ÂËÈßÍÈÅ ÍÎÂÎÃÎ ÂÑÅËÅÍÖÀ ÃÐÅÁÍÅÂÈÊÀ MNEMIOPSIS LEIDYI (À. AGASSIS, 1865) ÍÀ ÎÑÍÎÂÍÛÅ ÇÂÅÍÜß ÝÊÎÑÈÑÒÅÌÛ ÊÀÑÏÈÉÑÊÎÃÎ ÌÎÐßНа основании многолетних данных экологических мониторинговых исследований (1999-2016 гг.) изучено влияние относительно нового для Каспия вселенца -гребневика Mnemiopsis leidyi (A. Agassis, 1865) -на морские и анадромные виды рыб. В частности, проведен анализ неблагоприятного влияния гребневика Mnemiopsis leidyi на численность морских, проход-ных, полупроходных рыб и каспийского тюленя. Рассмотрены механизм, последователь-ность и степень влияния гребневика на ихтиофауну и морских млекопитающих через кормо-вую базу. Отмечается, что распределение мнемиопсиса соответствует сезонному распреде-лению зоопланктона и меропланктона, при этом как организм с агрессивной «r-стратегией» развития вида гребневик влияет на ихтиофауну как напрямую (путем поедания пелагической икры и личинок морских рыб), так и опосредованно (через кормовую базу). Резкое снижение уловов каспийской тюльки, в частности анчоусовидной и большеглазой тюльки, обусловле-но пищевой конкуренцией между данными видами и гребневиком Mnemiopsis leidyi. В свою очередь, сокращение численности каспийской тюльки привело к изменению рациона пита-ния осетровых (русского осетра, севрюги, белуги) и уменьшению их численности и промыс-ловых запасов. Намечены пути решения данной экологической проблемы, предложены меры по восстановлению запасов осетровых. Результаты научно-исследовательской работы могут быть использованы при прогнозировании запасов промысловых видов рыб Каспийского бассейна, а также для определении ущерба при ежегодных инвазиях мнемиопсиса (Mnemiopsis leidyi) в cеверной части Каспия (как наиболее продуктивном районе моря).
The global environmental changes that occurred in the Caspian Sea in the last decade, as well as the population outbreak of Mnemiopsis leidyi invasive from the Black Sea resulted in extinction of the most important food item of endemic sprat species, which caused biological wellbeing of ordinary sprat. The data of multi-year observations obtained in the course of studying the abundance and biology of the species have been generalized in order to improve methods and fishing gear and to organize ordinary sprat fishery. The article presents study results of ordinary sprat’s behavior. The analysis has revealed that ordinary sprat is less photophilous in strong light zones, compared to anchovy sprat, and forms exploitable concentrations under the light exposure from 45 min and longer. In the light field ordinary sprat species keep away from the light source and create a second ring at a distance 0.8-2.5 m from the light bulb. The fish aggregation in the light field of the DNaT 400 lamp occurs at smaller exposures than of the SC-102 lamp. The experiment with concurrent use of two different light sources demonstrates some preference of monochromatic light collecting in its light field up to 75% fishes of the total number. The contrast of light and shade created by a light source with a partially darkened bulb changes the fish behavior and increases the density of their aggregation in the light sector near the bulb of the lamp. According to the study results, the article presents considerations concerning new fishing gear in fishing industry that allow to regulate fish species composition in the catch in the Caspian Sea with an increase in the proportion of ordinary sprat.
The article presents annual statistical data of the Caspian Research Institute of Fishery. There has been kept track of the long term dynamics of the stocks of three species of Caspian sprat (anchovy, big-eyed kilka, sprat) and investigated a process of substituting a food item of sprats Eurytemora grimmi to a small-celled copepod species Acartia tonsa Dana. According to the research results, there has been determined growth potential of stocks of each species. Ctenophoran-Mnemiopsis has an adverse effect on sprat population by eating fish eggs and larvae. Ctenophoram - Mnemiopsis is a nutritional competitor to the full-grown fishes. The article gives recommendations on reclamation of stocks of the most perspective species - common sprat, whose biological characteristics helped not to suffer during Ctenophoram outburst and to increase its population during change of the main food item. Hydroacoustic survey data prove the intensive growth of common sprat biomass in the north-west part of the Middle Caspian. According to the results of the research it may be concluded that to realize the volumes of recommended sprat catch it is necessary to organize the marine fishery of common sprat at the Russian Middle Caspian shelf.
In the Caspian Sea sprats is the name of the numerous small fish of herring family (Clupeonella). One of Clupeonella representatives is anchovy sprat. It differs from other types of kilka not only in appearance, but also in its way of life. A brief biology of the species and the state of ecosystem of the Caspian pelagic zone has been described. Anchovy kilka has been a leading targeted fish for over 50 years. Invading the Caspian Sea by ctenophore-mnemiopsis has undermined the stocks of anchovy sprats and caused the crisis of anchovy kilka fishing. Recently, there has been observed a slight increase in the number of the species. The survey materials confirm the increase in the number of juveniles in the studied catches. There are presented the materials from the last six years (2014–2019) of observations (eight research runs), shown the dynamics of abundance, biomass, spawning females, and total fertility of the population. Spawning efficiency and survival rate in early ontogenesis are analyzed. The relationship between kilka survival rate and the number of emerging generations is stated. Recommendations on restoring the stock of anchovy kilka are given. To restore the ecosystem of the Caspian Sea there has been proposed the invasion of a new species – ctenophore-beroe (Beroe ovata), which, due to the structure of its mouthparts eats only jelly-skinned (ctenophore-mnemiopsis, exceptionally) and can significantly reduce the number of ctenophore-mnemiopsis
More than 50 years sprat fishing has been a leading trend in the Caspian Sea. Annual catch of sprat species (common kilka, anchovy, big-eyed kilka) reached 440.000 tons. The main fishery object was anchovy sprat. In recent decades anchovy sprat stock has decreased. Common sprat has become main fishery object. There took place redistribution of sprat concentrations in the sea. Middle part of the Caspian Sea has become the general area of commercial stocks of sprat distribution. The cause of these changes was ecosystem deterioration of the Caspian Sea. In this regard, there was made an attempt to generalize the material of many years of ecological monitoring and to determine the degree of impact of outside environment factors on the pelagial ecosystem of the Caspian Sea and forming sprat stocks. According to the analysis results, seismic activity, sea pollution with chemical wastes or petroleum products, sharp increase of the number of unwanted invaders can be considered the factors affecting the habitat of aquatic bioresearches. Closed water reservoirs, circulation of tides, specific ecological features can become the result of transforming separate anomalies into major catastrophes, which will result in decreasing fish productivity of the sea area. One of the problem solutions concerning common sprat commercial stocks decrease is a purposeful struggle against an invader ctenophore-mnemiopsis - the main rival in feeding adult planktivorous fishes and a direct predator for fish spawn and larvae
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