A note on the growth‐rate and year‐class strength of bream, Abramis brama (L.), in three eutrophic lakes, England

Three models of the sieving mechanism of the branchial sieve were used to predict the ability to retain zooplankton of three sympatric cyprinids: common bream, white bream and roach. The model predictions were tested with filter-feeding experiments, using three size classes of each species. Results of experiments in darkness corroborated closely with the reducible-channel model for common bream (retention in the medial channels on the gill arches; the diameter of these channels can be reduced with the lateral rakers), rather well with the unreducible-channel model for white bream (the channel diameter cannot be reduced) and possibly with the saw-tooth model for roach (retention on the gill slits). Common bream can adjust the mesh size of its branchial sieve, thus achieving a higher flexibility in food uptake than the other two species. In light experiments, roach and the small common and white bream switched to particulate intake, characterized by a lower retention ability and a higher filtering rate than during gulping. The retention ability was used to calculate the percentage of the available zooplankton energy that the three cyprinids can retain as a function of the fish's length. This retained energy percentage decreases sigmoidly with increasing fish length. At any length between 10-50 cm, common bream has the highest retained energy percentage, white bream the lowest and roach is intermediate. The population of common bream will therefore be at an advantage in the competition for food when zooplankton is a major food source, like in eutrophic lakes.

Copepods are one of the main fish ectoparasites, commonly known among the parasitism world, due either to their special adaptive capacity or to induce stress in aquatic organisms, as well as the parasitism result is overall a physiological rate affecting the survival of host. This study was, therefore, aimed to investigate the ectoparasite copepods, infesting 277 fishes including 135 of Cyprinus carpio (Linnaeus, 1758) and 142 of Abramis brama (Linnaeus,1758), collected from Beni-Haroun dam (Mila, northeast Algeria). The fishes were subjected to some morphometric measurements and gill dissections and the harvested parasites were afterwards stored and identified. The parasitic indices were determined to examine the effect of size, sex, season and microhabitat on the parasitic infestation by using χ2 test. The gill examination of individuals of the two host fishes led to harvest 135 copepods, including 127 individuals from Cyprinus carpio belonging to five species (Ergasilus sieboldi, Ergasilus briani, Neoergasilus japonicus, Neoergasilus longispinosis and Lernaea cyprinacae) and 8 individuals from Abramis brama belonging to the two species: Ergasilus sieboldi and Ergasilus briani, and hence Cyprinus carpio is the most infested species than Abramis brama. This study also revealed that the parasitic index values were high in C. carpio (P=53.00%; A=3.7; I=6.93) compared to A. brama (P=12,0.5%, A=0.12, I=1.00) during autumn, since no effect on infestation was noticed during winter. The statistical analysis has revealed the effect of season on parasitic copepod infestation in the two fish species (χ2Obs =30,959 > χ20.05 = 7,815, ddl =3), Since there was no effect due to the microhabitat and sex.

In this thesis the retention mechanism of the branchial sieve of three sympatric cyprinid fish species, the common bream (Abramis brama), the white bream (Blicca bjoerkna) and the roach (Rutilus rutilus) , is studied. In eutrophic lakes zooplankton is an important food resource and common bream is dominant. Previous research indicated that common bream retains zooplankters in the medial channels on its gill arches. The mesh size of its branchial sieve can be reduced by rotating the lateral rakers into these channels. In this thesis it is shown that of the three species under study the channels of common bream are comparatively the widest. Because of its extra long and pointed lateral gill rakers common bream is most suited to reduce its channels, in the way described above. The retention ability of the fishes was calculated from the decline in density of zooplankton as a function of its size in experimental tanks with filter-feeding fish. These data were compared with the predictions from three retention models. In these predictions the influence of the shape of the zooplankton on retention was taken into account. This aspect was studied in a separate experiment with industrial sieves (with square meshes). The ratio of body width and depth of the zooplankton, proved to be a crucial size parameter for retention. It was concluded from the filter-feeding experiments that the channel model with adjustable mesh size can be applied to common bream. The same model can be applied to white bream, but without the possibility to adjust the mesh size. Roach probably retains zooplankton on its gill slits, according to the saw-tooth model. The presence of abductor muscles for the lateral rakers, which allows them to rotate, is a prerequisite for the application of the reducible-channel model. A detailed micro anatomical study showed that all lateral gill rakers of each gill arch of common bream have such muscles. In white bream and roach, however, these muscles are only present on the lateral gill rakers of the first gill arch. Therefore, the reducible- channel model cannot be applied to these species. During the uptake of zooplankton by suction feeding, the gill arches move along with the expanding head. These gill arch movements affect the relative position of the gill rakers on either side of each gill slit. The gill arch movements were studied in X-ray films of white bream and common bream. A novel, 3D method of analysis was developed to analyze these films quantitatively. The lateral gill rakers of both species proved to be long enough to reach across the gill slit (i.e. into the medial channels), even when the width of the gill slits was maximal. The sideward movement of the lateral gill rakers out of the centre of the medial channels was considerable. The adjustable branchial sieve is not rigid. Due to the mucus encapsulement of trapped particles, the conical shape of the lateral gill rakers and the possibility to rotate them sidewards, the reducible-channel mechanism still functions well. The filter-feeding effectiveness of the three species was quantified in terms of energy. Common bream proved to have a higher energy gain from filter-feeding than its relatives white bream and roach. This difference is probably a crucial factor explaining the dominance of common bream in eutrophic lakes. The present research corroborates the fundamental idea that there is a strong relation between the functional morphology and the ecological niche of a species. Furthermore, small morphological differences between related species (the presence of abductor muscles of the lateral gill rakers) can be used to explain and predict interspecific differences in the exploitation of food sources and the performance in the ecosystem.

Ligula intestinalis (Linnaeus, 1758) is a tapeworm parasite with a worldwide distribution that uses a wide variety of fish species as its second intermediate host. In the present study, we investigated the prevalence and population genetic structure of plerocercoids of L. intestinalis in five common cyprinoid species, roach Rutilus rutilus (Linnaeus), freshwater bream Abramis brama (Linnaeus), white bream Blicca bjoerkna (Linnaeus), bleak Alburnus alburnus (Linnaeus), and rudd Scardinius erythrophthalmus (Linnaeus), collected in six water bodies of the Czech Republic (Milada, Most, Medard, Jordán, Římov and Lipno). Of the six study sites, the highest frequency of parasitism was recorded in Lake Medard (15%). The overall prevalence rate among the species was as follows: roach > rudd ≥ freshwater bream > bleak > white bream. Two mitochondrial genes (cytb and COI) were used to compare the population genetic structure of parasite populations using selected samples from the five fish species. The results of the phylogenetic analysis indicated that all populations of L. intestinalis were placed in Clade A, previously identified as the most common in Europe. At a finer scale, haplotype network and PCoA analyses indicated the possible emergence of host specificity of several mtDNA haplotypes to the freshwater bream. Moreover, pairwise Fixation indices (FST) revealed a significant genetic structure between the parasite population in freshwater bream and other host species. Parasite populations in roach not only showed the highest rate of prevalence but also depicted a maximum number of shared haplotypes with populations from bleak and rudd. Our results suggest that recent ecological differentiation might have influenced tapeworm populations at a fine evolutionary scale. Thus, the differences in prevalence between fish host species in different lakes might be influenced not only by the parasite's ecology, but also by its genetic diversity.

The tapeworm Ligula intestinalis (Linnaeus, 1758) is a broadly distributed parasite with high epidemiological significance. Its first intermediate hosts are crustaceans, its second intermediate hosts are fish, while the definitive hosts are various fish-eating birds and, as recently shown, ringed seals. The stages of Ligula intestinalis in fish are called plerocercoids. In this study we obtained new data on the occurrence of plerocercoids of L. intestinalis in freshwater bream Abramis brama in six lakes in Karelia (Northwestern Russia). Infection of freshwater bream with L. intestinalis in Lakes Onego, Ladoga, Svyatozero and Konchezero was found for the first time. In Lake Syamozero, where long-term parasitological monitoring has been conducted, infection indices of bream with L. intestinalis were similar to those recorded in 1975. Using two mitochondrial genes (cytb and COI), we examined the genetic structure of L. intestinalis from different localities. Phylogenetic analysis showed that all specimens of of L. intestinalis examined in our study belonged to “Lineage A” identified by Nazarizadeh et al. (2023). We found 40 haplotypes of concatenated cox1+ cytb in freshwater bream from different water bodies in Northwestern Russia and Europe. Plerocercoids from Karelian lakes were represented by 22 haplotypes, of which 21 were unique. Our molecular analysis of L. intestinalis from freshwater bream in European part of the Palearctic shows that this parasite lineage is genetically diverse and rapidly expanding.

The bioaccumulation potential of parasites resulting from the competition for chemical elements including heavy metals represents a valuable instrument of evaluating the functions of the parasite-host system. In the present study, the effect of the parasite-host system of Ligula intestinalis and Abramis brama on the mercury concentrations and fish muscle composition of infected and non-infected fish was evaluated. Nine parasitized and seven non-parasitized fish were studied. The total mercury content in the samples was determined by the atomic-absorption spectrophotometry method. Standard indicators of the chemical composition of muscles (dry matter, crude protein, fats, and ash) and a spectrum of fatty acids were evaluated. The mean mercury concentration in the biomass of plerocercoids was 0.045 ± 0.025 mg·kg-1; about × 7 lower compared to fish muscles. The mean mercury concentration in the muscles of infected and non-infected fish was 0.36 ± 0.11 and 0.24 ± 0.1, respectively. There was no difference in the composition of fish muscles and the spectrum of fatty acids. Ligula intestinalis takes nutrients from the fish body but according to the results of our study, the withdrawal of the monitored nutrients was uniform without any selection. Mercury is not efficiently accumulated by plerocercoids of Ligula intestinalis. This study brings novel data for this heavy metal and for this parasite host system.

Summary The model of the International Society of Limnology (SIL) Plankton Ecology working group (hereafter the PEG model) is a verbal model describing the patterns and driving factors of seasonal phytoplankton and zooplankton succession in oligotrophic and eutrophic lakes (Sommer et al., 1986). Despite being a citation classic, tests of the PEG model with respect to differences in zooplankton biomass dynamics between oligotrophic and eutrophic lakes are lacking. Here, I use the long‐term data from Lake Constance, which during the last 100 year changed from an (ultra‐) oligotrophic lake to a eutrophic lake and back to an oligotrophic lake to analyse trophic status differences in zooplankton biomass seasonality. Using data from one lake allows one to study trophic influences on biomass dynamics without the confounding effects of lake geographical setting and lake morphology, which complicate comparative dynamics in eutrophic versus oligotrophic lakes. However, environmental changes due to other driving factors, for example climate change, may possibly alter biomass dynamics as well. Data from Lake Constance do not support the differences in zooplankton seasonality in respect to peak timing between eutrophic and oligotrophic lakes suggested by the PEG model. Rather total zooplankton biomass, as well as cladoceran and copepod biomass showed a peak in May/June during all trophic conditions. Biomass dynamics of cladocerans during spring were more strongly influenced by water temperature than by trophic state. Furthermore, analyses of the geographical setting of the lakes considered in Sommer et al. (1986) suggest that the proposed differences in zooplankton seasonality between eutrophic and oligotrophic lakes are at least partially due to the confounding effect of lake altitudinal setting; the oligotrophic lakes were located at higher altitude than the eutrophic lakes. As a consequence of the results from Lake Constance, and the bias detected in the Sommer et al. (1986) study, a modified PEG model is proposed which considers low water temperature and not food limitation as the most important factor reducing zooplankton growth rate during early spring in both oligotrophic and eutrophic lakes.

In situ growth of heterotrophic nanoflagellates (HNF) in Lake Donghu, a eutrophic shallow lake in mainland China, was studied from January 1999 to March 2000 using a modified Weisse protocol. The study results indicated that the growth rates of HNF showed pronounced seasonal variation (-0.37-1.25 d(-1)), reaching the maximum during spring to early summer. When the water temperature was higher than 25.5 degreesC, HNF growth was inversely proportional to water temperature. There was an effect by bacterial abundance and autotrophic picoplankton on HNF growth that depended on location. HNF biomass was the highest in late spring, and the HNF production ranged from -2.25 to 35.45 mg l(-1) d(-1) with mean of 3.17 mg l(-1) d(-1). When considered in the context of biomass and production data for zooplankton in Lake Donghu, it was evident that HNF contributed significantly to the total zooplankton production in Lake Donghu. These in situ studies indicate that temperature and food supply are the major determinants of HNF abundance and productivity.

Influence of temperature during four following spawning seasons on the spawning effectiveness of common bream, Abramis brama (L.) under natural and controlled conditions

Background. Aparasitological survey was carried out in a shallow, eutrophic Oświn Lake (north-eastern Poland), within the Seven Islands ornithological reserve. We intended to compare the occurrence of eyeflukes in fish from two, distinctly demarcated pools of the lake, differing in environmental conditions. The effect of fish size and sampling season on the infection parameters was analysed and the present state of the parasite faunas was compared to the literature data. Materials and methods. Within 1998 and 1999, a total of 1091 fishes representing 8 dominant species was examined. Samples were collected four times a year (in May, July, August, and October) simultaneously from the eastern- and western parts of the lake. Results. Diplostomum spp. occurred in all the fish examined, numerously in roach, rudd, white bream, and carp bream. The parasite was less abundant in northern pike, crucian carp, and European perch. Tylodelphys clavata was found in roach and it sporadically occurred in northern pike, rudd, white bream, carp bream, and European perch. The infection rates of rudd and white bream in the eastern part of the lake were significantly higher than the respective values from the western part. The infection with both eyeflukes did not correlate with the fish length. The infection of roach, rudd, white bream, and carp bream with Diplostomum spp. and roach with T. clavata varied significantly in different experimental periods. Conclusion. The eyeflukes as well as P. cuticola were sensitive indicators, reflecting the environmental pressure-both, spatial differences in ecological conditions within the same water body and long-term alterations. Diplostomum spp. did not accumulate with the age of fish. No seasonality was found for Diplostomum spp. or Tylodelphys clavata. Locationdependent occurrence of Diplostomum spp. in rudd and white bream indicated a limited fish interchange between the two pools of the lake.

The livers of bream (Abramis brama) from the Oder River near the town of Schwedt (Germany), were investigated for their content of macrophage centres (MCs). The investigations were carried out for a period of 18 months.The mean MC‐number was 20 ± 14 MCs per mm2 in females and 13 ± 11 MCs per mm2 in males. These values are regarded as normal for adult bream of the Oder River. The diameter of the MCs ranged between 8 and 160 μm for both sexes.The study showed positive correlations between the MC‐response and water pH as well as water temperature. Different correlations existed between males and females and the water temperature. The use of histological analyses of liver MC‐size and ‐number as a biomarker is discussed. Currently the MC‐response is only regarded as an unspecific biomarker on environmental stress.

Infection of British freshwater fishes with the plerocercoid larva of Ligula intestinalis is most widespread and common in the roach. It is less common in gudgeon, rudd, bream, and minnow, and rare in dace. Roach fry may become infected within the first 3 months of life. Infections in roach exceeding 13 cm in length are infrequent. Data concerning the size distribution of plerocercoids in fish of different size are given. In infected roach, the liver weight, packed cell volume of erythrocytes and hemoglobin are reduced. Gonadal retardation is a universal consequence of infection. It is severe in roach, rudd, bream, dace, and minnow but is less marked in gudgeon. Plerocercoid larvae of pseudophyllidean cestodes of the family Ligulidae have been recorded from the body cavity of numerous species of freshwater fishes in many parts of the Northern Hemisphere. Early accounts concerning the genus Ligula were reviewed by Cooper (1918). He recognized only the single species L. intestinalis (L.) and listed 63 species of host fish from many different families. More recently, Russian workers have suggested that at least two genera and five species, including L. intestinalis, are involved and that the plerocercoid larva of each species is restricted to a relatively narrow range of host species (Bykhovskaya-Pavlovskaya et al. 1962). In Britain the plerocercoids of Ligula intestinalis (L.) have been previously recorded from the roach (Rutilus rttilus) (Baylis 1928, 1939; Smyth, 1947; Kerr, 1948), gudgeon (Gobio gobio) (Baylis, 1939), brown trout (Salmo trutta) (Baylis, 1939) and minnow (Phoxinus phoxinu s) (Archer, pers. comm.). The only other member of the family Ligulidae which has been found in British fish is Schistocephalus solidus (Muller), the plerocercoid larva of which is restricted to the stickleback (Gasterosteus aculeatus). Despite numerous records of plerocercoids of Ligula intestinalis parasitizing fish, there have been relatively few accounts of the incidence and intensity of the infections encountered. Kosheva (1956) and Zitnian (1964) have provided data concerning infections in bream (Abramis brama) and roach respecReceived for publication 20 November 1967. * Present address: Department of Biology, Rice University, Houston, Texas. tively in Eastern Europe; Pitt and Grundmann (1957) described infections of the yellow perch (Perca flavescens) in the USA, and Van Cleave and Mueller (1934) discussed its incidence in the fishes of Oneida Lake, New York. The presence of Ligula intestinalis pleroc rcoids has been shown by many workers to be associated with certain pathological effects in the fish host (see Dogiel et al., 1961). Most of these reports originate in the USSR where ligulosis presents a serious problem in p sciculture. Blood changes associated with L. intestinalis infection, in particular an increase in the monocyte and polymorphonuclear leukocyte counts, have been described in the gudgeon by Sadkovskaya (1953) and in the crucian carp (Carassius carassius) by Shpolyanskaya (1953). The latter author also noted a fall in blood hemoglobin; a similar decrease was observed by Kosheva (1956) in bream infected with Ligula intestinalis and with the related species Digramma interrupta. Kosareva (1961) found that in roach, bream, and white bream (Blicca bjoernka) infected with L. intestinalis and D. interrupta the liver glycogen levels tended to be lower than in noninfected fish. Kosheva (1956) stated that infections with either species of tapeworm tended to depress the fat content of bream, and Pitt and Grundmann (1957) found that the rate of growth of yellow perch infected with L. intestinalis was lower than that for noninfected individuals. A commonly noted feature of infection with L. intestinalis plerocercoids is the inhibition of host gonad development. The most specific references to this condition are those of Kerr (1948), Kirschenblat (1951), and

Adaptive evolution in response to varying environments, leading to population divergence, is among the most intriguing processes of speciation. However, the extent to which these adaptive processes effectively drive population divergence amidst ongoing gene flow remains controversial. Our study addresses this by analyzing population genetic structure, gene flow, and genomic divergence between lineages of a tapeworm parasite (Ligula intestinalis) isolated from sympatric fish hosts. This parasite, which must overcome host immunological defenses for successful infection, significantly impacts host health. Utilizing genome-wide Single Nucleotide Polymorphisms (SNPs) and transcriptome data, we investigated whether host species impose distinct selection pressures on parasite populations. Genetic clustering analyses revealed clear divergence, with parasites from bream (Abramis brama) forming a distinct genetic cluster separate from those infecting roach (Rutilus rutilus), rudd (Scardinius erythrophthalmus), and bleak (Alburnus alburnus). Demographic modeling indicated isolation with continuous gene flow as the most plausible scenario for this divergence. Selection analyses identified 896 SNPs under selection, displaying low to moderate nucleotide diversity and genetic divergence compared with neutral loci. Transcriptome profiling supported these findings, revealing distinct gene expression profiles between parasite populations. Examination of selected SNPs and differentially expressed genes identified candidate genes linked to immune evasion mechanisms, potentially driving ecological speciation. This research highlights the interplay of host specificity, population demography, and disruptive selection in ecological speciation. By dissecting genomic factors, our study improves the understanding of mechanisms facilitating population divergence despite ongoing gene flow.

The data from studies of the antimicrobial properties of blood serum, the content of total lipids, and antioxidant activity of immunocompetent tissues and organs of breams Abramis brama infected with plerocercoids Ligula intestinalis depending on the phase of development of the parasite are presented. The quantitative characteristics of the studied parameters are determined.

Fourteen easily recognizable stages of embryonic, larval and juvenile development are described in order to provide a model for early ontogeny in bream (Abramis brama L.). The proposed developmental stages are applicable both in laboratory studies and in field work, enabling quantitative description of the course and rate of bream early ontogeny and give the possibility to use statistical analysis (i.e. comparison of mean length-at-developmental stage). Early development of bream was studied in a wide range of rearing temperatures (13.5-34.0°C) and food supply (four food regimes), the two most important environmental factors influencing fish survival and development. In the lowest and highest test temperatures, the same stage was observed in fish of significantly smaller size. Fish reared under different feeding regimes differed in size when they reached particular developmental stages. Generally, fish kept in disadvantageous conditions (i.e. semi-lethal temperature, poor quality of food) attained the successive developmental stages at smaller size.