The stability of the ecosystem directly affects the water quality and safety, fishery production, and people’s quality of life along the route. In this study, extensive biological information on five dominant species of prey fish, including Hemiculter leucisculus (Basilewsky, 1855), Acheilognathus macropterus (Bleeker), Rhodeus sericeus (Pallas,1776), Pseudorasbora parva (Temminck & Schlegel, 1846), and Squalidus argentatus (Sauvage & Dabry de Thiersant, 1874), was collected in the lower reaches of the Songhua River, and the population parameters and variation rules of these fish were evaluated. The results showed that at present, the fish resources in the lower reaches of the Songhua River were in an overexploited state. Although the growth rate of prey fish was accelerating, their growth potential was decreasing. In addition to the homogeneous structure of the fish community, it was increasingly evident that a high proportion of small-sized fish were present in the fish community. In addition, the growth length coefficients of the five prey fish species were all greater than 0.2, indicating that the prey fish were growing at a faster rate, and the range of the growth performance indicators were 3.49 ~ 4.37. Our data also demonstrated that the exploitation rates of Hemiculter leucisculus and Squalidus argentatus were both greater than 0.5, and the exploitation rates of all species were higher than Emax except for Pseudorasbora parva. Finally, based on the above results, the mesh size of all nets should be controlled above 45 mm to ensure the size of the main prey fish populations in the lower reaches of the Songhua River. In summary, these results provided variation rules and growth of prey fish resources in the lower reaches of the Songhua River. At the same time, the distribution of major commercial or endangered baiting grounds in the lower reaches of the Songhua River was determined, which was beneficial to the balance and integrity of the ecosystem.

This study examined the age, growth, and maturation of 1,064 Coregonus ussuriensis individuals that were collected monthly from the middle section of the Amur River, China between 2016 and 2018. The fork length (FL) ranged from 14.9 to 49.1 cm for males and 21.5 to 58.8 cm for females, and the body weight (BW) ranged from 72.6 to 1,348.7 g for males and 107.9 to 2,854.9 g for females. The relationship of BW and FL was expressed as: BW♂ = 0.0324 × FL2.708; BW♀ = 0.014 × FL2.963. The sample ages ranged from 2 to 8 years for males and 2 to 9 years for females. We used the von Bertalanffy function based on otolith reading and observed FL data to simulate Coregonus ussuriensis growth, which has been suggested to be similar to that of other Salmonidae fishes. No significant difference in growth was determined between males and females. The monthly gonad somatic index (GSI) value ranged from 0.16% to 1.69% for males and from 0.73% to 16.15% for females, with a peak in November. Additionally, the reproductive staging suggested that the Coregonus ussuriensis spawning season was mainly in November and December. The size at maturity (FL50%) for males and females was 34.9 cm and 37.9 cm respectively, and the corresponding age (T50%) was 4.5 and 5.1, respectively. This study provides basic information for understanding the biological characteristics of Coregonus ussuriensis and should aid in the assessment and management of fishery resources.

Chum salmon (Oncorhynchus keta) migrate from freshwater to saltwater, and incur developmental, physiological and molecular adaptations as the salinity changes. The molecular regulation for salinity adaptation in chum salmon is currently not well defined. In this study, 1-g salmon were cultured under 0 (control group, D0), 8‰ (D8), 16‰ (D16), and 24‰ (D24) salinity conditions for 42 days. Na+/K+-ATPase and Ca2+/Mg2+-ATPase activities in the gill first increased and then decreased in response to higher salinity environments where D8 exhibited the highest Na+/K+ATPase and Ca2+/Mg2+-ATPase activity and D24 exhibited the lowest. Alkaline phosphatase (AKP) activity was elevated in all salinity treatment groups relative to controls, while no significant difference in acid phosphatase (ACP) activity was observed across treatment groups. De novo transcriptome sequencing in the D0 and D24 groups using RNA-Seq analysis identified 187,836 unigenes, of which 2,143 were differentially expressed in response to environmental salinity (71 up-regulated and 2,072 down-regulated). A total of 56,020 putative single nucleotide polymorphisms (SNPs) were also identified. The growth, development, osmoregulation and maturation factors of N-methyl-D-aspartate receptors (nmdas) expressed in memory formation, as well as insulin-like growth factor 1 (igf-1) and igf-binding proteins (igfbps) were further investigated using targeted qRT-PCR. The lowest expression of all these genes occurred in the low salinity environments (D8 or D16), while their highest expression occurred in the high salinity environments (D24). These results provide preliminary insight into salinity adaptation in chum salmon and a foundation for the development of marker-assisted breeding for this species.