The species diversity and spatial distribution of ichthyoplankton in the coastal and open waters of the northern part of the Black Sea and the southwestern part of the Sea of Azov in June 2020 are presented. The species diversity and abundance of ichthyoplankton changed in accordance with the regional conditions of warming of the upper layer of the sea during the formation of a vertical thermal structure of the summer type. The most favorable conditions for the survival of ichthyoplankton were observed in the open and coastal waters of the Western Sector of research in the initial phase of the summer hydrological season.

Климатические изменения в гидрологическом режиме Чёрного моря, отмечаемые с 1990-х гг., отразились на состоянии эпипелагических комплексов морских организмов, прежде всего на сезонной изменчивости их биологических циклов. Это оказало существенное влияние на фенологию нереста природных популяций рыб, видовое разнообразие и пространственное распределение ихтиопланктона, а также на устоявшиеся трофические взаимоотношения в планктонном сообществе. В конечном итоге характер взаимодействия между различными звеньями трофической цепи в эпипелагических комплексах, их сезонная и межгодовая изменчивость влияют на эффективность нереста рыб, прежде всего массовых промысловых видов, и в значительной степени определяют успех пополнения их будущих поколений. С целью изучения видового состава, численности и пространственного распределения ихтиопланктона в октябре 2016 г. (89-й рейс НИС «Профессор Водяницкий», 30 сентября — 19 октября) были проведены исследования в шельфовых и открытых водах Чёрного моря у Крымского полуострова, проанализированы не только икра и личинки рыб, но и биомасса мезо- и макропланктона. Пробы ихтио- и макропланктона отбирали сетью Богорова — Расса (площадь входного отверстия — 0,5 м²; ячея — 300 мкм) методом тотальных вертикальных ловов от дна до поверхности моря в области шельфа и от нижней границы кислородной зоны до поверхности моря в глубоководной части. Ихтиопланктон фиксировали 4%-ным раствором формалина и анализировали позже под микроскопом, определяя таксономический состав организмов и по возможности — наличие и состав пищи в кишечниках личинок рыб. Проанализированы данные о видовом составе и пространственном распределении ихтио-, мезо- и макропланктона, а также о питании личинок рыб Чёрного моря у Крымского полуострова в октябре 2016 г. Период съёмки соответствовал начальной фазе осеннего гидрологического сезона. Ихтиопланктон был представлен икрой и личинками 9 видов тепловодных и 6 видов умеренноводных рыб. Средняя численность икры рыб составляла 2,92, а личинок — 3,56 экз.·м−2. Низкая доля (30 %) мёртвой икры тепловодной хамсы Engraulis encrasicolus, а также наличие её разноразмерных личинок в море свидетельствовали о продолжении результативного нереста. Биомасса зоопланктона возрастала в направлении от шельфа к глубоководным районам. Мелкоразмерные фракции планктонных организмов преобладали на шельфе, обеспечивая здесь лучшие кормовые условия для выживания личинок рыб. Несмотря на значительную биомассу желетелых-планктофагов в октябре 2016 г., их влияние на ихтиопланктонные комплексы Чёрного моря, по-видимому, оставалось несущественным.

The data on species composition and abundance of ichthyoplankton in the open waters of the shelf and in the deep waters of the Black Sea in June 2018 are presented. Totally 17 species of fish eggs and larvae of ten families were identified. The mean number of eggs varied from 5.8 spec./m2 in the western sector of study area to 20.9 spec./m2 in the eastern sector; the number of larvae ranged from 1.7 to 9.7 spec./m2, respectively. In June 2018, a high spawning efficiency of Engraulis encrasicolus was observed: the ratio of the number of larvae to the number of eggs in the sea was an order of magnitude higher than in the 1970s, averaging 23.7%. A decrease in the minimum size of larvae of mass species of warm-water fish was noted, which is probably associated with a reduction in the periods of embryonic and postembryonic development with an increase in sea water temperature >27°C. The heterogeneity of the spatial distribution of the abundance of ichthyoplankton and phytoplankton biomass was expressed in their increase from shelf to deep-water sites in accordance with water temperature contrasts and the nature of circulation on the western and eastern peripheries of regional quasi-stationary anticyclonic gyres and, conversely, in their decrease outside the zone of influence of these hydrodynamic structures.

The species structure and spatial distribution of ichthyoplankton in July–August 2019 during the period of mass spawning of warm-water fish species are presented. The material was collected at the shelf and in the deep waters of the Black Sea off the Crimean Peninsula and along the Caucasian coast. In ichthyoplankton, eggs and larvae of 29 species of fish from 19 families were identified. The indices of species richness in the open sea and in the coastal areas were similar. European anchovy Engraulis encrasicolus (Linnaeus, 1758) dominated in ichthyoplankton, its maximum egg abundance reached 260 ind./m2, larvae abundance, 117 ind./m2. The highest spawning efficiency of E. encrasicolus was observed in coastal waters heated up to 26–27°C near the Caucasian coast and on the western shelf of the Crimean Peninsula, where the ratio of the number of larvae to the number of eggs was 30% in the samples. An increased population density of ichthyoplankton was also noted within the Crimean anticyclonic gyre, the shelf thermodynamic formation most stable in time. We assume that favorable conditions were formed there for spawning and survival of fish at the early stages of development.

Aim. Experimental assessment of the impact of electric fields generated by the Electronic Programmable Complex Fish‐protecting Facility on the Electric Principle (EP EPRCFF) on the most common fish species of the Black Sea. Material and Methods. Experimental studies were conducted at the marine test site in the Quarantine Bay of the Black Sea (May‐September, 2021). The following were investigated on larvae, juvenile and adult fish of common species: behavioral responses to electrical fields, survival during and after exposure to the electric field of the fish barrier and the effects of electricity on the reproductive system and biochemical parameters of fish. Results. The features of behavior and distribution of larvae, juvenile and adult fish in the electric field of a fish barrier in modes of different intensity have been analysed. Survival estimation of fish during and after electric influence of the fish barrier has been carried out. The data on the influence of the electric screen on the reproductive system of fish and the biochemical characteristics of their internal organs were analysed. Conclusion. The larvae of fish exhibit a clear “first reaction” to the impact of the electric fish screen. However, there is no avoidance effect (i.e. escape of fish larvae from the area of the working electrode system). The juveniles actively move from the operation zone of the fish‐protective device, avoiding the influence of the electric field. The maximum distance from the electrode system EPRCFF was recorded in adult fish. Significant differences were not revealed in the direct assessment of the survival rate of control and experimental groups of individuals of different species of marine fish after exposure to the electric fish barrier. Some differences on the reproductive system of the fish species studied in the structure of gonads and reactivity of cells were noted under the influence of the electric barrier. The biochemical status of the internal organs under electrical action differed in various species of marine fish.

The species structure, abundance, size composition, and trophic characteristics of fish larvae are presented for the northern part of the Black Sea within the Russian EEZ in July–August 2019. Larvae of 21 fish species belonging to 14 families are found in ichthyoplankton. The average larvae abundance varies from 17 ind./m2 in the deep-sea areas to 38 ind./m2 on the shelf. The dominant commercial species is the European anchovy Engraulis encrasicolus, which accounts for 77.4% of total larvae abundance. The maximum larvae abundance is noted off the coast of the Caucasus (148 ind./m2), high values (116 ind./m2) are recorded in the northwestern part. The larvae abundance follows the areas of high abundance of forage zooplankton (3305 ind./m3 off the Caucasian coast and 13,913 ind./m3 in the northwestern part of the sea). The high abundance, wide size range of fish larvae, and the abundance of food organisms in the intestines testified to a good food supply as the most important factor for their development and survival.

— The paper presents characteristics of the species composition, spatial distribution, and trophic interactions of ichthyoplankton and zooplankton in the spring–summer period 2017 dependent on intensity of the warming-up of the upper sea layer against the background of transition in a system of the surface currents from the winter-to summer-type of the circulation. In April–May, successful spawning of temperate-water fish species was prolonged due to low values of the sea surface temperature (8–9°С) on the shelf of Crimean Peninsula between Kerch Strait and the cape Sarych. The maximum abundance of Sprattus sprattus eggs was 162.8 egg/m 2 and larvae –116 ind/m 2 . Warm-water species occurred only in the western sector from Herakleian Peninsula to the Cape Tarkhankut, where the water temperature was at its maximum and peaked at 14–15°С. In June–July the delayed summer hydrological season hampered the warming-up of the upper quasi-homogenous layer, which thickness varied from 1–5 to 10–15 m. Notwithstanding the large number of species in the ichthyoplankton, maximum abundance did not exceed 50 egg/m 2 and 14 ind/m 2 for larvae. The proportion of larvae of temperate-water fish species averaged 32% and reached 40% in deep water areas. Juvenile stages of copepods of the cold-water complex were dominant in the larval fish diet both in the spring and summer seasons.

This paper is focused on the ichthyoplankton species composition and spatial distribution in the Black sea shelf zone and the deepwater regions off Crimean Peninsula and coast of the Caucasus during the spring hydrological season. Recent changes in the hydrological regime of the Black sea induced by the climate warming have led to an extension of the active and productive spawning of the dominant temperate-water species Sprattus sprattus until the end of the spring hydrological season. An intensive spawning of sprat was detected in March, April and May 2016, 2017 and 2019, which was confirmed by a predominance of younger age group larvae. The maximum number of eggs reached 224 ind./m2 , and the larvae - 116 ind./m2. The wide size range of larvae as well as the low proportion of larvae with empty guts evidenced a favorable fodder base for their survival. The reduction in the age and sizeweight composition of sprat stock in the Black sea observed since 2016 as a result of an increase in its commercial fishing on the shelf of the Crimean Peninsula, did not affect sprat’ spawning activity.

The study examines changes in the abundance (density) and distribution of European Sprattus sprattus (Linnaeus, 1758) eggs and larvae, as well as the phenology of its spawning depending on hydrologic factors and abundance of gelatinous macroplankton in the Black Sea in November 2016–2017. An increase in temperature of the sea surface layer observed since the beginning of the century and the enhanced heat content of the cold intermediate layer contributed to the preservation of warm-water mesozooplankton at the end of the fall hydrologic season, which provided favorable trophic conditions for all planktophages. The biomass of scyphomedusa Aurelia aurita was the third largest in the gelatinous macroplankton in 2016 and the largest in 2017 for the long-term annual research in fall months. Gelatinous macroplankton, however, did not have a significant impact on the survival of sprat in the early stages of its development. The average number of sprat eggs and larvae in November 2016 (184 and 11 ind/m2, respectively) and November 2017 (268 and 21 ind/m2, respectively) exceeded their maximum values recorded in the 1950s. The larvae did not differ in length composition in the beginning of the spawning season in 2016 and 2017.

Проанализированы данные, полученные в пяти комплексных экспедициях НИС «Профессор Водяницкий», которые были проведены с мая по сентябрь 2013 г., в летний нерестовый сезон. Представлена динамика видового разнообразия и пространственного распределения ихтиопланктона в связи с особенностями гидрологического режима в различных районах шельфовых и открытых вод Чёрного моря у Крымского полуострова. В ихтиопланктоне идентифицированы 17 видов икринок и личинок рыб из 13 семейств. Количество видов в ихтиопланктоне было максимальным в июне. Средняя численность икринок и личинок за нерестовый сезон составляла 50,5 и 9,2 экз.·м-2 соответственно. Наибольшая численность икринок (109,9 экз.·м-2) и личинок (9,2 экз.·м-2) наблюдалась в июле. Преобладали икринки и личинки хамсы Engraulis encrasicolus (Linnaeus, 1758): их доля в открытых водах в разгар нерестового сезона достигала 96 и 94 % соответственно. Максимальные показатели индексов видового разнообразия и выровненности (значимости каждого вида) наблюдались в мае, когда доминантные виды только начинали нерест и их численность в ихтиопланктоне была сравнительно низкой. Индексы видового богатства достигали наибольших величин в июне, когда количество видов в ихтиопланктоне было максимальным. Видовой состав и пространственное распределение ихтиопланктона зависели от термогидродинамической структуры морских вод в районах исследований, а выживанию личинок рыб способствовало снижение прессинга желетелого планктона на их кормовую базу. Наиболее значимыми факторами, определяющими термогидродинамическую структуру моря в летний нерестовый сезон 2013 г., являлись: региональные особенности режима прогрева поверхности моря в весенне-летний период и его выхолаживания в осенний гидрологический сезон; адвективный вклад поверхностных течений (прежде всего Основного черноморского течения); сгонно-нагонные процессы в прибрежных районах; локальные термохалинные неоднородности в областях смешения распреснённых речных вод с шельфовыми в северо-западной части моря и азовоморских вод в Керченском предпроливье — с водами открытого моря.

Data obtained in five complex cruises of RV “Professor Vodyanitsky”, carried out from May to September 2013, in the summer spawning season, are analyzed. The dynamics of ichthyoplankton species diversity and spatial distribution depending on hydrological regime peculiarities in different regions of the shelf and open waters of the Black Sea off the Crimean Peninsula is presented. Totally 17 species of fish eggs and larvae from 13 families were identified in ichthyoplankton, maximum number of species was observed in June. Average eggs and larvae numbers during the spawning season were 50.5 and 9.2 specimen per m², respectively. Maximum abundance of eggs and larvae was observed in July (109.9 and 9.2 specimen per m², respectively). Eggs and larvae of anchovy Engraulis encrasicolus (Linnaeus, 1758) dominated: their proportion in open waters in the height of the spawning season amounted 96 and 94 %, respectively. Maximum indexes of species diversity and evenness (significance of each species) were observed in May, at the beginning of spawning season, when dominant species had just started their spawning and their abundance in ichthyoplankton was comparatively low. Species richness indexes reached the maximum value in June, when maximum species number in ichthyoplankton was observed. Species composition and spatial distribution of ichthyoplankton depended on the thermohydrodynamic structure of the sea in the studied regions, and decline of jelly plankton pressing on ichthyoplankton fodder base contributed to the survival of fish larvae. The most significant factors determining the thermohydrodynamic structure of the sea in the summer spawning season 2013 were: regional peculiarities of the sea surface warming up regime in spring-summer period and its cooling in autumn hydrological season; advective contribution of surface currents (first of all, the Main Black Sea Current); wind-driven processes (upwelling and vortex dipole) in the coastal regions; local thermohaline inhomogeneities in the areas of freshened river water mixing with shelf waters in the north-western part of the Black Sea and the Sea of Azov waters at the beginning of the Kerch Strait mixing with open sea waters.

Исследован летний ихтиопланктон у берегов Крыма в 2017 г.Идентифицированы икра и личинки 31 вида рыб.Максимальные значения обилия были зарегистрированы в зонах действия циклонических круговоротов.Благодаря большому количеству видов и отсутствию

Photos of predation of sagitta (Chaetognatha) to fish larvae are presented for the first time for the Black Sea. While analyzing ichthyoplankton samples obtained in May and November in 2017 (94ᵗʰ and 98ᵗʰ cruises of RV “Professor Vodyanitsky”) several sagittas were found with their guts containing fish larvae. Fish larvae seem to have been captured by sagittas in the same way as sagittas in cannibalism cases, with both folding in half at the spot of capture. The data obtained allow us to assume sagittas in conditions of high abundance of eggs and fish larvae in the plankton to be not only a food competitor of larvae and juveniles of fish, as a fodder plankton consumer, but an active predator, being able to play a significant role in the elimination of ichthyoplankton.

Pattern of species composition and spatial distribution of the Black Sea ichthyoplankton in open waters near the Crimean Peninsula in different seasons of the year are based on the studies of materials collected during eight expeditions of the Research Vessel (R/V) ''Professor Vodyanitsky'' from 2011 to 2016. We noted an increase in abundance and species diversity of ichthyoplankton during the spawning season, indicating positive changes in the plankton community of the Black Sea. Favourable temperature conditions for embryonic and postembryonic development as well as rich nutrient items contributed to the spawning season prolongation of the mass fish species of the Black Sea in recent years. In previous years, larvae of the thermophilic species Engraulis encrasicolus, dominating in summer ichthyoplankton species, were collected from June to September, but in recent years, they have begun to appear from May to October. Intensive spawning of the temperate-water species Sprattus sprattus dominating in the winter ichthyoplankton, has been observed in November, at the end of the autumn hydrological season, while its larvae are detected all year round. Prolongation of the productive spawning of thermophilic and temperate-water fish species results in the simultaneous appearance of their larvae in the sea. An increased population of short-cycle species facilitates the appearance of new species of temperate-water Mediterranean predators, namely Trisopterus luscus and Molva macrophthalma. Their eggs and larvae are detected in the ichthyoplankton, indicating favourable conditions for their spawning and the survival of the early larval stages (c) 2018 Elsevier B.V. All rights reserved.