Discoplan ts

Histological sections were prepared from 1.5 cm-thick samples of the midshaft region following standard procedures [21] –[24] . Bone samples were embedded in epoxy resin EpoFix (Struers), the cutting surface was ground and polished with a Buehler low-speed Isomet with SiC grinding papers (SiC-800, SiC-1200; Struers) and fixed to a glass-slide with epoxy resin. Subsequently, 200 µm-thick sections were cut using a Struers Discoplan TS diamond saw, and finally ground and polished to a final thickness of 100 µm with the use of different SiC grinding papers (SiC-800, SiC-1200; Struers). All necessary permits were obtained for the described study, which complied with all relevant regulations (Vertebrate Paleontology Collection Department, Museo Nacional de Ciencias Naturales – CSIC, Madrid, Spain). All histological sections employed in the present paper are deposited in the Vertebrate Paleontology Collection of the MNCN and are available to researchers. High resolution images of the analyzed sections can be obtained from the corresponding author of this article.

Each otolith was embedded in epoxy resin (Epofix, Struers, Ballerup, Denmark), mounted on a glass slide, and cut and ground to expose the core on the sagittal plane using a grinding machine equipped with a diamond cup wheel (Discoplan-TS, Struers). In order to obtain the core and most exposed annuli, the sections were further polished with 6 and 1 mm diamond paste on an automated polishing wheel (Planopol-V, Struers). The otolith microstructure of Coregonus ussuriensis showed characteristics of annulus that are generally represented by a sequence of alternating opaque and translucent concentric zones. The opaque zones are deposited in the rapid growth summer season, whereas the translucent zones are deposited in the slow growth period of winter (Blacker, 1974 ). Each otolith was read twice and if the two counts differed, the otolith was recounted until the result was consistent.

Thirty‐two fish from seven lakes (4–5 for each lake) and one fish from an anadromous population were analyzed for relative composition of trace element (Sr/Ca) of their otoliths. The otoliths were placed into a rectangular plastic mold and embedded in epoxy resin (Epofix; Struers). The otoliths were subsequently grinded with sand paper of 240 grits, 600 grits, 1,200 grits, and 2,000 grits in turn on Struers grinder (Discoplan‐TS, Struers), until the core of sagittal plane was exposed. The ratio of Sr/Ca was determined by coupled plasma mass spectrometer (ICPMS, Agilent 7700x, Agilent Technologies), in center, margin, and middle position of the sagittal otoliths.