Seasonal dynamics in fish and squid trophic status in the pelagic Sea of Okhotsk, based on d13C and d15N stable isotope data analysis

Seasonal trophic dynamics was analyzed based on d13C and d15N stable isotope mass content in the Sea of Okhotsk 35 nekton species (fish and squid (Teuthida)). It was shown that considerable differences between species in stable isotope mass contents were associated with diet type. The d15N average values of the Sea of Okhotsk species were 4.5 ‰ in spring, 7.5 ‰ in summer, and 5.4 ‰ in autumn. The summer increase was caused by the appearance of juvenile fish and squid with low trophic status and minimal d15N values, and the autumn decline was due increased trophic status of growing up young individuals. Variability range in d15N values was 11.5 ‰ for all nekton species in summer including migrants into the Sea of Okhotsk from the Pacific Ocean. Relatively narrow variability range in d13C for nekton species, of 2.2–3.3 ‰, reflects seasonal homogeneity in the trophic web basement in the pelagic Sea of Okhotsk. The structure of the food web of the pelagic nekton, presented according to the d15N and d13C data, provides useful information on the pathways of organic matter transfer to the pelagic zone at the upper trophic levels and can be further used to construct trophodynamic models of the Sea of Okhotsk.