The article focuses on the study of changeability of morphological and physiological characteristics of 850 specimens of beluga ( Huso huso ) and sturgeon ( Acipenser gueldenstaedtii ) juveniles obtained at the Ikryaninskiy and Bertyulskiy sturgeon hatcheries in the Astrakhan region. There was determined weight of the body, heart, liver and spleen, as well as the relative mass of these organs - cardiosomatic, hepatosomatic indices and index of a spleen. Sturgeon juveniles were studied within 47-127 days, beluga juveniles within 40-116 days. Certain tendency was noted: cardiosomatic index decreased with aging in both sturgeon and beluga, i.e. the relative weight of the heart in ontogenesis changes inversely to the body weight, which corresponds to literature data. Comparison of the dynamics of the liver relative weight in sturgeon and beluga juveniles showed that it coincides at two segments: maximum raise to 50-60ths day from the birth and a gradual decrease in older juveniles. The value of spleen index in juveniles of both types rose with aging. It was found in the course of the study that the size of internal organs could change, getting smaller under unfavorable factors, in particular, when juveniles suffer from the lack of feeds. In such circumstances the dynamics of developing main internal organs drastically slows down, which reflects on physiological adequacy (quality) of juveniles. Thus, morphological and physiological indicators characterizing the degree of the vitals development can be used not only for characterizing species differences of the investigated group of fish at all stages of ontogenesis, but as important factor for assessing conditions of Russian sturgeons juvenile breeding and the degree of morphological and physiological maturity of the juveniles before releasing into the natural water body.
For more than 50 years anchovy sprat fishing in the Caspian Sea has been a leading trend. Yearly sprat catch reached 440,000 tons. The main fishing object was anchovy sprat represented in the Caspian Sea by a single population consisting of a large number of non-separated reproductively biological groups occupying the area of the Middle and Southern Caspian at a depth more than 15-20 m. Studying the size of population in sprat generations is impossible without knowing embryonic and post embryonic periods of species development, because main elimination of generations takes place in early ontogenesis. This problem has not been studied thus far and requires further investigation. The study is based on the results of scientific surveys made in 2010-2017 in the Middle and Southern part of the Caspian Sea by the researchers of Caspian scientific institute of fisheries. The study of early stages of development was carried out at water temperature of 19-20°C, samples were taken every hour. Fecundation was made by dry method in Petri dishes; impregnated roe was transferred into aquariums. Early stages of impregnated eggs development: processes of swelling and forming a plasma knob, blastomeres, stages of forming morula, blastula, gastrulation turning into organogenesis (from segmentation up to emerging prolarvae from membrane). Post-embryonic development is characterized by forming rudimental fins, mouth opening, movements that precede swimmability, pigmentation of eyeballs, orientation to light. For 11th-14th day after hatching there takes place the change of mixed nutrition to food taken from the water, which means qualitatively new, larval stage of development.
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