Size and age structure of the population of Monoporeia affinis (Amphipoda) in the Petrozavodsk Bay of Lake Onego

The Petrozavodsk Bay of Lake Onego is characterized by favorable temperature, food and oxygen conditions for the functioning of relict crustaceans Monoporeia affinis (Lindström, 1855) population. The analysis of the size-age population structure was performed for this species for the first time. In 2018, the spring and autumn state of the M. affinis population differed. The contribution to the total number of young crustaceans with a body length of 1.5–2.0 mm and 2.1–3.0 mm in May and September was distinguished. The number of these two size groups changed inversely from May to September, which indicated the growth processes of juveniles. The total contribution of young crustaceans to the total population at different stations varied within 40–50% in May and 20–40% in September. Based on the analysis of size-frequency histograms and the method of principal components, two age cohorts of crustaceans with a body length of 3.1–7.0 mm (the first cohort, age 1 year) and 7.1–10.0 mm (the second cohort, age 2 years) were identified. The proportion of the first cohort in the total population of M. affinis was on average 12% in May and September. The proportion of the second cohort in May was on average 2%, and in September it increased to 8%. The data obtained are in good agreement with two age cohorts of M. affinis in other lakes of Karelia.