拥挤胁迫后鲫鱼血液皮质醇和溶菌酶水平的变化及对病原的敏感性

分别在拥挤胁迫后第3、10、20、30天对鲫鱼(Carassiusauratus)血液皮质醇水平和血液溶菌酶水平进行了检测,并在每次采集血样后用嗜水气单胞菌(Aeromonashydrophila)进行人工感染以评估鱼体抵抗力。结果显示,血液皮质醇水平在胁迫后几天内显著升高(P<0.005),随后虽有所下降,但至第30天高密度组仍保持较高水平(216.32±11.91)ng/mL。实验初期,血液溶菌酶水平小幅升高,但经过长时间的拥挤后,高密度组血液溶菌酶活性受到极大抑制,显著低于同期对照组(P<0.01)。攻毒后的死亡率与胁迫时间成正相关,在最后一次采样期,高密度组死亡率高达58.3%,而同期对照组仅为16.7%,两者具统计学差异(P<0.01)。实验表明,经过拥挤胁迫后,鲫鱼血液皮质醇水平和溶菌酶水平均发生了变化,短期内,两者成正相关,而长期胁迫则表现为负相关。另外,拥挤胁迫也引起了鱼体抵抗力的下降。

Changes of cortisol and lysozyme levels in Carassius auratus blood after crowding stress and the fish sensitivity to pathogen

The problem of stress in both natural fisheries and in intensive cultivation systems is a major subject of scientific investigation. It is reported that one of the fish's physiological response to different stresses is a stimulation of the hypothalamic-pituitary-interrenal (HPI) axis, with a increase in the concentration of circulating cortisol, which can be considered as both general and endocrine indicator of stress in fish. As is known, teleost fish possess a variety of specific and non-specific defence mechanisms against invading organisms. When a pathogen penetrates the physical barriers of the animal, the first defence lines it encounters are those of the non-specific immune system. Chemical defences attack the pathogen or may opsonise it for further destruction by the cellular components of the non-specific immune system. Therefore, the non-specific immune system of fish is important in resistance to disease and is worthy of more attention. Lysozyme is a well-characterized protein, having an antibacterial effect (particularly on Gram-positive bacteria) both by attacking the bacterial cell wall, thereby causing lysis, and by stimulating phagocytosis of bacteria, so lysozyme as an innate immune component plays an important role in resisting the invasion of pathogenic microbes. Crowding stress, a kind of chronic stress, is so common in aquaculture that in recent years it has received much attention. In view of the important effects of cortisol and lysozyme on fish, the two indexes were therefore chosen to test after crowding stress. In the present study, about 160 fish were randomly allocated into 13 fiberglass tanks (12 fish per tank): one tank for control group before crowding (unrelated control group), four tanks for control groups (7.5 g/L), four tanks for low density groups (15 g/L) and four tanks for high density groups (80 g/L). Cortisol and lysozyme levels in Carassius auratus blood were measured at the 3rd, 10th, 20th and 30th days after crowding stress, and the mortalities due to Aeromonas hydrophila infection were also counted post every sampling. The results indicated that the blood cortisol level increased remarkably at third day after stress (P<0.005), then decreased slightly, but still maintained a high concentration (216.32 ng/mL average) in high density group at 30th day. At third day after crowding, blood lysozyme level had a little elevation, but it was inhibited markedly by long term stress in comparison with control group (P<0.01). Mortality due to A. hydrophila infection had a positive correlation with crowding time. At the last sampling, the average mortality of high density group was (58.3%,) which was statistically higher than that of control group (16.7% only). It is suggested from the present study that, blood cortisol level and lysozyme level are a positive correlation after short term stress, whereas have a negative one post long term stress. In addition, the resistance ability to disease is also depressed by crowding stress.