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<article xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ali="http://www.niso.org/schemas/ali/1.0/" article-type="research-article" dtd-version="1.2" xml:lang="en"><front><journal-meta><journal-id journal-id-type="publisher-id">Journal of Ichthyology</journal-id><journal-title-group><journal-title xml:lang="en">Journal of Ichthyology</journal-title><trans-title-group xml:lang="ru"><trans-title>Вопросы ихтиологии</trans-title></trans-title-group></journal-title-group><issn publication-format="print">0042-8752</issn><issn publication-format="electronic">3034-5146</issn><publisher><publisher-name xml:lang="en">The Russian Academy of Sciences</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="publisher-id">683959</article-id><article-id pub-id-type="doi">10.31857/S0042875225020081</article-id><article-id pub-id-type="edn">CVLLVM</article-id><article-categories><subj-group subj-group-type="toc-heading" xml:lang="en"><subject>Articles</subject></subj-group><subj-group subj-group-type="toc-heading" xml:lang="ru"><subject>Статьи</subject></subj-group><subj-group subj-group-type="article-type"><subject>Research Article</subject></subj-group></article-categories><title-group><article-title xml:lang="en">Infestation of young northern pike <italic>Esox Lucius</italic> (esocidae) with macroparasites in different reaches of the river continuum</article-title><trans-title-group xml:lang="ru"><trans-title>Заражённость молоди щуки <italic>Esox lucius</italic> (esocidae) макропаразитами на разных участках речного континуума</trans-title></trans-title-group></title-group><contrib-group><contrib contrib-type="author"><name-alternatives><name xml:lang="en"><surname>Zhokhov</surname><given-names>А. Е.</given-names></name><name xml:lang="ru"><surname>Жохов</surname><given-names>А. Е.</given-names></name></name-alternatives><address><country country="RU">Russian Federation</country></address><email>zhokhov@ibiw.ru</email><xref ref-type="aff" rid="aff1"/></contrib><contrib contrib-type="author"><name-alternatives><name xml:lang="en"><surname>Mikheev</surname><given-names>V. N.</given-names></name><name xml:lang="ru"><surname>Михеев</surname><given-names>В. Н.</given-names></name></name-alternatives><address><country country="RU">Russian Federation</country></address><email>zhokhov@ibiw.ru</email><xref ref-type="aff" rid="aff2"/></contrib></contrib-group><aff-alternatives id="aff1"><aff><institution xml:lang="en">Papanin Institute of Biology for Inland Waters, Russian Academy of Sciences</institution></aff><aff><institution xml:lang="ru">Институт биологии внутренних вод РАН — ИБВВ РАН</institution></aff></aff-alternatives><aff-alternatives id="aff2"><aff><institution xml:lang="en">Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences</institution></aff><aff><institution xml:lang="ru">Институт проблем экологии и эволюции РАН — ИПЭЭ РАН</institution></aff></aff-alternatives><pub-date date-type="pub" iso-8601-date="2025-03-01" publication-format="electronic"><day>01</day><month>03</month><year>2025</year></pub-date><volume>65</volume><issue>2</issue><fpage>217</fpage><lpage>226</lpage><history><date date-type="received" iso-8601-date="2025-06-10"><day>10</day><month>06</month><year>2025</year></date></history><permissions><copyright-statement xml:lang="en">Copyright ©; 2025, Russian Academy of Sciences</copyright-statement><copyright-statement xml:lang="ru">Copyright ©; 2025, Российская академия наук</copyright-statement><copyright-year>2025</copyright-year><copyright-holder xml:lang="en">Russian Academy of Sciences</copyright-holder><copyright-holder xml:lang="ru">Российская академия наук</copyright-holder></permissions><self-uri xlink:href="https://transsyst.ru/0042-8752/article/view/683959">https://transsyst.ru/0042-8752/article/view/683959</self-uri><abstract xml:lang="en"><p>The composition of infracommunities and quantitative characteristics of infestation of underyearlings of northern pike <italic>Esox lucius</italic> with macroparasites have been studied in the small Ild River (a second-order tributary of the Rybinsk Reservoir) from different reaches of the river continuum (from the upper reaches to the mouth). The number of parasite species, mean abundance, and prevalence were significantly higher in the lower reaches than in the upper reaches of the river. Pike samples from different reaches significantly differed in the infection variability between the individuals. The highest coefficient of variation in the number of parasites in one fish individual was observed in the sample from the upper reaches (162%), while it was significantly lower in individuals from the middle and lower reaches (89 and 57%, respectively). Among various parasites from the lower part of the river, we have recorded manipulators of host behavior, which increase the fish availability for predators. These include several trematode species (Trematoda) of the family Diplostomidae. Underyearlings from the sample from the upper reaches were most variable in size, dominated by small individuals. The specimens from the lower reaches were characterized by a minimum variability in body length. The largest (on average) individuals were recorded in the sample from the middle reaches. Parasites are presumably a significant factor influencing the pike population structure and migratory activity, which is usually considered a sedentary species.</p></abstract><trans-abstract xml:lang="ru"><p>В малой реке Ильдь (приток Рыбинского водохранилища второго порядка) исследован состав инфрасообществ и количественные характеристики заражённости макропаразитами сеголеток щуки <italic>Esox lucius</italic> из разных частей речного континуума (от верховья до устья). Число видов паразитов, индекс обилия и экстенсивность заражения были существенно выше в низовье по сравнению с верховьем реки. Выборки щук из разных участков значительно различались по изменчивости заражённости между особями. Самый высокий коэффициент вариации числа паразитов в одной рыбе наблюдался в выборке из верхнего участка — 162%, у особей из среднего и нижнего участков он значительно снижался соответственно до 89 и 57%. Среди разнообразных паразитов нижнего участка реки обнаружены манипуляторы поведением хозяев, повышающие доступность рыб для хищников. К ним относятся несколько видов трематод (Trematoda) семейства Diplostomidae. Наиболее изменчивыми по размерам с преобладанием мелких особей были сеголетки в выборке из верховья. Рыбы из низовья характеризовались минимальной изменчивостью по длине тела. Наиболее крупные (в среднем) особи составляли выборку из среднего течения. Паразиты предположительно могут быть значительным фактором, влияющим на структуру популяции и миграционную активность щуки, которую обычно считают оседлым видом.</p></trans-abstract><kwd-group xml:lang="en"><kwd>macroparasites</kwd><kwd>underyearlings of northern pike Esox lucius</kwd><kwd>river continuum</kwd><kwd>heterogeneity of the environment</kwd><kwd>intrapopulation polymorphism</kwd></kwd-group><kwd-group xml:lang="ru"><kwd>макропаразиты</kwd><kwd>сеголетки щуки Esox lucius</kwd><kwd>речной континуум</kwd><kwd>гетерогенность среды</kwd><kwd>внутрипопуляционный полиморфизм</kwd></kwd-group><funding-group><award-group><funding-source><institution-wrap><institution xml:lang="ru">Российский научный фонд</institution></institution-wrap><institution-wrap><institution xml:lang="en">Russian Science Foundation</institution></institution-wrap></funding-source><award-id>24-14-00111</award-id></award-group></funding-group></article-meta></front><body></body><back><ref-list><ref id="B1"><label>1.</label><mixed-citation>Быховская-Павловская И.Е. 1985. 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