Proceedings of the Zoological Institute RAS Vol. 324, No. 4, 2020, pp. 449–458 10.31610/trudyzin/2020.324.4.449 УДК 597 Vertical distribution, seasonal differentiation and trajectory of movement of zooplankton and pelagic fish of Lake Baikal V.G. Sideleva Zoological Institute of the Russian Academy of Sciences, Universitetskaya Emb. 1, 199034 Saint Petersburg, Russia; e-mails: [email protected], [email protected] Submitted October 18, 2020; revised November 22, 2020; accepted November 30, 2020. ABSTRACT In Lake Baikal, the majority (87%) of fish biomass is generated in the pelagic zone. Two species of Comephorus, three species of Cottocomephorus and Coregonus migratorius inhabit in the pelagic zone of the lake. Currently, only C. migratorius is considered a resource species, the harvest of which accounts for about 50% of the total fish harvest. Food items for pelagic fish are copepods Epischura baikalensis and pelagic amphipods Macrohectopus branickii. The seasonal distribution and formation of zooplankton concentrations determine the food migrations of fish. The patterns of the seasonal distribution of zooplankton and pelagic fish were described by M.M. Kozhov (1954, 1964). Comparative analysis of the abundance of zooplankton in South Baikal over time, showed that dur­ ing the period of maximum development of E. baikalensis in the months of July, August, and October, zooplankton concentration in the decade 1997 to 2007 was twice more than in 1951 (Kozhov 1954; Kiprushina 2010). The distribution of E. baikalensis is directly dependent on the water temperature. Yet there is an inverse relationship in the surface layer 0–25 meters. During warming up of the surface water layers, which had been observed in 2002 year E. baikalensis left the upper zone and migrated deeper, to a layer of 50–100 meters. In other months of the year, no changes in the average values of the E. baikalensis abundance were revealed. At present, when the ecosys­ tem of Lake Baikal is changing, there is a need to restart such complex studies, and to use the results obtained by M.M. Kozhov for comparative analysis. Key words: Comephorus, Coregonus migratorius, Cottocomephorus, Lake Baikal, pelagic fish, zooplankton Вертикальное распределение, сезонная дифференциация и траектория перемещения зоопланктона и пелагических рыб озера Байкал В.Г. Сиделева Зоологический институт Российской академии наук, Университетская наб. 1, 199034 Санкт-Петербург, Россия; e-mails: [email protected], [email protected] Представлена 18 октября 2020; после доработки 22 ноября 2020; принята 30 ноября 2020 РЕЗЮМЕ В озере Байкал большая часть (87%) биомассы рыб создается в обширной пелагиали. В пелагиали озера обитают два вида Comephorus, три вида Cottocomephorus и Coregonus migratorius. В настоящее время из этих видов ресурсным считается только C. migratorius, вылов которого составляет около 50% от общего улова рыбы. Корм для пелагических рыб – веслоногие рачки Epischura baikalensis и пелагическая амфипода Macrohectopus branickii. Сезонное распределение и формирование концентраций зоопланк тона опре­ деляют пищевые миграции рыб. Закономерности сезонного распределения зоопланктона и пелаги­ ческих рыб описаны М.М. Кожовым (1954, 1964). Сравнительный анализ численности зоопланктона южного Байкала с течением времени показал, что в период максимального развития E. baikalensis в июле–октябре концентрация зоопланктона за десятилетие с 1997 по 2007 гг. вдвое больше, чем в 1951 г. 450 V.G. Sideleva (Кожов 1954; Кипрушина 2010). Распространение E. baikalensis напрямую зависит от температуры воды. Но в поверхностном слое 0–25 м наблюдается обратная зависимость. Во время прогрева поверхностных слоев воды, которое наблюдалось в 2002 г., E. baikalensis покидает верхнюю зону и уходит глубже, в слой 50–100 м. В остальные месяцы года изменений средних значений численности E. baikalensis не выявлено. В настоящее время, когда происходит изменение экосистемы оз. Байкал, возникла необходимость воспроизведения подобных комплексных исследований и использовать результаты, полученные М.М. Кожовым, для сравнительного анализа. Ключевые слова: Comephorus, Coregonus migratorius, Cottocomephorus, зоопланктон, оз. Байкал, пела­ гические рыбы INTRODUCTION of which was prohibited in 1971 (Khanaev et al. 2016; Sideleva 2020). The ecosystem of Lake Baikal is regarded as an Research by M.M. Kozhov (1954, 1964) regard­ unique source of the biological resources. Their con­ ing the relationship between the consumption of dition, assessment and methods of rational use have zooplankton and the state of pelagic fish population always posed a challenge. In the scientific community can serve as background data for a comparative as­ there is a view that changes in natural ecosystems sessment of changes in the Baikal ecosystem. occur in “catastrophically short time scales.” In this regard, there is a need for the availability of sound SEASONAL DISTRIBUTION information about the state of biological resources OF ZOOPLANKTON AND PELAGIC FISH during the period of stable states of ecosystems. Such Winter ice period data is utilized as background information for com­ parative analysis and forecasting likely ecological risks during large ecosystems changes. Lake Baikal zooplankton was represented mainly The ecosystem of Lake Baikal is characterized by by the species E. baikalensis, which had a distinct sea­ a high degree of entropy, a state close to equilibrium. sonal distribution (Fig. 1). In the sub­ice period from The large pelagic zone, the deepest part of which is January to March, the number of adult and copepodite characterized by stable environmental conditions, stages of crustaceans in the 0–250 meter layer reached determines the stable functioning of organisms at 100 000–150 000 ind/m2 (400–600 ind/m3), bio­ different trophic levels. The trophic level of meso­ mass – 7–9 g/m2 (0.028–0.036 g/m3) (Kozhov 1954). zooplankton in pelagic zone of Baikal is mainly At this time, about 30% of adults of E. baikalensis represented by the species Epischura baikalensis had been concentrated at the intermediate layer of (Copepoda), the trophic level of macroplankton – by 150–250 meters. The pelagic amphipod M. branickii the species Macrohectopus branickii (Amphipoda) was concentrated in the 200–300 meter layer, and (Kozhov 1954, 1964, 1969). Pelagic fish form two performed migrations to the surface sub­ice layers of trophic levels: the level of five species of cottoid fish water at night. (three species of the genus Cottocomephorus and Planktivorous fish: C. migratorius, C. grewingkii two species of the genus Comephorus), consume zoo­ and C. inermis had been found in wintering grounds, plankton and their own juveniles, and the trophic in the area of the submerged coastal slope, at a depth level of predatory fish is represented by one species – of 200–300 meters near vast shallow waters (Fig. 2). Coregonus migratorius, feeding on zooplankton and Pelagic Comephorus during the ice period were dis­ cottoids of the previous level (Sideleva 2003). persed in the water column, but formed higher popu­ In recent years, changes had been noted in the lation density at a depth of 150–350 meters. During coastal phytobenthic and pelagic phytoplankton the ice period, Comephorus were caught together with communities of Lake Baikal. While there is also a omul (C. migratorius). The water temperature at this decline in the number of the Baikal omul (C. migra- depth had reached 3.5–3.6 °C, which was 2.5 times torius). Additionally, within the past 50 years, the warmer than at a depth of 50 meters. In March, zoo­ po pulation of the Cottocomephorus grewingkii, has plankton started to move to the surface water layers not been able to recover, the commercial harvesting 0 to 50 meters (Fig. 1). Zooplankton and pelagic fish of Lake Baikal 451 Fig. 1. Vertical distribution of Epischura baikalensis in the pelagic zone of South Baikal in winter (under ice) (Kozhov 1954). Spring period along the coast. During the May, C. grewingkii span­ ners in breeding plumage migrated to the littoral of Under influence of wind and sun, ice of Lake Bai­ the lake, and spawned in rocky areas. kal usually melts in the springtime period from April In the second half of May and June, zooplankton to May, thus commencing to the spring turnover. was dispersed throughout the water column of the Soon, at the end of April and the first half of May, lake and did not form dense accumulations, which when the temperature in the entire water column of are very important for fish nutrition. In the shallow Lake Baikal reaches 3.5 °C, zooplankton accumula­ water, the water started to warm up. Already at the tes in the surface layer (Fig. 3). beginning of June, the abundance and biomass of The pelagic fish C. migratorius, C. grewingkii and zooplankton in shallow water reached significant C. inermis started to migrate to the shallower regions values up to 1.6 g/m3 (Kozhov 1954). This was one closer to the coast (Fig. 4). Separate flocks migrated of the most important reasons for the mass migra­ 452 V.G. Sideleva Fig. 2. Distribution of pelagic fish during the ice period in the pelagic zone of Lake Baikal. tion of the Baikal omul into the littoral of the lake persed over the entire 0–250 meters layer. The total (Fig. 5). biomass of E. baikalensis in this layer was 39 g/m2 (or 0.16 g/m3). Currently, the daily vertical migrations of Summer period zooplankton were clearly expressed. Macroplankton (M. branickii) also had been distributed in the 0–300 meters layer and, following E. baikalensis, made ver­ In July there was a clear thermal stratification. In tical diurnal movements. the upper layers of the lake, the temperature reached In the summer, C. migratorius moved away from 9 °C. Clear vertical differentiation congruent with temperature has been observed in plankton distribu­ shallow waters to the open areas of the lake, into tion. The summer reproduction of E. baikalensis also layers of water rich by zooplankton (Fig. 8). In the started in July. In the upper water layer 0 to 50 meters darkness, C. migratorius stayed closer to the surface the number of nauplii was up to 75% (Fig. 6). In to­ than during the day. This was attributed to the ver­ tal, the layer 0–250 meters contained 1 800 000 ind/ tical migrations of zooplankton into the upper water m2 (or 7200 ind/m3) (Kozhov 1954). In 1947–1949, layers. 1951 years, the number of nauplii reached 4 000 Until completion of spawning, Cottocomephorus 000–6 000 000 individuals. At the end of July, the adhered to the coastal area and actively fed upon number of copepodite stage crustaceans increased zooplankton. In July – August larvae of Cottocome- sharply (Kozhov 1954). The maximum number of phorus appeared and migrated as a school along the copepodites in the 0–250 m layer in 1947–1949 and coast. For C. migratorius and adult Cottocomephorus 1951 was 2 000 000–4 000 000 ind/m2 (on average larvae had been the main food source at this time. 1 250 000–1 450 000 ind/m2) (Fig. 6). At least In open areas of the pelagic zone, a high level of zoo­ 70–80% of crustaceans accumulated in the layer 0 to plankton biomass was sustained till the first half of 50 meters. At the end of July, August and September, October. Therefore, the period from mid­July to the the annual maximum development of E. baikalensis end of September was the main feeding period for was observed (Fig. 7). Crustaceans were evenly dis­ planktivorous fish in the open pelagic zone of the lake. Zooplankton and pelagic fish of Lake Baikal 453 Fig. 3. Distribution of Epischura baikalensis in the 0–250 m layer, above a depth of 800 m in spring (Kozhov 1954). Autumnal period in the depth of 200–300 m, and remained there for the winter. In autumn, in the second half of October and In 1947–1949 and 1951, throughout the year, the November, the upper layers of the lake were cooled, total number of E. baikalensis adults and copepodites the wind circulation intensified, and by the end of varied at 14.5 times, from 100 000 in January to November, autumnal homoeothermic regime was 1 450 000 ind/m2 in September. The total biomass established. Plankton scattered and went to the deep at this time changed at 5.6 times, from 7 to 39 g/m2. zone of 100–500 meters (Fig. 9). Macrohectopus 50 years later, in the early 2000s, the total number of branickii migrated to the bottom water layers with E. baikalensis in the upper layer of 0–25 m changed fading of vertical migrations. more than 70 times – from 37 470 to 2 670 200 ind/ White­fish C. migratorius feeding in the pelagic m2. The average number of E. baikalensis in 1951 zone moved closer to the shallow waters. The mass development of zooplankton was gradually dying was 405 820 ± 59.7 ind/m2 (Kozhov 1954). This out (Fig. 10). Closer to the winter period, the part of value was two times less than the average long­term zooplankton was concentrated at a depth of 150–300 value of the abundance of zooplankton in 1997–2007 meters. Planktonic Cottocomephorus again gathered (Kiprushina 2010). 454 V.G. Sideleva Fig. 4. Spring migration of pelagic fish to the coastal area of the lake. Fig. 5. Concentration of pelagic fish in the coastal zone of Lake Baikal – “halt”. Zooplankton and pelagic fish of Lake Baikal 455 Fig. 6. Maximum numerical and uniform distribution of zooplankton in the water column in summer (Kozhov 1954). Fig. 7. Dynamics of seasonal changes in the abundance and biomass of zooplankton in the open pelagic zone of Lake Baikal (Kozhov 1954). 456 V.G. Sideleva Fig. 8. Dispersion of omul (Coregunus migratorius) in the upper layers of the pelagic zone in summer. Fig. 9. Distribution of zooplankton in autumn (Kozhov 1954). Zooplankton and pelagic fish of Lake Baikal 457 Fig. 10. Coregunus migratorius and Cottocomephorus in the coastal pelagic part of Lake Baikal in autumn. CONCLUSION Comephorus and 3 species of sculpins genus Cotto- comephorus could be found in the pelagic zone of Lake One of the main food items for pelagic fish of Lake Baikal, which are able to use lower concentrations of Baikal is the copepod Epischura baikalensis. In the zooplankton than omul (C. migratorius). summer, its biomass reached 74–100% of the total Comparative analysis of the abundance of zoo­ zooplankton biomass of the lake. In shallow waters, plankton in Southern Baikal in different years and sometimes in the open areas of the lake, Cyclops showed that during the period of maximum develop­ kolensis had been found together with E. baikalensis. ment of E. baikalensis in July, August, and October, In the deep­water pelagic zone of the lake, the pelagic its number in 1997–2007 was twice more than in amphipod Macrohectopus branickii was a valuable 1951 (Kozhov 1954; Kiprushina 2010). The distri­ food item for fishes. bution of E. baikalensis was related to water tempe­ It was shown in the experiment that C. migrato- rature, and in the surface layer from 0 to 25 meters rius actively fed when number of copepodite stages had an inverse relationship. During the warming up reached 30–35 specimens per liter (Potakuev 1954). of the surface layers of water, which was observed in Such concentrations of zooplankton were formed in 1947–1949, 1951 and some years of the last decade, spring and summer at night, when 60–70% of crusta­ E. baicalensis left the upper zone and went deeper, to ceans gathered in the surface water layer 0 to 5 meters. a layer of 50–100 meters. In other months of the year, The abundance of copepodite stages of E. baikalensis no changes in the average values of the E. baikalensis in these layers could reach 150 to 300 specimens per abundance were revealed (Kozhov 1954). liter. In addition to E. baikalensis, the omul fed on Review of research from 1948 to 2007 showed pelagic amphipods M. branickii, which able to catch that the life strategy of pelagic fish of Lake Baikal, in individual specimens, regardless of concentration. including vertical distribution, seasonal differentia­ An assessment of the forage base of Lake Baikal tion, and trajectories of movement, had been associ­ showed that the main fish production was formed in ated with seasonal redistribution and concentrations the pelagic zone. In addition to omul, two species of of food items. 458 V.G. Sideleva The study of M.M. Kozhov involved an integral REFERENCES approach to solve the problem of the relationship be­ tween aquatic organisms in the pelagic zone of Lake Khanaev I.V., Dzyuba E.V., Kravtsova L.S. and Gra­ Baikal. In one complex had been studied the annual chev M.A. 2016. 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