Effects of Predation Stress on the Physiological Responses of Juvenile Four Major Chinese Carps

Abstract

As low-level aquatic vertebrates, fish are highly dependent on the water environment, and general activities such as growth, foraging, and reproduction are easily affected by changes in the external environment. Changes in environmental factors can lead to different degrees of stress response in fish, and trigger a series of physiological changes, which then affect the stability of the organism's internal environment. Predation is one of the main environmental factors affecting the survival of individuals. In nature, almost all species face the risk of predation. Brief encounters with predators can reduce feeding and other health-related activities in prey fish and/or trigger primary and secondary stress responses, including the release of stress substances into the bloodstream. In predation stress, after initially sensing stress, fish initiate a stress response to overcome the stress and restore homeostasis. The degree of physiological stress depends primarily on the intensity and duration of the stress. If the appearance of predators is intermittent, then the physiological state of the prey fish returns to normal quickly, which allows the stress response to promote physiological changes in the prey fish to better adapt to the environment. However, repeated or persistent and unavoidable stress situations cause the normal physiological response mechanisms of prey fish to become compromised. Physiological stress may have long-term negative effects on the immune system, growth, or reproduction, and may reduce the adaptability and survivability of prey fish in the environment. Many studies have confirmed that predation stress can cause physiological stress in fish. Different species of fish and even different groups of the same species vary greatly in the degree of stress and stress mode. More species-specific studies are required to determine the effects of different levels of predation stress on physiological stress in fish.Black carp (Mylopharyngodon piceus), grass carp (Ctenopharyngodon idellus), silver carp (Hypophthalmichthys molitrix), and bighead carp (Aristichthys nobilis) are known as the four major Chinese carps. As common fish species in Chinese inland watersheds, the four major Chinese carps are ecologically and economically valuable. Over the years, many reasons such as hydraulic construction, environmental pollution, and overfishing have led to sharp declines in wild populations. In addition, the prevalence of predators in natural waters also threaten population growth. It remains unclear how the juveniles of the four major Chinese carps adjust their physiological processes to cope with predation stress. We investigate the physiological and energy metabolism adaptations by black carp, grass carp, silver carp, and bighead carp to predatory stress. We selected the common local enemies of natural waters, the snakehead carp (Channa argus) and the southern catfish (Silurus meridionalis) as predators. The levels of serum cortisol and biochemical parameters in the juveniles of the four major Chinese carps under the stress of no-predation (control), low-predation (indirect stress) and high-predation (direct stress) over 0 d, 7 d, and 14 d were investigated. Changes in the biochemical parameters were analyzed. The effects of different predation stress levels on serum cortisol and biochemical parameters were also analyzed. The results showed: (1) under different levels of predation stress, the biochemical parameters and serum cortisol levels of juveniles of the four major Chinese carps varied to different degrees but the trends were consistent; (2) the serum cortisol levels of juveniles of the four major Chinese carps increased significantly with the degree of predation stress and the stress duration, and showed the following patterns: non-predation group < low predation group < high predation group, 0 d < 7 d < 14 d. (3) Among the biochemical parameters, serum total protein concentration and cholesterol concentration were relatively stable and did not vary significantly. Glucose concentration and alkaline phosphatase increased with predation stress, while triglyceride had a decreasing trend. The results showed that juveniles of the four major Chinese carps adjust their physiological responses to enhance their own survivability according to the predation risk. After the predation stress treatment, the juveniles of the four major Chinese carps all underwent a stress response. Compared with indirect predation, direct predation had a more significant effect on the physiological response of fish, and the degree of stress increased with the stress duration. Among the detection parameters, serum total protein and cholesterol may not be sensitive parameters for stress in fish under predation stress. The most significant changes were in cortisol and glucose and may compensate for the increased energy demand by the organism during stress. The adaptation of physiological stress and energy metabolism to predation stress in juveniles of the four major Chinese carps under predation stress conditions provides a theoretical basis for the stress responses of an organism to environmental changes, and can also provide a scientific reference for exploring the ecological interactions between predator and prey