Mercury distribution in the great cormorant (Phalacrocorax carbo) from the Krogulna ponds and Nysa Klodzka River (Poland).

Mercury belongs among the highly hazardous substances present in the environment and represents a great health risk for many animals including predatory and piscivorous birds. The aim of this study was to assess the total mercury content in the main detoxifying organs (liver and kidney) and in the muscles of adult great cormorants (Phalacrocorax carbo) caught at two localities (the Krogulna ponds and the Nysa Klodzka River) in southern Poland. The aim was to compare the locality with the iron ore mining history (the Krogulna ponds) with a site without such load (the Nysa Klodzka River). The total mercury content in the great cormorants decreases as follows: kidneys > liver > muscle in both monitored localities and significantly differs between the localities. The average mercury content varied from 0.58 ± 0.38 mg/kg (muscle) to 1.39 ± 1.42 mg/kg (kidney) in the cormorant from the Krogulna ponds and from 1.09 ± 0.40 mg/kg (muscle) to 3.12 ± 1.55 mg/kg (kidney) in the cormorant from the Nysa Klodzka River. The accumulation of mercury does not correlate with the mining history as it was higher at the Nysa Klodzka River, but it is probably influenced by the different fish stocks in these two localities (omnivorous fish in the Krogulna ponds versus predatory fish in the Nysa Klodzka River).

Characteristics and Cryopreservation of Semen of Sex-Reversed Females of Salmonid Fish.

Sex reversal has been used as a breeding strategy by salmonid fish to produce genetically and phenotypically single sex populations. Production of all-female fish has great importance for the creation of monosex female triploids of salmonid fish, which are valued for their sterility, lack of female maturation, and larger commercial size. Among salmonids, the majority of rainbow trout (Oncorhynchus mykiss) production is based on all-female production with a high proportion of all-female triploid production in Europe. The main aim of this review is to present the recent knowledge regarding sex-reversed females (SRFs) of salmonid fish. We discuss the methods of sex reversal as well as their effects on the morphology and histology of the reproductive tract. We focus on the characteristics of SRF semen as well as the factors determining semen quality. The lower quality of SRF sperm compared to that of normal males has resulted in the need for the artificial maturation of semen. Most importantly, methods of semen storage-both short-term and long-term (cryopreservation)-that can improve hatchery operations are presented with the special emphasis on recent progress in development of efficient cryopreservation procedures and use of cryopreserved semen in hatchery practice. Moreover, we also address the emerging knowledge concerning the proteomic investigations of salmonid sperm, focusing primarily on the proteomic comparison of normal male and SRF testicular semen and presenting changes in SRF rainbow trout sperm proteome after in vitro incubation in artificial seminal plasma.

Characterization of carp seminal plasma Wap65-2 and its participation in the testicular immune response and temperature acclimation.

Two functionally distinct isoforms of warm-temperature acclimation related 65-kDa protein (Wap65-1 and Wap65-2) with a role in the immune response are present in fish. To our knowledge, contrary to Wap65-1, Wap65-2 has neither been isolated nor functionally characterized in carp especially in reproductive system. The aim of this study was to characterize Wap65-2 and ascertain its functions in immune response and temperature acclimation within reproductive system. Wap65-2 corresponded to one of the most abundant proteins in carp seminal plasma, with a high immunologic similarity to their counterparts in seminal plasma of other fish species and a wide tissue distribution, with predominant expression in the liver. The immunohistochemical localization of Wap65-2 to spermatogonia, Leydig cells, and the epithelium of blood vessels within the testis suggests its role in iron metabolism during spermatogenesis and maintenance of blood-testis barrier integrity. Wap65-2 secretion by the epithelial cells of the spermatic duct and its presence around spermatozoa suggests its involvement in the protection of spermatozoa against damage caused by heme released from erythrocytes following hemorrhage and inflammation. Our results revealed an isoform-specific response of Wap65 to temperature acclimation and Aeromonas salmonicida infection which alters blood-testis barrier integrity. Wap65-2 seems to be related to the immune response against bacteria, while Wap65-1 seems to be involved in temperature acclimation. This study expands the understanding of the mechanism of carp testicular immunity against bacterial challenge and temperature changes, in which Wap65-2 seems to be involved and highlights their potential usefulness as biomarkers of inflammation and temperature acclimation.

Feed contamination with zearalenone promotes growth but affects the immune system of rainbow trout.

To investigate the effects of feed contamination with zearalenone (ZEN) at the current European Commission (EC) guidance value (2 mg⋅kg-1 feed) on the growth and health of rainbow trout, we performed a long-term feeding trial under aquaculture conditions. It started with the external feeding of the fish larvae, and continued for 96 weeks, at which point the fish had reached market size. To assess the growth of fish and their feeding efficiency throughout this period, the fish were regularly weighed and measured, and their feed consumption was monitored. Additionally, to investigate potential health effects, after 72 weeks of the exposure to ZEN, the fishes' blood was analyzed for major hematological and biochemical indices, and their head kidney, spleen, and liver were examined for morphological, histopathological, cytological, and molecular changes. Finally, to gain insight into the metabolism and distribution of ZEN in fish, the content of free and glucuronidated forms of ZEN and its major metabolites was measured in the intestine, liver, and muscles of the exposed fish. The feed-borne exposure of rainbow trout to ZEN at a dose of 2 mg⋅kg-1 feed resulted in higher feeding efficiency and growth rate, most probably due to the anabolic properties of the ZEN metabolite. Importantly for the consumers of fish, despite absorption and metabolism of ZEN in the digestive system of the fish that had been exposed for 72 weeks, the residuals of ZEN were not transferred to the fishes' muscles, which rules out a potential risk to human health related to the consumption of fish meat. However, the increased growth of fish fed with the contaminated feed may come at some cost, as the exposure to ZEN was associated with modulation of key components of the adaptive and innate immune systems. Moreover, the trunk kidney of ZEN-fed fish showed massive inflammation that was likely caused by pathogen infection. These findings raise concerns about fish health under the current recommended EC guidance values.

Acclimation to cold and warm temperatures is associated with differential expression of male carp blood proteins involved in acute phase and stress responses, and lipid metabolism.

The environmental temperature affects plasma biochemical indicators, antioxidant status and hematological and immunological parameters in fish. So far, only single blood proteins have been identified in response to temperature changes. The aim of this study was to compare the proteome of carp blood plasma from males acclimated to warm (30 °C) and cold (10 °C) temperatures by two-dimensional differential gel electrophoresis followed by MALDI-TOF/TOF mass spectrometry. A total of 47 spots were found to be differentially regulated by temperature (>1.2-fold change, p < 0.05): 25 protein spots were more abundant in warm-acclimated males and 22 were enriched in cold-acclimated males. The majority of differentially regulated proteins were associated with acute phase response signalling involved in: i) activation of the complement system (complement C3-H1), ii) neutralization of proteolytic enzymes (inter-alpha inhibitor H3, fetuin, serpinA1, antithrombin, alpha2-macroglobulin), iii) scavenging of free hemoglobin and radicals (haptoglobin, Wap65 kDa), iv) clot-formation (fibrinogen beta and alpha chain, T-kininogen) and v) the host's immune response modulation (ApoA1 and ApoA2). However, quite different sets of these proteins or proteoforms were involved in response to cold and warm temperatures. In addition, cold acclimation seems to be related to the proteins involved in lipid metabolism (apolipoproteins A and 14 kDa) and stress response (corticosteroid binding globulin). We discovered a strongly regulated protein Cap31 upon cold acclimation, which can serve as a potential blood biomarker of cold response in carp. These studies significantly extend our knowledge concerning mechanisms underlying thermal adaptation in poikilotherms.

Helminths of European smelt Osmerus eperlanus (Linnaeus, 1758) in Lake Hancza and the Vistula Lagoon, with special regard to their zoonotic threats.

Reports published in recent years on the increased risk in the Baltic Sea of fish infection with larvae of nematodes potentially pathogenic to humans have prompted a study of European smelt Osmerus eperlanus with respect to the presence of these parasites in the Vistula Lagoon. Additionally, samples of this fish from Lake Hancza were comparatively considered. The body cavity, the surface of internal organs, the alimentary tract and the swimbladder of fish from two environmentally different water bodies were taken into account in the analysis. Only the tapeworm Proteocephalus longicollis was found in the alimentary tract of the fish from Lake Hancza. Depending on the period of study, the prevalence ranged from 60% to 100%, and the mean intensity from 11.5 to 42.0. The helminth fauna of smelt from the Vistula Lagoon was more diverse: Cystidicola farionis occurred with a prevalence from 25.9 to 75.0% and a mean intensity of infection from 2.7 to 66.0%, Proteocephalus longicollis and cystacanths of Corynosoma with a prevalence ranging in different years from 12.0 to 44.0% and from 14.8 to 50.0%, respectively. Zoonotic parasites, such as nematode larvae of the Anisakidae family (Contracaecum sp., Anisakis simplex), occurred in the intestine with a prevalence of 31%, 72% and 22% in consecutive years of the study.

Temporal Changes in Gametogenesis of the Invasive Chinese Pond Mussel Sinanodonta woodiana (Lea, 1834) (Bivalvia: Unionidae) from the Konin Lakes System (Central Poland).

Gametogenesis and the temporal changes occurring in the ovaries and testes throughout the reproductive cycle in the invasive alien bivalve, Chinese pond mussel Sinanodonta woodiana (Lea), from the heated Konin lakes system (central Poland) were studied using histological techniques. S. woodiana was confirmed to be a gonochoristic species with overall sex ratio of 1:1. The examined morphological parameters of Chinese pond mussel spermatozoa, i.e. 42 µm mean total length; 4.3 µm mean head length and the maximum size of previtellogenic (34-43 µm) and vitellogenic oocytes (75-83 µm) are consistent with values established for closely related members of the Unionidae family. Our results suggest that S. woodiana in the Konin lakes system are able to spawn throughout March to October, with a season of higher reproductive activity in females extending from March to April. This type of reproductive biology may contribute to the Chinese pond mussel's success in thriving in freshwater ecosystems.

Molecular cytogenetic study of the European bitterling Rhodeus amarus (Teleostei: Cyprinidae: Acheilognathinae).

The European bitterlings (Rhodeus amarus) from the Eastern locations were cytogenetically examined by conventional and molecular techniques. All analyzed individuals presented invariably the same chromosomal constitution of 2n = 48, with 8 metacentrics + 20 submetacentrics + 20 subtelo-acrocentrics and C-banding positive heterochromatin at the pericentromeric regions in most of the chromosomes. Moreover, some of the chromosomes had short arms entirely built with heterochromatin. GC-rich Ag-NORs (nucleolus organizer regions) were located at the short arms of two submetacentric chromosomes, and the length polymorphism of these regions was found. Multiple location of 28S rDNA sequences with fluorescence in situ hybridization signals was observed on the long and/or short arms of three submetacentric chromosomes including NOR regions and short arms of three to five acrocentric chromosomes in the studied fish. 5S rDNA sites were found on the short arms of two subtelocentric chromosomes, and telomeric repeats were localized at the ends of all chromosomes. Provided results have expanded our knowledge concerning genetic characteristics of the European bitterlings that may be profitable in the conservation programs of this endangered species.

Paternal-effect-genes revealed through sperm cryopreservation in Perca fluviatilis.

Knowledge about paternal-effect-genes (PEGs) (genes whose expression in the progeny is influenced by paternal factors present in the sperm) in fish is very limited. To explore this issue, we used milt cryopreservation as a specific challenge test for sperm cells, thus enabling selection amidst cryo-sensitivity. We created two groups of Eurasian perch (Perca fluviatilis) as a model - eggs fertilized either with fresh (Fresh group) or cryopreserved (Cryo group) milt from the same male followed by phenotypic-transcriptomic examination of consequences of cryopreservation in obtained progeny (at larval stages). Most of the phenotypical observations were similar in both groups, except the final weight which was higher in the Cryo group. Milt cryopreservation appeared to act as a "positive selection" factor, upregulating most PEGs in the Cryo group. Transcriptomic profile of freshly hatched larvae sourced genes involved in the development of visual perception and we identified them as PEGs. Consequently, larvae from the Cryo group exhibited enhanced eyesight, potentially contributing to more efficient foraging and weight gain compared to the Fresh group. This study unveils, for the first time, the significant influence of the paternal genome on the development of the visual system in fish, highlighting pde6g, opn1lw1, and rbp4l as novel PEGs.

Ovarian alterations in wild northern pike Esox lucius females.

The aim of the present study was to analyse the occurrence of macroscopically visible ovary alterations in 2 populations of northern pike Esox lucius L. originating from lakes in the Mazurian Lake District (NE Poland). The alterations were characterised by ovary tissue that was morphologically malformed, in part or in whole, and contained immature oocytes, i.e. trophoplastic or previtellogenic oocytes instead of vitellogenic oocytes. These alterations were found only in the ovaries, and no morphological alterations of the testes were noted. Macroscopic and histological analyses were carried out in order to classify the observed alterations in the ovaries. Three types of alterations were identified in which morphological malformations as well as histological investigation of the ovaries were considered. An analysis of the size and age of the fish in relation to the occurrence of alterations as well as of the macroscopic and histological nature of the alteration types was made. The data obtained revealed no lake or age dependency of the observed alterations. Based on the results obtained, we suggest that the presence of endocrine disruptors in the environment or/and genetic factors could be responsible for these kinds of gonad anomalies. However, our results did not allow us to determine the aetiology of the alterations.

Telomerase Activity in Somatic Tissues and Ovaries of Diploid and Triploid Rainbow Trout (Oncorhynchus mykiss) Females.

Telomerase activity has been found in the somatic tissues of rainbow trout. The enzyme is essential for maintaining telomere length but also assures homeostasis of the fish organs, playing an important role during tissue regeneration. The unique morphological and physiological characteristics of triploid rainbow trout, when compared to diploid specimens, make them a promising model for studies concerning telomerase activity. Thus, in this study, we examined the expression of the Tert gene in various organs of subadult and adult diploid and triploid rainbow trout females. Upregulated Tert mRNA transcription was observed in all the examined somatic tissues sampled from the triploid fish when compared to diploid individuals. Contrastingly, Tert expression in the ovaries was significantly decreased in the triploid specimens. Within the diploids, the highest expression of Tert was observed in the liver and in the ovaries of the subadult individuals. In the triploids, Tert expression was increased in the somatic tissues, while the ovaries exhibited lower activity of telomerase compared to other organs and decreased compared to the ovaries in the diploids. The ovaries of triploid individuals were underdeveloped, consisting of only a few oocytes. The lack of germ cells, which are usually characterized by high Tert expression, might be responsible for the decrease in telomerase activity in the triploid ovaries. The increase in Tert expression in triploid somatic tissues suggests that they require higher telomerase activity to cope with environmental stress and maintain internal homeostasis.

Differences in predator-avoidance behavior between two invasive gobies and their native competitors.

Globally, fish are frequently introduced beyond their native range. Some, like Ponto-Caspian gobies, are becoming invasive, achieving high colonization rates and constituting frequent prey for native predators. However, little is known about the effectiveness of antipredator behaviors of the invaders, which may shape their role in the invaded community and contribute to the invasion success. We compared antipredator behaviors of invasive gobies and native fish species after their detection by the predator, when the danger becomes direct. We studied 2 fish pairs, each consisting of an invasive and native species co-occurring in the environment and belonging to the same prey guild: (1) the racer goby Babka gymnotrachelus versus European bullhead Cottus gobio, (2) the monkey goby Neogobius fluviatilis versus gudgeon Gobio gobio, facing a naïve predator (the Eurasian perch Perca fluviatilis). We analyzed behaviors of single prey individuals (escaping, staying in shelter, and activity) and single predators (activity, searching, following, capturing, and latency to prey consumption). In the predator presence, the bullhead was less active and more often managed to escape after capture than the racer goby. The gudgeon escaped before the capture more often than the monkey goby. The predator succeeded later with the bullhead compared to racer goby, whereas no differences in ingestion time occurred between the gudgeon and monkey goby. The results suggest that, in terms of hunting effort of native predators, the invasive gobies are equivalent to or more profitable prey than their native analogs, which can facilitate the integration of the gobies into local food webs.

Cystidicola farionis, a Swim Bladder Parasite of European Smelt: Characterization of the Nematode Trehalose Strategy.

The molecular identification of Cystidicola farionis (a swim bladder nematode of European smelt from the Vistula Lagoon in Poland) was performed. Their prevalence level was determined, and changes in the trehalose synthesis pathway in larvae and adult nematodes were demonstrated. The trehalose level was almost four times higher in adult nematodes than in larvae. In contrast, the activity of both enzymes (trehalose 6-phosphate synthase, TPS and trehalose 6-phosphate phosphatase, TPP) involved in the synthesis of trehalose was higher in larvae than in adults under optimal conditions. The optimum pH for TPS isolated from larvae and adults was pH 7.0. The optimum pH for TPP from larvae and adults was pH 7.0 and pH 8.0, respectively. The optimal temperature was 20 °C, and Mg2+ ions were an activator for trehalose-synthetizing enzymes from both sources. Enzymes isolated from adult nematodes were less susceptible to divalent ion chelator and inorganic phosphate than larval enzymes. The dynamic transformation of trehalose in the nematode developing inside the swim bladder of the smelt appears to be an important metabolic pathway in the nematode survival strategy. These studies are aimed at a better understanding of the issue of the metabolic adaptation of parasites, which, in the future, may indirectly contribute to the elimination of the parasite from aquacultures, which will impact public health.

Application of high hydrostatic pressure (HHP) shock to induce triploid development in the European grayling (Thymallus thymallus L.).

Use of parental individuals from allochthonous lineages to produce stocking specimens resulted in genetic contamination of many local European grayling (Thymallus thymallus) populations. Stocking programs including triploid individuals that are not able to reproduce with stocks located in natural habitats due to being reproductively sterile could contribute to populations of the European grayling and prevent regional lineages from the genetic pollution. The main goal of the present research, therefore, was to provide conditions for triploidization of the European grayling and examine gonads of triploid individuals. High Hydrostatic Pressure (HHP) shock (9000 psi/5 min.) applied to activated and incubated grayling eggs at 10 °C between the 17 min. 30 sec. and 20 minute timepoints subsequent to the time of insemination resulted in inhibition of the second polar body release and triploidization. Histological analysis confirmed that all 1-yr-old cytogenetically confirmed triploids had gonadal tissues indicative of sterility with the gonads being composed of connective tissue including fibroblasts, adipocytes and degenerated epithelial structures without there being any differentiated germ cells detected. There were no differences in the body length and Fulton's condition factor between 1 year old diploid and triploid grayling, however, the average body weight was markedly less in triploid than diploid specimens. Although, 1-yr-old triploid European female and male graylings did not have fully developed gonads, before these triploid specimens are deemed safe, as a result of being reproductively sterile for stocking in natural grayling habitats, there needs to be further examination of the 2- and 3-year-old triploid specimens.

Gyrodactylus proterorhini Ergens, 1967 (Monogenoidea, Gyrodactylidae) in gobiids from the Vistula River--the first record of the parasite in Poland.

During a parasitological survey of two non-native fish species-racer goby Neogobius gymnotrachelus and monkey goby Neogobius fluviatilis-in the Wloclawek Reservoir on the lower Vistula River, the monogenean Gyrodactylus proterorhini was recorded for the first time in Poland, and for the first time, the racer goby was listed as the parasite host. Specimens of G. proterorhini were detected on body surface, fins, and gills of the fish studied. In 2006, the monkey goby was infected with the prevalence 41%, racer goby with the prevalence of 47%, at maximal intensity of 6 and 9 parasites per fish, (respectively). The infection level in relation to the fish size and sampling season was also discussed. The presented study supports the hypothesis of progressive introduction of the parasite with gobiids to the colonized areas.

Perfluoroalkyl Substance Contamination Levels of Pike (Esox lucius L.) and Roach (Rutilus rutilus L.) from Selected Masurian Lakes in Eastern Europe.

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are of increased concern because of their bioaccumulation in humans and the biota, the health risk they pose to humans and other animals, and their persistence in the environment. In the present study, the occurrence of PFAS in selected tissues from pike (Esox lucius L.) and roach (Rutilus rutilus L.) collected from two lakes in the Masurian Lake District (Poland) in eastern Europe was addressed. Ten PFAS were analyzed in the tissue of the brain, liver, kidneys, gonads, and muscles by micro-high-performance liquid chromatography coupled with tandem mass spectrometry. Only perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA), perfluorononanoic acid, and perfluorodecanoic acid were detected above the limit of quantification. Concentrations in pike and roach from Elckie Lake and Hancza Lake were estimated to be 14/27 and 4.4/3.2 ng/g wet weight, respectively. The contributions of PFOS and PFOA to the sum of PFAS calculated for particular tissues for each fish species were higher than those of the other analyzed compounds: PFOS was found to be predominant in fish from Elckie Lake, whereas PFOA predominated in fish from Hancza Lake. It was noted that PFAS concentrations in tissues declined in the following order: kidney > gonads ≈ brain > liver > muscle. The sum of the greatest estimated PFAS concentration was 9.7 ng/g wet weight in kidneys of pike collected from Hancza Lake. No correlation was noted between PFAS concentration and fish size. The information provided in our study gives a better understanding of the potential dependencies in PFAS distribution and accumulation in biota. Environ Toxicol Chem 2021;40:3317-3327. © 2021 SETAC.

Isolation and characterization of transferrin from common carp (Cyprinus carpio L) seminal plasma.

Transferrin (Tf) in fish is recognized as a component of non-specific humoral defense mechanisms against bacteria. It is a major protein of common carp seminal plasma but its structure and localization in carp testis is unknown. In this study we developed a simple and efficient three-step purification procedure consisting of affinity chromatography (Con A-Sepharose), hydrophobic interaction chromatography (Phenyl Sepharose) and gel filtration (Superdex 200). The molecular mass of Tf has been determined to be 73.6 kDa and isoelectric point 5.1. The peculiar characteristics of carp transferrin were the lack of carbohydrate component and binding of iron ions by only one functional iron-binding site. Western blot analysis revealed a strong similarity of carp seminal plasma Tf to carp blood Tf and Tf from seminal plasma of other cyprinids but a lower similarity to salmonid and percid fishes. Tf was localized to the blood vessels of the carp testis which strongly suggest that most Tf of carp seminal plasma originates from blood. In conclusion, seminal plasma Tf has a unique structure and is similar or identical to blood Tf.

First record of Nippotaenia mogurndae Yamaguti and Miyata, 1940 (Cestoda, Nippotaeniidae), a parasite introduced with Chinese sleeper to Poland.

A parasitological survey of an Asian fish, Chinese sleeper Perccottus glenii, inhabiting the Wloclawek Reservoir on the lower Vistula River (central Poland) revealed the occurrence of an exotic cestode Nippotaenia mogurndae Yamaguti and Miyata, 1940, which spreads spontaneously with a host to European waters. This paper documented the first record of the parasite in Poland. Specimens were described, and the effect of fish size and season on the infection level was analyzed. Within the 3 years, 112 fish were examined. Samples were collected by electrofishing in the upper part of the Wloclawek Reservoir three times (seasonally) in 2006 and 2007 and once in autumn 2008. Parasitological indices (prevalence, intensity of infection, and abundance) in particular samples as well as in two size groups of fish were calculated. Statistical tests were applied to analyze the results (Spearman's correlation coefficient and Mann-Whitney's U test). The infection level of Chinese sleeper with N. mogurndae in the Wloclawek Reservoir (prevalence 54.7%, mean intensity of infection 7.2, maximum 29) was comparable to infection noted in the area of the natural range of the host. The infection of smaller fish (less than 80 mm, TL) increased gradually with increasing body size. The infection of bigger specimens was in general higher, but the parasite recruitment was of a more incidental character. The spreading of N. mogurndae with Chinese sleeper is still in progress. The Wloclawek Reservoir provides suitable conditions for parasite development.

Semen biology of vendace (Coregonus albula L.).

The objective of this study was to describe the morphometry and motility parameters of vendace (Coregonus albula) spermatozoa. Morphometric parameters of vendace sperm head and tail were of values similar to rainbow trout. The effects of pH, sodium, potassium and calcium ion concentrations on computer-assisted sperm analysis (CASA) sperm motility characteristics were tested. Vendace sperm was motile in a wide pH range of 6.0-10.5 with the optimum pH established at 9.0. Increases in potassium and calcium ions caused decreases in the percentage of motile sperm. The CASA parameters and erratic sperm movement pattern of vendace spermatozoa were similar to whitefish (C. lavaretus) sperm motility, suggesting that there is a coregonid-specific sperm motility pattern.

Free amino acids as indicators of nutritional status of silver bream (Vimba vimba), when using commercial and purified diets.

Studies on larval rearing of silver bream (Vimba vimba), a migratory cyprinid fish have addressed on limited scale larval and juvenile rearing using commercial and semipurified diets along with live feeds, such as brine shrimp Artemia nauplii. The objectives of the present study were (1) to determine whether experimental, protein-, peptide-, free amino acid-based diets are adequate for larval silver bream, a stomachless fish, (2) to evaluate whether commercial and purified diets are comparable as the first/exclusive feed for growth and survival of silver bream, and (3) to examine whether free amino acid concentrations in fish body are potential indicators of availability of amino acid sources. We report here the differences in diets acceptance, fish growth and diet utilization in silver bream in comparison to other cyprinid fishes. We specifically address the response in free amino acids in the body to dietary treatments. Experimental diets included: a commercial Aglo Norse feed, casein-gelatin based diet (CG), free amino acid mixture diet (FAA), dipeptide (PP), dipeptide-protein (PP50) based diet, and dipeptide diet without arginine (NoArg). In addition, live Artemia were offered to 3 groups and "fasting" control treatment was included during 3 week long trial. Fish offered Artemia overperformed those offered formulated diets both in terms of mass (80.7+/-26.3 mg) and survival (97.2%). We also indicate that commercial and purified diets are comparable as the first/exclusive feed for growth and survival of silver bream. Our experiment also showed that the whole body free amino acid concentrations of 9 indispensable amino acids (IDAA) out of 10 (His, Thr, Arg, Val, Met, Ile, Leu, Trp, Lys) in the PP50 group was the highest among 7 diet treatments and the totalfree amino acid concentration, total dispensable amino acids (DAA) and total IDAA of the PP50 diet fed fish showed the same trend. This may indicate that diets based on 50% of dipeptides and 50% of protein are adequate for silver bream larvae as they result in similar growth, survival and whole body IDAA concentrations as in fish fed CG (protein-based) diet fed fish.