Screening of several evaluation indicators for alkali resistance in fish

The vast expanse of saline-alkali waters worldwide poses significant challenges to fish survival. To identify key evaluation indicators for fish alkali tolerance and guide the selection of new alkali-tolerant fish varieties, this study selected the alkali forms of Crucian carp and Amur ide from Dali Lake (50 mM alkalinity) as research subjects. These alkali and freshwater populations underwent 7-day alkalinity stress treatments (10, 30, 50 mM), with systematic monitoring of hematological and serum biochemical indicators. Results showed that as alkalinity increased, the fish initiated corresponding immune responses, with Crucian carp showing a significantly stronger response than Amur ide. Furthermore, the freshwater forms of both fish species were more significantly affected by the external environment compared to their alkali forms, as evidenced by the significant increases in white blood cell count and neutrophil count in the high-alkalinity groups to varying degrees. Meanwhile, erythrocyte parameters rose significantly under high alkalinity, with Crucian carp exhibiting a more pronounced increase, indicating that fish respond to harsh environments by enhancing their oxygen transport efficiency. Additionally, changes in the serum concentrations of Na⁺ and Cl^- ions reflected differences in the osmoregulatory capacity of the fish, with both alkali forms fish species demonstrating stronger overall ion regulatory abilities. In high-alkaline environments, fish also exhibited impaired ammonia excretion and elevated urea nitrogen levels, particularly notable in the two freshwater fish species, suggesting weaker ammonia detoxification capabilities. The general downregulation of glutamate dehydrogenase activity implied that neither fish species employed the glutamine synthesis pathway for ammonia detoxification. The blood glucose levels of Crucian carp decreased more significantly overall compared to those of Amur ide, while the total cholesterol levels of Amur ide rose more markedly than those of Crucian carp, reflecting differences in energy metabolism pathways between the two fish species under alkali stress. The multi-indicator comprehensive evaluation system constructed in this study, encompassing hematological parameters, serum ion concentrations, and enzyme activities, provides a scientific basis for a holistic assessment of fish adaptability to alkaline environments. The research not only facilitates the selection of new alkali-tolerant fish varieties but also offers crucial technical support for the utilization of aquaculture in developing saline-alkali waters.