. Oligochaeta, as bioindicators of water quality. The ichthyofauna of the Danube Basin rivers in Montenegro comprises 20 fish species. Hydromorphological alterations of watercourses, such as dam construction, have a significant effect on fish populations. Thus, the dam on the Piva river has a negative effect on the fish population, presenting a barrier to migration routes, as is the case with huchen or Danube salmon (Hucho hucho).

The surface area of Montenegro’s portion of the Black Sea drainage basin is 7075 km2, or 0.9% of the entire basin, and is made up of the rivers Tara, Piva, Lim, Ćehotina and Ibar, and Lake Plav. The major polluters of watercourses are wastewaters and solid waste from inhabited settlements. According to water categorization regulations, the rivers of the Danube Basin in Montenegro are, based on the status quo, classified into four quality classes. Thus, the mid- and lower courses of the larger rivers fall within quality classes II and III. The water quality of the Lim, downstream of Berane, worsens at its exit from Montenegro and falls into quality class III. The Ibar profile downstream of Bać is largely out of a prescribed class. The Ćehotina downstream of Pljevlja is in water quality class III, and occasionally out of a prescribed class. The Tara belongs to the Durmitor National Park, but its water quality is class I only at its most upstream profile; its profiles downstream of Kolašin are from class II to class III. These findings are confirmed by analyses of their aquatic macroinvertebrate communities, i.e. Oligochaeta, as bioindicators of water quality. The ichthyofauna of the Danube Basin rivers in Montenegro comprises 20 fish species. Hydromorphological alterations of watercourses, such as dam construction, have a significant effect on fish populations. Thus, the dam on the Piva river has a negative effect on the fish population, presenting a barrier to migration routes, as is the case with huchen or Danube salmon (Hucho hucho).

The length-weight relationship and condition of 35 individuals of the cactus roach, caught in June 2017 in the Perućac reservoir, were analysed. The average length (± SD) of the sampled individuals was 29.8 ± 6.1cm and the average weight (± SD) was 334 ± 215 g. Age ranged from 2+ to 5+ years, and almost half of individuals were aged 2+. The regres-sion coefficient of the length-weight relationship was b > 3, which indicates a positive allometric growth. The Fulton’s condition factor ranged from 0.94 to 1.27, with the mean value (± SD) of 1.09 ± 0.09. Allometric condition factor ranged from 0.49 to 0.62, with the mean value (± SD) of 0.56 ± 0.04.

The life-history traits and population dynamics of the black bullhead Ameiurus melas (Rafinesque) were studied monthly from August 2009 to December 2012 in Lake Sava in Belgrade (Serbia). This period included a mass mortality event in May 2011 and subsequent population recovery during 2012. Available evidence suggests the mass mortality was caused by European catfish virus that only affected the black bullhead. Changes in key life-history traits and the recovery potential (increase in catch-per-unit-effort, doubling of young-of-the-year to adult ratio, earlier maturation and increased fecundity) following the mass mortality event suggest population resilience typical of a recurring boom-bust pattern and demonstrate the species' capacity to recover rapidly and re-establish following disturbance. Repeated systematic mass removals of both juveniles and adults (using fyke nets) will be required if future fishery management measures aimed at limiting the population size are to be effective.

Značaj riba kao bioindikatora kvaliteta vode potvrđen je brojnim istraživanjima, a zasniva se na njihovoj dugovečnosti, mobilnosti, pripadnosti različitim funkcionalnim trofičkim grupama i osetljivosti na širok spektar antropogenih stresora. Cilj ovog istraživanja bilo je poređenje diverziteta vrsta riba u akumulacijama Garaši, Perućac, Zaovine, Ovčar i Međuvršje na osnovu Šenonovog indeksa diverziteta (H), kao i procena kvaliteta vode na osnovu poređenja vrednosti ovog indeksa sa rezultatima merenja fizičkih i hemijskih parametara vode. Uzorkovanje riba je vršeno mrežama (20-50 m dužine, 1.5-2 m visine i 28-100 mm promera okca) tokom leta 2017. godine. Ukupno je uzorkovano 795 jedinki riba koje pripadaju 21 vrsti. Multiparametarska sonda YSI 6600 V2-2 korišćena je za prikupljanje podataka o fizičkim, hemijskim i biološkim karakteristikama ispitivanih akumulacija. Praćene su vrednosti pH, rastvorenog kiseonika (DO) i hlorofila a. Prema vrednostima Šenonovog indeksa diverziteta za pomenute akumulacije, diverzitet naselja riba pokazao je sledeći trend: Perućac (H = 1.81) > Ovčar (H = 1.69) > Garaši (H = 1.52) > Zaovine (H = 1.45) > Međuvršje (H = 1.27). A kumulacije G araši, Perućac i O včar i male su k valitet vode koji spada u II-III klasu, dok je lošiji kvalitet (III-IV klasa) zabeležen za akumulacije Zaovine i Međuvršje. Procena kvaliteta vode na osnovu Šenonovog indeksa diverziteta poklapa se sa procenom kvaliteta vode baziranom na merenju pH vrednosti, rastvorenog kiseonika (DO) i hlorofila a, osim za akumulaciju Zaovine. Rezultati istraživanja sugerišu da kombinovanje Šenonovog indeksa diverziteta i merenja fizičkih i hemijskih parametara može pružiti pouzdanu procenu kvaliteta vode u istraživanim akumulacijama.

Ovaj rad ima za cilj da ustanovi koja je uloga zakona u dobrom upravljanju ribolovnim resursima. Zaštita i održivo korišćenje ribljeg fonda u Srbiji regulisano je istoimenim zakonom, a odvija se na 17 ribarskih područja i 29 ribarskih područja u zaštićenim prirodnim dobrima. Na 17 osnovnih ribarskih područja upravljaju „javna preduzeća“ ili privatne firme u statusu „doo“. Kod većih područja, posebno u Vojvodini, postoji više upravljača za jedno ribarsko područje. U zaštićenim prirodnim dobrima upravljači su ujedno i korisnici ribljeg fonda. U periodu od 2006. do 2017. godine Zakon o zaštiti i održivom korišćenju ribljeg fonda menjan je u dva navrata: 2009. i 2014. godine. Podzakonska akta su takođe podložna vrlo čestim promenama. U radu se prate efekti promene zakonske regulative na broj rekre ativnih i profesionalnih ribolovaca, broj ribočuvara i trendove u ulovima. Osnovni trendovi su da broj rekreativnih ribolovaca u navedenom periodu opada, broj profesionalnih riba ra stagnira, dok broj ribočuvara raste. Ulov ima tendenciju opadanja za sve vrste koje se statististički prate (šaran, som, smuđ i deverika od autohtonih vrsta, i babuška i tolstolobik od alohtonih). Odnos ulova u rekreativnom i komercijalnom ribolovu pokazuje značajne promene u poslednjih desetak godina: osim za tolstolobika, kod koga dominira komer cijalni ulov, kod ostalih vrsta je došlo do zamene u smislu da rekreativni ulov preuzima dominaciju u poslednjih nekoliko godina

Heavy metals have high potential to enter and accumulate in food chains and therefore are considered as critical contaminants of aquatic ecosystems (Erdoğrul and Erbilir 2007). Given that fish are situated at the top of the food chain they can accumulate large amount of heavy metals (Yilmaz et al. 2007). Heavy metals are taken up through different organs of the fish because of the affinity between them, and then are concentrated at different levels in different organs of the body (Bervoets et al. 2001). Accumulation of heavy metals in muscle tissues was the most studied, given that muscle tissue is the main fish part that is consumed by humans (Begum et al. 2013; Storelli et al. 2006). However, it is important to analyze other tissues since muscles are not always a good indicator of the whole fish body contamination (Has-Schön et al. 2006). The Danube River is the second longest river in Europe and runs through Serbia at a length of 587.4 km. The biggest industrial capacities are largely located on the Danube river banks - Belgrade, Novi Sad, Pančevo and Smederevo (Petrović 2015). The main problem is that none of them has any kind of system for treating municipal wastewaters (Veljković 2005). Within this study, published articles regarding heavy metal accumulation in different tissues of fish in the Danube in Serbia were analyzed in order to evaluate which species, tissues and elements were the most represented in the analyses. The main aim was to assess present practices in research of heavy metal pollution in this region, as well as to identify key gaps regarding the particular river sections, types of pollutants, tissues, and trophic and functional groups of fish communities that were inadequately represented in research.

Crni američki patuljasti som Ameiurus melas (Rafinesque, 1820) je jedna od najzastupljenijih i najuspešnijih alohtonih vrsta riba koja naseljava kopnene vode Evrope. Istraživanja koja su sprovedena u proteklih nekoliko godina ukazuju na to da je ovo dominantna invazivna vrsta i u mnogim kopnenim vodama u Srbiji, uključujući i Savsko jezero (Jaćimović, 2015). Zdravstveno stanje populacije crnog američkog patuljastog soma u Savskom jezeru procenjeno je na osnovu analize prevalencije i intenziteta parazitiranosti kod 2349 jedinki u uzorcima prikupljenim tokom 2011. i 2012. godine. Prevalencija parazitiranosti predstavljena je kao broj zaraženih jedinki, tj. procentualni udeo zaraženih jedinki, dok je intenzitet parazitiranosti predstavljen kao broj parazita u svakoj analiziranoj jedinki. Osim kod jedne jedinke kod koje je nađena cista Trematoda, svi uočeni endoparaziti pripadali su razdelu Nematoda. Najverovatnije se radilo o vrsti Philometra rischta. Zdravstveno stanje populacije tokom 2012. godine bilo je znatno bolje u odnosu na 2011. godinu. Jedinke starosti 3+ bile su najzaraženije endoparazitima, kako u svakoj pojedinačnoj godini, tako i u celom uzorku. Iako bi se moglo očekivati da nezaražene jedinke imaju veće vrednosti Fultonovog faktora kondicije (K) u poređenju sa zaraženim, rezultati istraživanja u Savkom jezeru pokazali su da takva pravilnost ne postoji.

Istraživanje populacione dinamike crnog američkog patuljastog soma Ameiurus melas (Rafinesque, 1820) u Savskom jezeru realizovano je u periodu od avgusta 2009. do decembra 2012. godine. U cilju pronalaženja načina za smanjenje i kontrolu brojnosti populacija ove invazivne alohtone vrste (pre svega u stajaćim vodama), važno je ispitati i selektivnost ribolovnog alata (vrša). Selektivnost vrša procenjena je na osnovu odnosa ulovljenih jedinki crnog američkog patuljastog soma i drugih vrsta riba, za svaki mesec pojedinačno. Ukupno je uhvaćeno 13866 jedinki, od čega su jedinke crnog američkog patuljastog soma činile 94.2%, grgeča (Perca fluviatilis) 3.2% i sunčice (Lepomis gibbosus) 2.1%. Ulov štuke (Esox lucius), deverike (Abramis brama), babuške (Carassius gibelio), šarana (Cyprinus carpio), bodorke (Rutilus rutilus)), smuđa (Sander lucioperca) i glavoča (Gobiidae) bio je na nivou statističke greške (< 1%). Rezultati istraživanja pokazuju da se vrše mogu primenjivati u kontroli brojnosti populacija crnog američkog patuljastog soma, jer osim što su selektivne, efikasne i lake za rukovanje, imaju minimalan negativni uticaj na druge vrste riba.

Human society reaps immense benefits from rivers and in the course of the last century man has considerably altered rivers, using them as waterways, for irrigation, energy production, water intakes, commercial and recreational fisheries, etc. As a consequence of anthropogenic effects, degradation of river courses has occurred and there is great need for preserving and restoring their physical, chemical and biological integrity. The Danube is an international river that connects 17 countries and has been under anthropogenic influence since ancient times. Intensive exploitation of fish in the early Holocene led to human settlements remaining for longer periods along the Danube. Numerous Neolithic archaeological sites demonstrate that fishing was the main economic activity of humans on the Danube. Alterations to the river's course in order to secure navigation through the Djerdap gorge were conducted already in the late 19th century, while significant changes in this part of the Danube occurred after the construction of the Iron Gate I and Iron Gate II dams, in 1970 and 1984, at Danube rkm 943 or rkm 863, respectively. This led to interruption of the Danube's longitudinal continuity and to the curtailment of migration routes of anadromous fish species that migrate from the Black Sea up the Danube for spawning. In the past, the beluga sturgeon (Huso huso) migrated all the way to Straubingin Germany (rkm 2327), and the Black Sea shad (Alosa immaculata) went as far as Budapest (rkm 1650). Overfishing and illegal fishing of sturgeon species for their highly prized caviar, water and sediment pollution, and physical alteration to the bottom and banks of the Danube, as well as the restriction of migration due to dam construction led to a significant decrease in population sizes of sturgeons and shads, which now have only 17.4 kilometers for spawning in the Serbian part of the Danube. At the same time, there was the introduction of non-native (allochthonous) fish species for a

Some recent data show that five gobiid species have spread outside their native range and expanded their distribution area in Serbia along the rivers Danube, Tisza, Sava,and Velika Morava. These are: monkey goby Neogobius fluviatilis, racer goby Neogobius gymnotrachelus, bighead goby Neogobius kessleri,round goby Neogobius melanostomus, and tubenose goby Proterorhinus semilunaris. The aim of this study was to compare the efficiency of two different methods for fish sampling, as well as to assess influence of the diel period on catch of gobiids in the Serbian stretch of the Danube River. During October 2012 and September 2013, the sampling was performed by electrofishing and beach seining in four riverside locations of the Danube River,as follows: at NoviSad (river km 1257), Belgrade(river km 1173),Tekija (river km 956),and Prahovo (river km 862),with the water depth being of up to120cm. At each location,the sampling was performed at 17:30,18:30,19:30 and 20:30. A total of 551 gobies were caught. The highest number of specimens (238) was registered at Tekija,followed by the sites at Belgrade(192) and Prahovo (109), and only 12 specimens caught at NoviSad.The catch of the round goby and monkey goby was more efficient by beach seining than by electrofishing, while the catch of the tubenose goby was more efficient by electrofishing. Both methods had similar efficiency in terms of the catch of the bighead goby and racer goby.The species diversity and the number of caught specimens were the highest at 18:30.All five species were recorded at the sites of Belgrade,Tekija and Prahovo,while only the monkey goby and racer goby were caught at NoviSad. The methods used in this study showed good efficiency in catch of gobiids, especially at dusk. Despite the difficulty in using these methods in certain habitats, we assume that they could be highly relevant in the regular monitoring of gobiids along the inshore parts of the rivers.

The effects of physical factors on fish catch in the Serbian part of the Danube River were studied for period of six decades. The data on total catch for the Danube River from river kilometre 845 to river kilometre 1433 were collected from Statistical Office of the Republic of Serbia, while water level and water temperature data were collected from 16 water gauge stations along the investigated part of the Danube River for the period 1948-2009. Cross-correlation functions have been used to analyse the functional connection between Danube water level, water temperature and fish catch while ARMA model which combines cyclic (deterministic) and random (stochastic) components of the analysed sequences was used for the forecasts. The cross-correlation function showed negative correlation between water level and temperature as well as between water temperature and catch and positive correlation between water level and catch. The Danube water level and catch were coherent at the periods of 2.06, 4.13, 6.2, 10.33, 20.66 years, while the cross correlation function between these time series did not show phase lag. The results of reconstruction and forecast of water level, temperature, and catch of fish in the Danube River, obtained by summing the cyclic and stochastic components, was used for the forecast till 2029. In 2016, seven years after, the initial forecasts were made, validity of the model was checked by obtaining data for water temperature, water level and fish catch in the Danube River for the period 2010-2015. Model gave the best prediction for water temperature; average standard error was 1.6 times higher for predicted value than for model value while for fish catch and water level they were 1.96 and 4.97, respectively. Methods used in this work could be powerful tool for prediction of fish catch and serve as the basis for better fisheries management.

The Amur sleeper (Perccottus glenii), an indigenous species to the Russian Far East, northern China and the northern part of the Korean Peninsula, is an invasive alien species to European waters. It may have negative impact on eggs and larvae of native fish species, aquatic macroinvertebrate and larval amphibians. There were few findings of the Amur sleeper specimens in Serbia during 2001-2003, while the first record of established population was in the Danube River flood plain (1139 river km) in 2005. Our study included 85 specimens from the Danube River canal near Veliko Gradigte, Serbia, caught in two consecutive years (November 2015 and October 2016), with the aim to assess the weight-length relationship of this population. Body length of specimens ranged from 35.78 mm to 140.68 mm, while body weight ranged from 0.5 g to 40 g. The age of specimens was 0+, 1+ and 2+. Length-weight analysis did not show differences between regression slopes of samples from the two years, with b values of 3.317 and 3.113 for 2015 and 2016, respectively.

The black bullhead (Ameiurus melas Rafinesque, 1820) is one of the most abundant and successful non-native fish species in European freshwaters. Recent studies indicate that it is also one of the most dominant invasive fish species in Serbian waters. The aim of this research was to determine feeding habits of the black bullhead in Lake Sava, as it is known that this species may negatively affect the native fauna through predation or competition for food/space. Black bullhead samples were collected monthly from 2009 to 2012, for four days in a row. Gastro-intestinal contents were analyzed in 2349 specimens. The analysis was done macroscopically, where possible, or under a binocular microscope. Prey categories were identified to the lowest possible taxonomic level. For the diet analysis, we used the following indices: the vacuity index, frequency of occurrence, numerical abundance, and prominence value. In addition, the seasonal trophic niche breadth was calculated according to the Shannon’s diversity index. Vacuity index ranged between 72.86% and 100%. The diet spectrum consisted of 14 different prey categories from five groups: Mollusca, Crustacea, Insecta, Teleostei, and plants. Fish were the main prey in all seasons, followed by aquatic invertebrates. Plant material and terrestrial insects were used as food in relatively small quantities. Fish-egg predation was also detected. Our research confirmed that this species is an opportunistic generalist, foraging on the most abundant and available prey. Ontogenetic diet shift was also detected. The lowest value of trophic diversity was found in the largest black bullheads, while the highest was found in the medium sized individuals. The widest niche breadth was recorded in the spring of 2011.

Several fish species in the family Salmonidae have evolved multiple life history forms that exploit different degrees of anadromy (McMillan, Pess, & Katz, 2007) and one of these is rainbow trout (Oncorhynchus mykiss). In some cases, anadromous O. mykiss undergo smoltification and migrate, however those migrations are completely within freshwater (Holecek, Scarnecchia, & Miller, 2012). Silvering of juvenile salmonids as a part of smoltification is a key determinant of migratory behavior (Haner, Faler, Schrock, Rondorf, & Maule, 1995). The silvering of the anadromous trout, rainbow trout and salmons, which functions as cryptic colorations in open water (Björnsson, Stefansson, & McCormick, 2011), is the change that occurs during the process of smoltification. However, biological changes that take place during smoltification do not preclude continued life in fresh water (Björnsson, Einarsdottir, & Power, 2012). Spawning of rainbow trout in Serbia has never been recorded. In recreational fishing, the rainbow trout has some significance only in those rivers where aquaculture facilities are located (Simonović, 2001). However, they are occasionally caught in the Danube (M. Nikčević, M., personal observation) and are assumed to be escapees from aquaculture. This study reports on the occurrence of smoltification in rainbow trout O. mykiss along the main bed of the Danube River in its Serbian section.

The concentrations of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se and Zn in the muscle, gills, liver and intestine of the wels catfish (Silurus glanis) from the Danube River were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The aim of the study was to determine whether in complex muscle/skin, gill filament/gill arch, proximal/distal liver and proximal/median/distal intestine samples, particular components differ in concentrations of the analyzed elements. Results indicated that there were no differences in the accumulation of different elements between the proximal and distal liver segments and between the proximal and median intestine sections. Conversely, elemental accumulation patterns in muscle and skin differed significantly. Significant differences were also observed between the gill arch and filaments, as well as between the distal and the two upper intestine sections. Findings indicated the importance of detailed reporting of tissue sampling, i. e. whether the skin was included in the muscle sample, as well as if the gill arch and filaments were analyzed together. Due to a potential bias that can be produced by different muscle/skin or gill arch/filament ratios included in the sample, we strongly recommend that they should not be analyzed together. Results of the present study might be of interest to the scientific community and stakeholders involved in aquatic ecosystem monitoring programs.

Crni američki patuljasti som (Ameiurus melas Rafinesque, 1820) je najzastupljenija i najuspešnija alohtona vrsta ribe koja naseljava kopnene vode Evrope. Istraživanja koja su sprovedena u proteklih nekoliko godina ukazuju na to da je ovo dominantna invazivna vrsta i u mnogim kopnenim vodama u Srbiji. Cilj istraživanja se odnosio na ispitivanje populacionih atributa patuljastog soma u Savskom jezeru, kao i na bioakumulaciju teških metala i mikroelemenata u različitim tkivima. Terenska istraživanja vršena su u periodu od 2009. do 2012. godine. Jedinke su uzorkovane jednom mesečno, četiri dana za redom. Laboratorijska obrada prikupljenih uzoraka obuhvatila je merenje totalne dužine tela (TL) i ukupne težine tela ulovljenih primeraka. Nakon disekcije jedinki merena je težina tela bez utrobe, kao i masa jetre i gonada.Struktura populacije utvrđena je kombinovanjem dve metode – analizom starosti jedinki i analizom dužinskih frekvencija. Starost jedinki određena je takođe uz pomoć dve metode – očitavanjem naraštajnih zona otolita i Batačarija metodom. Analiza uzrasne strukture populacije pokazala je prisustvo pet uzrasnih kategorija: od 0+ do 4+. Klasa 1+ bila je najdominatnija uzrasna klasa, kako po godinama, tako i po sezonama uzorkovanja. Prosečna totalna dužina jedinki iznosila je 13,3 cm, a prosečna težina 38,26 g. Prosečna relativna težina jedinki u Savskom jezeru, u celokupnom uzorku, iznosila je 84,78 ± 1,09 g. Utvrđeno je prisustvo ukupno 8 kohorti. Na osnovu analize dužinsko-težinskih odnosa izračunate su i vrednosti faktora alometrije b koje su se kretale od 2,93 do 3,16, dok je vrednost ovog faktora u celom poduzorku iznosila 3,00. Analizom krive dužinsko-težinskog odnosa utvrđeno je da tačka infleksije ne postoji. Ribolovni napor (CPUE) je korišćen za analizu distribucije i relativne gustine jedinki, kako po staništima, tako i po godinama uzorkovanja i sezonama. Tokom 2009, 2010. i 2012. godine CPUE vrednosti varirale su od 0,4 ± 0,5 do 41,5 ± 22,1 jedinki po, abundant and successful non-native fish species in European freshwaters. Recent studies indicate that the black bullhead is one of the most dominant invasive fish species in Serbian waters as well. The aim of this research was to determine the population characteristics of this species in Sava Lake, as well as bioaccumulation of heavy metals and trace elements in different tissues. Fieldwork was conducted from 2009 to 2012. Black bullhead samples were collected monthly, for four days in a row. Once in the laboratory, specimens were measured for their total length (TL) and body weight (W). After dissection, the eviscerated body weight was measured, as well as liver and gonad weights. Population structure was determined by combining two methods – analyzing age and length-frequency distributions. Age was determined by combining otolith examination and Bhattacharya’s method. Age structure analysis has revealed the presence of five age classes: from 0+ to 4+. The class 1+ was the most dominant, by both year and season of sampling. The average total length was 13.3 cm and the average weight was 38.26 g. The mean value of the relative weight, across the entire sample, was 84.78 ± 1.09. The research revealed the presence of eight cohorts, in total. The length-weight allometric factor b ranged from 2.93 to 3.16, while the value of this factor, across the entire sample, was 3.00.The analysis of the length-weight relationship curve showed an absence of inflection points. Catch-per-unit-effort (CPUE) was used as an indicator of the black bullhead distribution and abundance in different habitats, years of sampling, and seasons. In 2009, 2010, and 2012, CPUE values fluctuated and ranged from 0.4 ± 0.5 to 41.5 ± 22.1 ind./net/night, while in 2011 they were low and rather uniform, ranging from 0.2 ± 0.3 to 3.4 ± 4.5 ind./net/night. CPUE values were highest in the macrophyte vegetation zone, as well as during autumn and spring...