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Volume 45 Issue 10
Oct.  2021
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Dietary zinc requirement of juvenile largemouth bass (Micropterus salmoides)

  • Corresponding author: LENG Xiangjun, xjleng@shou.edu.cn
  • Received Date: 2020-09-07
    Accepted Date: 2021-03-21
    Available Online: 2021-09-02
  • To determine the dietary zinc requirement of juvenile largemouth bass (Micropterus salmoides), casein, gelatin and fish meal were used as the main protein sources and ZnSO4·H2O as zinc source to produce a semi-purified diet. Then, zinc was added into the basal diet at levels of 0, 25, 50, 100 and 200 mg/kg, to obtain five diets containing zinc of 24.8, 48.8, 78.9, 126.1 and 223.6 mg/kg dry matter, respectively. Largemouth bass with initial body mass of (10.99±0.07) g were fed one of the five diets for 8 weeks. The results showed that the supplementation of 25 mg/kg Zn significantly increased the weight gain rate (WGR), and reduced feed coefficient ratio (FCR) of largemouth bass, which tended to stabilize when higher levels of Zn were added. When dietary Zn reached 25-49 mg/kg, the serum activity of CuZn-SOD and T-SOD remained stable, while Zn-79 group showed the highest AKP activity. The zinc content in whole fish and vertebrae increased with the increasing dietary Zn, and showed no significant increase when dietary Zn content was higher than 126 mg/kg (Zn-126 group). The iron content in whole body, iron and manganese contents in vertebra, and the retention of iron and zinc in whole body decreased with the increasing dietary Zn. In conclusion, the supplementation of Zn in diet improved the weight gain and feed utilization, promoted serum immunity and Zn deposit in whole body and vertebra. Based on the WGR, FCR, whole body zinc and vertebra zinc, the broken-line analysis indicated that the dietary requirement of Zn was 45.5, 44.6, 121.8 and 130.5 mg/kg dry matter, respectively.
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Dietary zinc requirement of juvenile largemouth bass (Micropterus salmoides)

    Corresponding author: LENG Xiangjun, xjleng@shou.edu.cn
  • 1. National Demonstration Center for Experimental Fisheries Science Education, Shanghai Ocean University, Shanghai    201306
  • 2. Centre for Research on Environmental Ecology and Fish Nutrition (CREEFN) of the Ministry of Agriculture and Rural Affairs, Shanghai Ocean University, Shanghai 201306
  • 3. Shanghai Collaborative Innovation for Aquatic Animal Genetics and Breeding, Shanghai Ocean University, Shanghai    201306

Abstract: To determine the dietary zinc requirement of juvenile largemouth bass (Micropterus salmoides), casein, gelatin and fish meal were used as the main protein sources and ZnSO4·H2O as zinc source to produce a semi-purified diet. Then, zinc was added into the basal diet at levels of 0, 25, 50, 100 and 200 mg/kg, to obtain five diets containing zinc of 24.8, 48.8, 78.9, 126.1 and 223.6 mg/kg dry matter, respectively. Largemouth bass with initial body mass of (10.99±0.07) g were fed one of the five diets for 8 weeks. The results showed that the supplementation of 25 mg/kg Zn significantly increased the weight gain rate (WGR), and reduced feed coefficient ratio (FCR) of largemouth bass, which tended to stabilize when higher levels of Zn were added. When dietary Zn reached 25-49 mg/kg, the serum activity of CuZn-SOD and T-SOD remained stable, while Zn-79 group showed the highest AKP activity. The zinc content in whole fish and vertebrae increased with the increasing dietary Zn, and showed no significant increase when dietary Zn content was higher than 126 mg/kg (Zn-126 group). The iron content in whole body, iron and manganese contents in vertebra, and the retention of iron and zinc in whole body decreased with the increasing dietary Zn. In conclusion, the supplementation of Zn in diet improved the weight gain and feed utilization, promoted serum immunity and Zn deposit in whole body and vertebra. Based on the WGR, FCR, whole body zinc and vertebra zinc, the broken-line analysis indicated that the dietary requirement of Zn was 45.5, 44.6, 121.8 and 130.5 mg/kg dry matter, respectively.

  • 锌是鱼类生长必需的微量元素,是鱼类体内多种酶的辅助因子,在脱氢酶、肽酶、醛缩酶和磷酸酶等金属酶中起着至关重要的作用[1]。锌参与调节许多重要的生理过程,包括蛋白质、脂类、糖类和核酸的合成和降解[2]。锌的缺乏会导致鱼类免疫能力下降[3],脂质过氧化[4],DNA损伤和氧化应激,甚至死亡[5]。在对虹鳟(Oncorhynchus mykiss)的研究中,锌缺乏会导致白内障、鳍和皮肤溃烂、生长速率和组织锌浓度降低,死亡率升高[6]。尼罗罗非鱼(Oreochromis niloticus)[7]则表现为食欲减退,饲料利用率低,血清抗氧化能力降低。锌缺乏可以激活下丘脑-垂体-肾上腺皮质轴,产生糖皮质激素如皮质醇,影响营养物质新陈代谢,甚至引起细胞死亡[8]

    虽然鱼类可以从水中吸收Zn,但在高密度人工养殖下,水中Zn浓度不足以满足鱼类对锌的需求量[9],因此有必要在饲料中补充Zn。目前,已经报道了一些鱼类对锌的需求量,比如,军曹鱼(Rachycentron canadum)[2](以蛋氨酸锌为锌源)、建鲤(Cyprinus carpio var. Jian)[10](以乳酸锌为锌源)对锌的需求量分别为47~54 mg/kg、48.7 mg/kg。尼罗罗非鱼[11]、虹鳟[12]和黄颡鱼(Pelteobagrus fulvidraco)[13]对锌的需求量分别为94.27、80和17.12~20.86 mg/kg (均以硫酸锌为锌源)。以上结果表明不同鱼类对锌的需求量存在较大差异,且与锌源相关。

    大口黑鲈(Micropterus salmoides),又名加州鲈,是一种生长快、养殖周期短、肉多刺少、味道鲜美、易捕捞、适温较广的名贵肉食性鱼类。近年来,大口黑鲈养殖发展十分迅速,2019年全国的养殖产量已达47.8万t[14]。目前,关于大口黑鲈营养需求的研究,主要集中在宏量营养素方面,如蛋白质[15-16]、脂肪[17-18]、氨基酸[19]等,而关于微量元素需求量的研究尚未见报道。鉴于锌在营养代谢中的重要性,考虑到生产中多为添加无机锌,故本研究以硫酸锌为锌源,在半精制饲料中补充不同水平的锌,考察对大口黑鲈生长、饲料利用、锌沉积和血清相关生化指标的影响,确定大口黑鲈对饲料中锌的需求量,为大口黑鲈饲料的科学配制提供理论基础。

1.   材料与方法
  • 以酪蛋白、明胶、鱼粉、大豆浓缩蛋白、豆粕和玉米蛋白为蛋白质源,以鱼油、豆油为脂肪源,制作了粗蛋白质含量为50.7%~50.8%、粗脂肪含量为12.5%~12.6%的半精制基础饲料(表1)。以硫酸锌(ZnSO4·H2O,37.4% Zn,分析纯)为锌源,在基础饲料中添加0、25、50、100和200 mg/kg锌,配制5种实验饲料,经电感耦合等离子体原子发射光谱仪(ICP-MS,美国赛默飞世尔科技公司)测定,5种实验饲料实际锌含量(以干物质计算)分别为24.8、48.8、78.8、126.1和223.6 mg/kg,分别命名为Zn-25,Zn-49,Zn-79,Zn-126,Zn-224。各主要原料经粉碎后过60目筛,逐级混匀,用单螺杆挤压机(SLP-45,中国水产科学研究院渔业机械仪器研究所)制成直径为2 mm的沉性颗粒饲料[制粒温度(85 ± 5) °C],饲料于通风阴凉处晾干,置于4 °C冰箱中保存备用。

    项目 items饲料 diets
    Zn-25Zn-49Zn-79Zn-126Zn-224
    原料/(g/kg) ingredients
     酪蛋白 casein 160.0 160.0 160.0 160.0 160.0
     明胶 gelatin 40.0 40.0 40.0 40.0 40.0
     鱼粉 fish meal 170.0 170.0 170.0 170.0 170.0
     大豆浓缩蛋白 soybean protein concentrate 100.0 100.0 100.0 100.0 100.0
     豆粕 soybean meal 100.0 100.0 100.0 100.0 100.0
     糊精 dextrin 110.0 110.0 110.0 110.0 110.0
     α-淀粉 α-starch 40.0 40.0 40.0 40.0 40.0
     玉米蛋白 corn gluten meal 90.0 90.0 90.0 90.0 90.0
     鱿鱼膏 squid paste 20.0 20.0 20.0 20.0 20.0
     啤酒酵母 beer yeast 20.0 20.0 20.0 20.0 20.0
     鱼油 fish oil 50.0 50.0 50.0 50.0 50.0
     豆油 soybean oil 30.0 30.0 30.0 30.0 30.0
     大豆磷脂 soybean phospholipid 30.0 30.0 30.0 30.0 30.0
     微晶纤维素 microcrystalline cellulose 10.0 10.0 10.0 10.0 10.0
     磷酸二氢钙 Ca(H2PO4)2 20.0 20.0 20.0 20.0 20.0
     多维 vitamin premix1) 5.0 5.0 5.0 5.0 5.0
     多矿 mineral premix2) 5.0 5.0 5.0 5.0 5.0
     硫酸锌 ZnSO4·H2O 0 0.07 0.14 0.29 0.58
     合计 total 1 000.0 1 000.0 1 000.0 1 000.0 1 000.0
    营养水平 proximate composition
     粗蛋白质/(g/kg) crude protein 507.5 508.5 508.1 507.3 507.5
     粗脂肪/(g/kg) crude lipid 126.0 125.1 125.7 125.6 125.6
     灰分/(g/kg) ash 70.1 70.3 70.4 70.7 70.6
     水分/(g/kg) moisture 96.1 95.3 95.4 96.4 95.7
     锌(干物质)/(mg/kg) Zn (dry matter) 24.8 48.8 78.8 126.1 223.6
    铁(干物质)/(mg/kg) Fe (dry matter) 304.8 301.6 307.2 302.9 305.8
    注:1) 维生素预混料向饲料提供的维生素(mg or IU/kg diet)为维生素 A 10 000 IU,维生素 D3 3 000 IU,维生素 E 150 IU,维生素 K3 12.17 mg,维生素 B1 20 mg,维生素 B2 20 mg,维生素 B3 100 mg,维生素 B6 22 mg,维生素 B12 0.15 mg,维生素 C 1 000 mg,生物素 0.6 mg,叶酸 8 mg,肌醇 500 mg。2) 矿物质预混料向饲料提供的矿物元素(mg/kg diet)(不含锌)为铜 5.25,锰 21,铁 120,镁 150,碘 0.6,硒 0.53,钴 0.45
    Notes: 1) vitamin premix supplied vitamins for the diets(mg or IU/kg diet) are vitamin A 10 000 IU, vitamin D3 3 000 IU, vitamin E 150 IU, vitamin K3 12.17 mg, vitamin B1 20 mg, vitamin B2 20 mg, vitamin B3 100 mg, vitamin B6 22 mg, vitamin B12 0.15 mg,vitamin C 1 000 mg, biotin 0.6 mg, folic acid 8 mg, inositol 500 mg. 2) mineral premix supplied minerals for the diets (mg/kg diet) (without zinc) are Cu 5.25, Mn 21, Fe 120, Mg 150, I 0.6, Se 0.53, Co 0.45

    Table 1.  Composition and nutrient levels of diets (air-dry basis)

  • 实验所用鲈鱼采购于浙江湖州鲈鱼养殖场,养殖实验在上海海洋大学滨海基地进行。实验为期8周,正式实验前先用基础饲料驯化4周,养殖模式为室内网箱养殖,取15个网箱(网箱规格1.2 m × 1.5 m × 1.2 m),放置于2口室内水泥池(5.0 m × 3.0 m × 1.2 m)中。挑选300尾体质健壮、规格均匀、平均初始体质量为(10.99 ± 0.07) g的大口黑鲈幼鱼,随机分组至15口网箱中,每个网箱放20尾鱼,实验共5个处理,每个处理设3个重复。每天投喂饲料2次(08:00、16:00),达到饱食状态。实验期间,每5 d吸污换水1次,水体溶解氧> 5 mg/L,水温26 ~ 30 °C,pH 7.2 ~ 7.9,氨氮含量< 0.2 mg/L,亚硝酸盐含量< 0.1 mg/L,锌含量< 10 μg/L。

  • 实验开始前,取10尾鱼作为初始样本。在养殖实验结束后,鱼禁食24 h,对每个网箱中的鱼尾数与体质量进行统计,以确定增重率和饲料效率。每个网箱随机取2尾鱼用于全鱼常规营养成分与矿物质含量的测定。另外,每个网箱随机取4尾鱼进行体长、体质量的测定。于尾部静脉处进行血液样本采集,离心10 min (3 000 r/min),取血清于−80 °C冰箱保存,然后对鱼进行解剖,对其肌肉、内脏、肝脏进行称量采集,将解剖后的鱼体放入微波炉内加热60 s,分离出脊椎骨,用蒸馏水清洗干净,105 °C烘干,−20 °C保存。

  • 对原料、饲料及鱼体进行常规成分测定。采用105 °C常压干燥法测定水分含量,通过凯氏定氮仪(2300自动凯氏定氮仪,FOSS,瑞典)测定粗蛋白质含量,通过氯仿—甲醇抽提法测定粗脂肪含量,通过550 °C马弗炉高温灼烧法(SXL-1008马弗炉,上海精宏实验设备有限公司)测定灰分。

  • 碱性磷酸酶(AKP)活性测定采用对硝基苯磷酸盐法;总超氧化物歧化酶(T-SOD)和铜锌超氧化物歧化酶(CuZn-SOD)活性测定均采用黄嘌呤氧化酶法;谷草转氨酶(AST)和谷丙转氨酶(ALT)活性测定均采用赖氏法。试剂盒均由南京建成生物技术研究所提供。

  • 全鱼、脊椎骨的矿物质元素含量测定采用电感耦合等离子体发射光谱法[20]。称取样品0.2 g于微波消解内罐中,加入10 mL硝酸,加盖放置1 h,旋紧罐盖,放入微波消解仪变温加热消解8 h 35 min。冷却后取出,缓慢打开罐盖排气,用少量水冲洗内盖,将消解罐放在超声水浴箱中超声脱气5 min,用水定容至50 mL。将定容后的样品溶液注入电感耦合等离子体原子发射光谱仪(ICP-MS,美国赛默飞世尔科技公司)测定,以元素的特征谱线波长定性,待测元素谱线信号强度与元素浓度呈正比进行定量分析。

  • 成活率(survival rate, SR, %) =Nt/N0×100%

    增重率(weight gain rate, WGR, %) =(WtW0)/W0×100%

    饲料系数(feed conversion rate, FCR)=Wf/(WtW0)

    肝体比(hepatosomatic index, HSI, %)=Wh/W×100%

    脏体比(viscerosomatic index, VSI, %)=Wv/W×100%

    肥满度(condition factor, CF, g/cm3)=W/L3×100

    矿物质沉积率(mineral retention rate, MR, %)= (Wt×WtmW0×W0m) / (Wf ×Wfm)×100%

    式中,Nt为终末尾数;N0为初始尾数;Wt为终末体质量(g);W0为初始体质量(g);Wf为摄入饲料量(g);Wh为鱼肝脏重(g);Wv为鱼内脏重(g);W为鱼体质量(g);L为鱼体长(cm);Wtm为终末全鱼矿物质含量 (mg/kg);W0m为初始全鱼矿物质含量 (mg/kg);Wfm为饲料矿物质含量 (mg/kg)。

    数据均以平均值±标准差(mean±SD)表示,采用SPSS 24.0统计软件进行单因素方差分析(One-Way ANOVA),并采用Duncan氏多重比较,P<0.05为差异显著。采用折线模型,对饲料锌含量与大口黑鲈增重率、饲料系数、全鱼锌和骨锌之间的关系进行回归分析。

2.   结果
  • 各组的成活率、肥满度、脏体比和肝体比均无显著差异(P>0.05)。大口黑鲈的增重率随着饲料锌含量的增加呈先上升后平稳的趋势,饲料系数则呈现先下降后稳定的趋势 (表2)。根据折线模型,大口黑鲈对饲料中锌的适宜需求量为45.5 mg/kg (增重率)(图1)、44.6 mg/kg (饲料系数)(图2)。

    项目 items  饲料 diets
    Zn-25Zn-49Zn-79Zn-126Zn-224
    初始均重/g IBW 11.0±0.10 10.97±0.03 10.98±0.08 11.02±0.08 10.97±0.10
    终末均重/g FBW 41.2±1.4b 44.4±1.1a 43.8±0.8a 43.4±0.9a 44.1±0.4a
    成活率/% SR 93.3±5.8 93.3±7.6 93.3±2.9 95.0±8.7 90.0±17.3
    增重率/% WGR 274.3±12.3b 303.2±9.5a 297.9±7.4a 294.9±8.0a 300.6±3.6a
    饲料系数 FCR 1.15±0.03b 1.03±0.02a 1.07±0.04a 1.05±0.05a 1.04±0.02a
    肥满度/(g/cm3) CF 2.1±0.1 2.0±0.2 2.0±0.2 2.0±0.2 2.1±0.2
    脏体比/% VSI 7.1±0.9 8.0±2.1 7.6±1.2 7.8±0.7 8.2±1.3
    肝体比/% HSI 1.5±0.5 1.6±1.3 1.7±0.9 1.8±0.6 1.9±0.9
    注:同一行上标字母不同表示差异显著(P<0.05),下同
    Notes:different superscript letters in the same row indicate significant differences(P<0.05), the same below

    Table 2.  Effects of dietary zinc level on growth of M. salmoides

    Figure 1.  Relationship between weight gain rate of M. salmoides and dietary zinc level

    Figure 2.  Relationship between feed conversion rate of M. salmoides and dietary zinc level

  • 饲料中补充锌对大口黑鲈肌肉、全鱼的水分、粗蛋白、灰分和粗脂肪含量均无显著影响(P>0.05)(表3)。

    项目 item饲料 diets
    Zn-25Zn-49Zn-79Zn-126Zn-224
    肌肉 muscle
     水分 moisture 78.2±0.4 77.9±0.3 77.7±0.2 77.9±0.3 78.0±0.3
     粗蛋白质 crude protein 19.5±0.3 19.6±0.3 19.8±0.2 19.5±0.4 19.5±0.4
     灰分 ash 1.25±0.03 1.29±0.03 1.32±0.03 1.37±0.10 1.43±0.20
     粗脂肪 crude lipid 0.86±0.04 0.84±0.01 0.84±0.06 0.85±0.06 0.83±0.05
    全鱼 whole body
     水分 moisture 72.5±0.3 72.5±0.1 72.3±0.1 72.2±0.1 72.2±0.2
     粗蛋白质 crude protein 17.7±0.3 17.2±0.2 17.5±0.4 17.5±0.3 17.7±0.1
     灰分 ash 3.57±0.14 3.46±0.17 3.50±0.02 3.67±0.13 3.39±0.01
     粗脂肪 crude lipid 3.05±0.56 3.35±0.26 3.39±0.74 3.40±0.02 3.52±0.17

    Table 3.  Effects of dietary zinc on muscle and whole body composition of M. salmoides %

  • 随着饲料锌含量增加,血清ALT活性呈上升的趋势;AST和CuZn-SOD、T-SOD活性在饲料锌含量分别达到79和49 mg/kg后,保持基本稳定;而AKP活性随饲料锌含量增加而上升,但Zn-49组与Zn-79、Zn-126和Zn-224组间差异不显著(P>0.05)(表4)。

    项目 item    饲料 diets
    Zn-25Zn-49Zn-79Zn-126Zn-224
    碱性磷酸酶/(U/L) AKP 5.8±0.7b 6.5±0.5ab 7.1±0. 3a 6.6±0.4ab 6.4±0.2ab
    总超氧化物歧化酶/(U/mL) T-SOD 77.9±1.1b 82.6±3.0a 80.4±1.1ab 81.8±0.2a 80.2±2.3ab
    铜锌超氧化物歧化酶/(U/mL) CuZn-SOD 63.3±1.4b 72.8±2.1a 67.6±3.1ab 68.6±3.5ab 67.8±2.9ab
    谷草转氨酶/(U/L) AST-GOT 25.0±0.9b 25.0±1.7b 27.8±0.9a 28.8±1.3a 27.3±0.7a
    谷丙转氨酶/(U/L) ALT-GPT 18.3±1.6c 20.8±0.5b 21.5±1.1b 22.7±1.2ab 23.9±1.3a

    Table 4.  Effects of dietary zinc on serum bio-chemical index of M. salmoides

  • 随着饲料中锌含量的增加,大口黑鲈全鱼和脊椎骨中的锌含量上升,当饲料锌含量达到126 mg/kg后(Zn-126组),全鱼和脊椎骨中的锌含量不再显著增加(P>0.05)。而全鱼铁、骨铁、骨锰含量、全鱼铁沉积率和锌沉积率则表现为随饲料锌含量增加而下降的规律(表5)。以全鱼锌和骨锌为评价指标,基于折线模型确定的大口黑鲈幼鱼对饲料中锌的适宜需求量分别为121.8 mg/kg (图3)和130.5 mg/kg (图4)。

    项目 item    饲料 diets
    Zn-25Zn-49Zn-79Zn-126Zn-224
    全鱼 whole fish
     铁/(mg/kg) Fe 90.4±2.3a 74.6±2.2b 65.4±1.9c 56.0±2.2d 55.5±2.2d
     铁沉积率/% Fe retention 29.2±1.3a 29.6±4.3a 18.6±0.7b 16.0±1.2b 15.5±0.1b
     锌/(mg/kg) Zn 32.5±1.7d 41.7±1.7c 48.0±2.0b 52.0±1.8a 54.3±2.0a
     锌沉积率/%  Zn retention 88.7±3.3a 76.1±5.0b 55.3±3.6c 40.2±2.6d 24.4±0.5e
    脊椎骨 vertebra
     锰/(mg/kg) Mn 75.9±1.4a 71.9±1.4b 65.4±2.9c 59.6±2.6d 59.9±0.7d
     铁/(mg/kg) Fe 136.9±2.7a 131.1±3.6b 118.2±2.8c 95.3±2.6d 93.8±2.1d
     锌/(mg/kg) Zn 66.3±1.4d 71.6±1.6c 78.3±2.6b 81.3±2.0ab 85.4±3.6a

    Table 5.  Effects of dietary zinc on mineral contents in whole body and vertebra of M. salmoides

    Figure 3.  Relationship between zinc level in whole body of M. salmoides and dietary zinc level

    Figure 4.  Relationship between zinc level in vertebra of M. salmoides and dietary zinc level

3.   讨论
  • Ogino等[6]发现,当饲料中锌含量为1 mg/kg时,虹鳟生长缓慢,死亡率高,出现白内障、鳍与皮肤腐烂,当锌含量在5 mg/kg以上后,可防止这些缺乏症状的出现。当饲料锌含量低于5 mg/kg时,尼罗罗非鱼也表现出生长缓慢,死亡率高,厌食等症状,当锌含量高于30 mg/kg时,存活率与增重率显著提高[21]。但在黄颡鱼[13](基础饲料含锌7.6 mg/kg)、露斯塔野鲮(Labeo rohita)[22](基础饲料含锌9.5 mg/kg)、斜带石斑鱼(Epinephelus coioides)[23](基础饲料含锌10.0 mg/kg)的研究中,对照组鱼体均无明显锌缺乏症状出现。在本实验中,尽管对照组(未补充锌)的鱼体生长性显著低于其他各组,但未出现明显的锌缺乏症。可能是由于本实验基础饲料为半精制饲料,鱼粉的用量为170 g/kg,使得基础饲料中锌含量达25 mg/kg,防止了锌缺乏症状的出现。

    本研究中,以增重率和饲料系数为指标,基于折线模型确定的大口黑鲈幼鱼对饲料锌的适宜需求量分别为45.5和44.6 mg/kg,低于同样以硫酸锌为锌源的虹鳟(80 mg/kg)[12]、草鱼(Ctenopharyngodon idella)(55.1 mg/kg)[24]、大黄鱼(Larimichthys crocea)(59.6 mg/kg)[25]、高于黄颡鱼(17.12~20.86 mg/kg)[13]、西伯利亚鲟(Acipenser baerii)(29.15 mg/kg)[26]、斜带石斑鱼(28.9 mg/kg)[23]和尼罗罗非鱼(37.2 mg/kg)[7]。锌需求量的不同,与鱼的种类、生长阶段、锌源以及基础饲料中锌的含量和可利用性等因素有关。

  • 锌参与调节许多重要的生理过程,其中包括蛋白质、脂类、糖类和核酸的合成和降解[2]。研究表明,提高饲料锌含量可以提高黄颡鱼的体蛋白质含量,但是体脂肪和灰分含量有所降低[13]。在虹鳟中,锌缺乏显著增加了鱼体水分含量,降低了蛋白质和灰分含量[27]。本实验中,随着饲料中锌含量的增加,大口黑鲈全鱼与肌肉的粗脂肪、粗蛋白质、灰分、水分均无明显差异,与在皱纹盘鲍(Haliotis discus hannai) [28]、建鲤[10]上的研究结果相一致,可能是基础饲料中的锌防止了缺乏症出现的同时,也满足了维持体成分和肌肉成分的需要。

  • 在生物体中,碱性磷酸酶 (AKP) 是一种重要的代谢调控酶,存在于高等动物几乎全部的组织中,直接参与磷酸集团的转移和代谢,对骨骼矿化起到重要的作用[29],而锌正是AKP的组成成分。在本实验中,基础饲料组(Zn-25组)的大口黑鲈血清AKP活性最低,当饲料锌添加量为25 mg/kg(Zn-49组)后,AKP活性不再显著上升。在虹鳟[30]、斜带石斑鱼[31]、斑点叉尾鮰 (Ictalurus punctatus)[32]等研究中,也观察到AKP活性受到饲料锌含量的影响,分别在锌含量达到60.0、57.4和20.0 mg/kg后活性基本稳定。

    超氧化物歧化酶(SOD)是生物体内存在的一种抗氧化金属酶,是抗氧化酶防御体系中第一道也是最重要的一道屏障[33-34],目前在生物体中已发现3种不同的SOD亚型,CuZn- SOD、Mn-SOD和Fe-SOD。其中,CuZn-SOD在生物体中分布最广泛[35]。有研究表明,饲料中锌含量的增加,有助于提高血清SOD的活性。在大菱鲆实验中,当饲料锌含量低于75 mg/kg时,血清T-SOD活性随锌含量增加而增加[36]。黄颡鱼饲料的锌含量低于38.75 mg/kg[13]、露斯塔野鲮饲料的锌含量低于51.42 mg/kg[22]、建鲤饲料的锌含量低于40.80 mg/kg[37]时,也有相同的规律。本实验中,饲料中添加25 mg/kg锌(Zn-49组)显著提高了大口黑鲈血清T-SOD和CuZn-SOD活性,且在该组后保持稳定,表明在基础饲料中添加25 mg/kg锌已能满足鱼体正常SOD活性所需。

    谷丙转氨酶 (ALT)、谷草转氨酶 (AST) 是评价肝脏健康状况的一个重要指标,当肝脏正常的情况下,血清中的AST、ALT活性维持在一定含量,当肝脏发生病变时候,肝脏细胞受损导致大量AST、ALT进入血液中,从而引起血清中AST、ALT活性显著增加[38]。Lowe等[39]发现,锌进入血液后,首先在肝脏中沉积,肝脏是锌代谢最快的器官。本实验中,血清中AST、ALT活性均随着饲料锌含量的增加而上升,并具有与全鱼铁、骨铁、骨锰含量基本一致的变化趋势,可能是高含量锌会增加肝脏负担,也可能是随着锌补充量的增加,抑制了铁、锰元素的吸收,导致肝脏有受损的风险。在星斑川鲽(Platichthys stellatus)[40]中,当饲料锌含量为411 mg/kg时,血清AST、ALT活性显著升高,且肝脏出现充血等症状。本实验中,大口黑鲈血清AST、ALT活性分别为25.0~28.8 U/L、18.3~23.9 U/L。在尼罗罗非鱼[7]中,血清AST正常范围为33.5~63.7 U/L。在南方鲇(Silurus meridionalis)、虹鳟中,正常血清ALT分别为42.5 U/L[41]、29.5~30.7 U/L[42],肝脏肿大充血虹鳟的血清ALT活性上升为137 U/L[42]。相比之下,本实验血清中AST、ALT活性在正常范围之内,继续增加饲料中锌含量,可能增加肝脏受损的风险,今后有待结合肝脏组织学加以分析。

  • 通常,全鱼和脊椎中的矿物质含量可用于评估鱼类的矿物质营养状况[43]。在尼罗罗非鱼,骨锌含量随着饲料锌含量增加而增加,在饲料锌含量达到99.7 mg/kg后趋于稳定[7]。在大菱鲆、斜带石斑鱼和露斯塔野鲮中,当饲料锌含量达到106.5 mg/kg[36]、71.9 mg/kg[31]和51.4 mg/kg[22]之后,全鱼锌与骨锌不再显著增加。然而在一些报道中,全鱼锌含量一直随饲料锌含量增加而增加,如虹鳟[30]、尼罗罗非鱼[21],可能是因为这些实验中设置的饲料最高锌含量较低(分别为107.6 mg/kg、100 mg/kg),不能满足实验鱼锌沉积量所需。在本实验中,全鱼锌和脊椎骨中锌也随着饲料中锌含量的增加而增加,并在饲料锌达到126 mg/kg后,其含量不再显著增加,这个结果远高于根据WGR和FCR确定的锌需求量。当饲料中锌含量满足最大生长后,过多的锌元素被存储在体内,特别是脊椎骨中,方便机体在需要锌时可以迅速获取。以全鱼锌和骨锌为评价指标,基于折线模型确定的大口黑鲈幼鱼对饲料中锌的适宜需求量分别为121.8和130.5 mg/kg。

    在本实验中,随着饲料中锌含量的增加,全鱼铁、骨铁、骨锰的含量则表现出下降的趋势。在许多研究中也有类似的报道:随着饲料中锌含量的增加,尼罗罗非鱼骨骼中铁含量[7]、露斯塔野鲮全鱼的铁含量[22]、黄颡鱼全鱼中铁、锰含量[13]、斜带石斑鱼骨骼中锰含量[31]均表现出下降的规律。这可能是因为锌和其他阳离子的理化性质相似,这些矿物质之间通常存在竞争作用,包括胃肠道吸收过程中的竞争性抑制[27]

4.   总结
  • 在本实验条件下,在含锌为24.8 mg/kg的半精制饲料中补充无机锌,可提高大口黑鲈幼鱼的生长性能、血清免疫能力与全鱼锌、骨锌的积累,但是高添加量的锌会降低铁、锰元素的积累。以增重率、饲料系数、全鱼锌和骨锌为指标,基于折线模型确定大口黑鲈幼鱼对饲料中锌的需求量分别为45.5、44.6、121.8和130.5 mg/kg干物质。

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