The peculiarities of larval development of the European smelt Osmerus eperlanus eperlanus (L.)

The larval development of the European smelt Osmerus eperlanus eperlanus from hatching to full resorption of the vitelline sac has been described. On the basis of the morphological and dimensioning characteristics, three stages almost equal in the duration have been distinguished during present period. The duration of the larval stages was measured in relative units τ_s. Two palingeneses were discovered during larval development of the smelt: (1) the presence of the ciliary epithelium in the intestine and (2) plasmatic circulation of the blood without blood cell formation. These features are evidence of the antiquity of the origin of smelt and place them at the base of the suborder Salmonoidei.     

Embryonic development of the European smelt Osmerus eperlanus eperlanus (L.) (Neva population)

The embryonic development of the European smelt Osmerus eperlanus eperlanus has been described. The whole period of the development has been subdivided into 6 subperiods. More than 80 stages may be described for all of these periods. The development of the European smelt embryo was studied in the 5 different regimes of a constant temperature in the range 9.5–18.3°C. The duration of different stages under different temperature regimes was expressed in the relative units of τ₀ (the time of a single cleavage) and τ_s (the time of formation of a single pair of somites). It was established that the τ_s value had proportionally changed by embryogenesis. Due to this fact, the duration of the stages was expressed only as τ_s. The dependency of the speed of the European smelt embryogenesis on temperature has been shown. This dependence was expressed using the standard interval τ_s and described using the equation log τ_s(t) = 3.22665?0.13876t + 0.00297t ², where t is the temperature.     

European smelt Osmerus eperlanus in the eastern Gulf of Finland,Baltic Sea: Stock status and fishery

In the eastern Gulf of Finland, European smelt Osmerus eperlanus occurs as an anadromous ecological form that spawns in coastal, low-salinity zones and in several rivers entering the gulf, most importantly in the River Neva. Osmerus eperlanus is a key commercial fish species for the population of St. Petersburg, and the city's fish symbol. However, the state of the smelt stock has considerably deteriorated over the past few decades. Monitoring shows that annual catches during 1965–1992 averaged 2274 t, compared to 292 t in 2002–2017, although some improvement is inidicated by a gradual increase in catches in recent years (e.g., to 595 t in 2017). This paper addresses long-term and recent changes in the fisheries for smelt in the eastern Gulf of Finland, including both commercial and recreational fisheries, and the range of fishing gears used. The commercial smelt fishery is mainly conducted during the spawning period (April–May) on migrating fish, using trap nets of various designs and beach seines. There has been an increase in recreational fisheries for smelt; in years when ice cover on the gulf during winter (December–April) is steady, recreational fishers from St. Petersburg, using hook and line, may catch quantities that are comparable to those of commercial catches. Several factors may account for the stock dynamics and associated changes in catches of smelt: these include a decrease in prey availability in the gulf; loss and degradation of spawning and nursery habitats, partly associated with large-scale hydroengineering operations in Neva Bay; and illegal, unreported and unregulated (IUU) fishing which became widespread in post-Soviet decades. In spite of a gradual increase in smelt abundance in the most recent 15 years, a failure to accurately monitor and manage the extent of IUU and recreational fishing for smelt, to assess the status of the stock, or to forecast catches, increases the risk of overfishing of the spawning stock.     

The role of fish parasites in fresh-water ecosystems exemplified by a parasite of the smelt (Osmerus eperlanus)

The European smelt Osmerus eperlanus had been accidentally introduced into the ecosystem of the Syamozero Lake (Karelia). The population of this species has achieved a high density and caused serious changes in the structure and trophic relationships of fish community of the Syamozero ecosystem. The microsporidia Glugea hertwigi Weisenberg, 1921 has become a new and super-dominant parasite of the european smelt in this ecosystem. The invasion of microsporidia has caused a mass death of fishes, that has led to changes in population structure of the smelt and lowered a fish catch. The present study suggests to show a role of parasites in the ichthyocenosis structure regulation in freshwater ecosystem.     

Quality and quantity of smelt (Osmerus eperlanus L.) sperm in relation to time after hormonal stimulation

The effect of Ovaprim (salmon GnRH analogue and a dopamine antagonist) treatment on the quantity and quality of smelt (Osmerus eperlanus L.) sperm was studied in relation to time after hormonal stimulation. Sperm was obtained at 0, 24, 48 and 72 h after treatment (n=13/each time point). Computer Assisted Sperm Analysis (CASA) was used to evaluate sperm motility parameters and histological analysis was used to examine the testis morphology. Only a small volume of semen (1-5 μl) was collected at the beginning of the experiment (time 0) but it dramatically increased 24 h after hormonal treatment. A further increase in semen volume was recorded 48 h after hormonal stimulation. CASA parameters, such as percentage of motile cells, progressive motility, curvilinear velocity, straightlinear velocity, straightness and amplitude of lateral head displacement of stripped sperm increased 48 h after hormonal treatment, which indicates high quality of sperm. No further increase in sperm quality was recorded at 72 h. Compared to stripped semen, testicular semen was characterized by a slightly lower quality. In addition, histological analysis indicated that 24 h after hormonal treatment, a high number of spermatozoa was released from the testis. Ovaprim-stimulated smelt became clearly darker than the control fish. In conclusion, our results suggest that smelt semen should be collected 48 h after hormonal stimulation to ensure high quality of semen. This time may vary depending on maturation status of testis.     

State of populations of smelt Osmerus eperlanus from Vygozero and Syamozero that formed as a result of self-dispersion

Long-term investigations permitted the study of the process of self-dispersal and formation of populations of smelt Osmerus eperlanus in Vygozero and Samozero. We recorded that smelt successfully naturalized in new ecosystems, reached commercial numbers, and became the main object of feeding of predatory fish. Biological indices of the smelt (linear and weight growth, fecundity, feeding) were analyzed, and its role under new habitation conditions was determined. It was shown that the introduction of the new species involves the rearrangement of all links of the food chain and of the entire community.     

Glochidia of the freshwater mussel,Anodonta anatina, affecting the anadromous European smelt (Osmerus eperlanus) from the Eider estuary, Germany

During a 2-year survey on external disease conditions of fish between January 1988 and December 1989, 0.2% of 40011 smelt, Osmerus eperlanus , from 30 stations in the German Wadden Sea and estuaries of tributary rivers, were infected with glochidia of the freshwater mussel Anodonta anatina . Seasonality was marked, with glochidia only being observed in March and April 1989, when prevalences were 37% and 15% respectively, in fish 10–20cm long at the most heavily infected site. A marked increase in prevalence in fish 15cm in length and longer, suggested that only temporarily resident spawning fish (as opposed to resident juveniles) were infected. Infection was almost exclusively restricted to the Eider estuary, where prevalence decreased with increasing salinity. It is concluded that the glochidia carried by smelt returning from their freshwater spawning sites in the Eider River to the sea would have died, thus representing a loss to the 1989 cohort of A. anatina from the Eider. The potential importance of the smelt– A. anatina relationship as indicator for the detection of (a) spawning mussel populations and of (b) possible ecological effects of climate changes is noted.     

Photoreceptors and cone patterns in the retina of the smelt Osmerus eperlanus (L.) (Osmeridae: Teleostei)

The outer retina of the smelt Osmerus eperlanus, a visually orientated plankton feeder, of Lake Hiidenvesi (Finland), was examined using both light and transmission electron microscopy. Apart from rods, six morphologically different cone photoreceptor types were identified: short single cones, long single cones, unequal/equal double cones and triple cones (triangular and linear variety). Additionally, in the dorsal region, multiple cone arrangements consisting of up to five members occur. Long single cones and triple cones were observed only sporadically throughout the retina. The incidence of short single cones as a regular element of the cone mosaic is restricted to the ventrotemporal area. The dominant pattern in the Osmerus retina is a pure or a twisted row pattern occurring in all regions. Ventrotemporally, however, square patterns were found as well. The highest cone densities occur in the peripheral ventrotemporal retina. These results indicate that the ventrotemporal region plays an important role in the vision of the smelt. The findings are discussed with respect to the photic habitat conditions and behavioural ecology of the smelt in Lake Hiidenvesi.     

Determining the optimum temperature and salinity for larval culture,and describing a culture protocol for the conservation aquaculture for European smelt Osmerus eperlanus (L.)

Populations of anadromous European smelt Osmerus eperlanus (L.) are declining across its geographical range in northern Europe, but no practical culture techniques exist to develop stock enhancement programmes for this species. In this study, a culture protocol is described to rear fish from fertilised eggs to mature adults in 2 years involving the use of ‘green water’, live feed and artificial diets. The sequence of embryonic development for eggs incubated at 10°C/0 ppt was described and photographed. To determine the optimum conditions for larval culture, fertilised eggs were reared at a range of salinities (0–20 ppt) and temperatures (5–18°C) until first feeding. Best hatching success (ca. 97%), size at hatch (ca. 0.8 mm) and survival to first feeding (ca. 96%) of larvae were achieved under combined conditions of low salinity (0–0 ppt) and temperature (5–10°C). No larvae survived a salinity of 20 ppt. The time taken from fertilisation to hatch (FtH) and hatching duration (HD) were temperature-dependent ranging from 42 days FtH and 10 days HD at 5ºC, to 10 days FtH and 2 days HD at 18°C irrespective of salinity. The results indicate that conservation programmes could utilise existing salmonid hatchery facilities (i.e. freshwater, ≤10°C water temperature) for stock enhancement. Since on-growing of smelt involves the logistical and technical problems of live feed production, it is recommended that smelt enhancement programme utilise freshwater hatchery facilities to rear fish until hatching, and then stock out onto known spawning grounds in rivers allowing hatched larvae to drift into estuaries to complete the larval and juvenile phases. This approach would minimise the time spent in the hatchery post-hatching, eliminate the need for live food production, prevent the development of predator-naïve fish, and hence would mimic the natural life cycle of the species as closely as possible.     

Competitive food exploitation of smelt Osmerus eperlanus by great crested grebes Podiceps cristatus and perch Perca fluviatilis at Lake IJsselmeer,The Netherlands

The impact of predation by an avian predator (great crested grebe Podiceps cristatus) and a predatory fish (perch Perca fluviatilis) on a common resource, the small planktivorous fish smelt (Osmerus eperlanus) was studied in Lake IJsselmeer, The Netherlands, in July–October 1985–1988. At this time of the year the grebes are in simultaneous wing-moult and therefore temporarily flightless. Four factors limit the predation pressure by grebes: (1) an underwater visibility threshold of c. 40 cm Secchi depth is the lower limit at which large numbers attend the moulting site, (2) a lower density threshold of exploitable smelt biomass of c. 30 kg/ha determines the actual fishing areas, (3) a fish size threshold of 6.5 cm total length further limits prey availability, and (4) the vertical movements of the prey impose a strong (diel) time constraint on the birds, allowing only crepuscular foraging. Perch, the other main predator in this system, is less restricted in its foraging. Perch were found to be in direct competition with the grebes, preying heavily on the juvenile smelt. Over the 4 years of study 90% of all predation from 15 July to 15 October (72–233 kg/ha) was due to perch. The high level of predation by birds and fish was only possible due to a continuous immigration of smelt into the area (1.45 kg/ha per day). In three of the four years, however, the combined predation by fish and birds surpassed the immigration rate of the prey, which led to a strong reduction in smelt stocks in the study area. A conceptual model is developed to describe the different sets of constraints on the grebes' foraging. First, the state of eutrophication in relation to the weather condition determines the degree of algal blooms, and thus underwater visibility, in late summer. This is the major factor governing the numbers of grebes on the moulting area. Second, the size of the population of predatory fish determines the overall food availability (biomass and size distribution of smelt). These factors are partly interconnected and related to human action (pollution, fisheries). It is suggested that, despite deteriorating visibility conditions, the largest moulting site for grebes in Europe at Lake IJsselmeer exists because the stocks of predatory fish are kept low by overfishing.     

Food and growth of age-0 smelts, Osmerus eperlanus, in a Norwegian fjord lake

Pennate diatoms, especially Aslerionella spp. and Tabellaria fenestrata , were the most important food items for smelts in Lake Mjøsa during the first month of external feeding; rotifers and early stages of copepods were eaten occasionally. Later, during summer and autumn, crustacean zooplankton (cladocerans and copepods) were the most important food items; selected species were Polyphemus pediculus, Eudiaptomus gracilis, Limnocalanus macrurus, Heterocope appendiculata. Leplodora kindti , and Bythotrephes longimanus ; some smelts larger than 7.4 cm fed on Mysis relicta . In 1979 and 1980, mean lengths of smelts in October-November were 4.7 cm and 7.2 cm, respectively. A difference in length increment between the two years may have resulted from summer water temperatures being higher in 1980 than in 1979.     

Comparative study of bile-acid composition in the bile of the European vendace Coregonus albula L. and whitefish Coregonus lavaretus L. under conditions of technogenic water pollution

Changes of the bile acid composition were studied in bile of the vendace Coregonus albula L. and the whitefish Coregonus lavaretus L. under effect of sewers of an ore mining and processing plant, the main pollutin agents of the sewers being heavy metals. The revealed variability of the bile acid composition indicates different species-related sensitivity to the studied pollutants as well as a lower resistance of males as compared with females to this type of pollution. Mechanisms of effects of industrial sewers on the bile formation processes and a possibility of using parameters of the steroid lipid metabolism at evaluation of the state of water ecosystems are discussed.     

Food and food selection of cisco (Coregonus albula L.) in a dysoligotrophic lake

With minor exceptions, cisco (C. albula) in Lake Suomunjärvi fed on cladocerans and copepods. The food habits were different among two size classes of fish. The composition of cisco diet changed at different depths and times of the day, but the distribution of zooplankton was usually very similar in each period irrespective of the depth or diel period. Daphnia, Bosmina, Cyclops scutifer and Heterocope appendiculata were the main diet of cisco. Calculations of fish electivity indices showed that cisco selected usually large species of cladocerans and copepods and a small cladoceran, Bosmina coregoni. Body-size selection was clear for Daphnia and Bosmina.     

Isolation and characterization of 12 dinucleotide microsatellite loci in the anadromous fish Osmerus eperlanus (L.)

A subtractive hybridization method was used to isolate 12 dinucleotide microsatellite loci for the anadromous European smelt, Osmerus eperlanus (L.). Three to 17 (mean 8.08) alleles per locus were identified in the two populations screened, and observed heterozygosity ranged from 0.067 to 0.933. Loci in both populations showed significant deviations from Hardy-Weinberg expected frequencies. These 12 loci provide a good basis for investigation of O. eperlanus population structure.     

The size of a vendace, Coregonus albula L., stock in a deep lake basin monitored by hydroacoustic methods

An acoustic estimate of the number and biomass of a vendace stock (age >2 + years) in a deep basin of Lake Karjalan Pyhäjärvi was made in August 1985. The acoustic data were collected at night during the summer stagnation. The vendace were on the lake bottom during the day and rose by midnight to the hypolimnion; they did not rise to the thermocline or water layers above it. In the research area the mean number of vendace was 1900 fish ha ⁻¹ and the mean biomass was 76 kg ha ⁻¹. Total vendace biomass was 151 in the whole 200-ha research area.     

Reproductive cycle of vendace (Coregonus albula L.) in relation to some chemical and biochemical changes in the body

Analysis of the body constituents of vendace indicated that considerable depletion of muscle lipid reserves occurred during gonad formation. Annual changes in total body chemical composition, in the evolution of protein, cholesterol and GPT and GOT activities in the blood, were monitored in both sexes. Significant differences were found in muscle free amino acids between the sexes. Cholesterol level in blood of males was found to be double the amount of that in females. This may explain the earlier mortality of males in coregonid populations when based on atherosclerosis development.     

Control of glycerol production by rainbow smelt (Osmerus mordax) to provide freeze resistance and allow foraging at low winter temperatures

The rainbow smelt (Osmerus mordax) is a small anadromous fish that actively feeds under the ice at temperatures as low as the freeze point of seawater. Freezing is avoided through the production of both non-colligative antifreeze protein (AFP) and glycerol that acts in a colligative manner. Glycerol is constantly lost across the gills and skin, thus glycerol production must continue on a sustained basis at low winter temperatures. AFP begins to accumulate in early fall while water temperatures are still high. Glycerol production is triggered when water temperatures decrease to about 5 degrees C. Glycerol levels rapidly increase with carbon flow from dihydroxyacetone phosphate (DHAP) to glycerol 3-phosphate (G3P) to glycerol. Glucose/glycogen serves as the initial carbon source for glycerol accumulation with amino acids contributing thereafter. The period of glycerol accumulation is associated with increases in GPDH mRNA and PEPCK mRNA followed by elevations in protein synthesis and enzyme activities. Plasma glycerol levels may reach in excess of 500 mM in winter. The high freeze resistance allows rainbow smelt to invade water of low temperature and forage for food. The lower the temperature, the higher the glycerol must be, and the higher the glycerol the greater the loss to the environment through diffusion. During the winter, rainbow smelt feed upon protein rich invertebrates with glycerol production being fueled in part by dietary amino acids via the gluconeogenic pathway. At winter temperatures, glycerol is quantitatively more important than AFP in providing freeze resistance of blood; however, the importance of AFPs to other tissues is yet to be assessed. Glycerol levels rapidly plummet in the spring when water temperature is still close to 0 degrees C. During this period, freeze resistance must be provided by AFP alone. Overall, the phenomenon of glycerol production by rainbow smelt reveals an elegant connection of biochemistry to ecology that allows this species to exploit an otherwise unavailable food resource.     

On the European smelt <Emphasis Type="Italic">Osmerus eperlanus</Emphasis> introduced to Bolshoi Kisegach Lake in the South Urals

In the European smelt introduced to Bolshoi Kisegach Lake (the South Urals) in 1930 the following parameters are investigated: size-age parameters, feeding, fecundity, infestation with parasites, and distribution of the water area. The smelt occupied a free niche of the profundal zone with a permanent low temperature (4–10°C). In the introduced smelt, there is no predation, it is planktivorous, its parasite fauna is impoverished, and the age series is reduced. The age structure and preference of cold water of the isolated population confirm the leading role of the temperature factor in the origin of short-cycle forms.     

Foraging rhythm of great crested grebes Podiceps cristatus adjusted to diel variations in the vertical distribution of their prey Osmerus eperlanus in a shallow eutrophic lake in The Netherlands

Summary Great crested grebes Podiceps cristatus only foraged for an hour or two during dawn and again during dusk on Lake IJsselmeer in August-September. During this time of the year the adult birds are in wing moult and temporarily unable to fly. The food of grebes consisted almost exclusively of smelt Osmerus eperlanus, the most numberous pelagic fish. Simultaneous sonar registrations and trawl net fishing showed that smelt moved to the water surface during the twilight periods. During day and night they were concentrated near the bottom. We argue that grebes have the best foraging opportunities during twilight when much of their prey is near the surface, where light intensities allow the fish to be detected and captured. When the smelt are in the upper water layers the distance to the covered to get the prey (i.e. diving time and cost) is also least.