| The breeding scale of hybrid snakehead (Channa argus♂×Channa maculate♀)waslarger and larger in south of China. Three important factors witch lead to low survivalrate was studied, including prey density, stocking density and cannibalism. Theexperimental results were as follows:1. The effects of prey density (0.1, 0.3, 1, 3 and 10 ind/mL) on growth and survivalin different body sizes (0.68, 1.50, 3.20 cm) of hybrid snakehead larval were investigated.Growth of three sizes hybrid snakehead larvae increased with prey density from 0.1 to 1ind/mL. The specific growth rate (SGR) was the highest at 1 ind/mL. When prey densitywas higher than 1 ind/mL, SGR of sizeⅠlarvae decreased, while no significant(P＜0.05)changes were observed in those of size-Ⅱand size-Ⅲ. Prey densities did not significantlyaffect survival rate, and survival rate was all high in different size groups. The differenceof body size was not sensitive to prey density. The suitable prey density was 1 ind/mL inall treatments.2. Hybrid snakehead Larval were stocked and reared in an outdoor tank at a rate of0.5, 1, 2, 4 and 8 ind/L for 15 days. The results showed that final weight SGR and dayweight gain of snakehead larvae improved with prey density increased from 0.5 ind/L to4ind/L, reached the highest at the stocking density 4 ind/L. The SGR decreased withstocking density increased when SGR reach the highest. The larval in high density groups(200 and 400 ind/m~2) had great variation in size and growth. The survival rate decreasedwith stocking density increased.3. A 60 day experiment was conducted to test the effects of stocking densities (25,50, 100 and 200 ind/m~3) on growth, survival, feed conversion rate, muscle component andpart of immunity function in hybrid snakehead juveniles (26.51g) in twelve 60cmdiameter concrete baskets with volume of 200 liters. The results showed that the stockingdensity of hybrid snakehead juveniles had significant effect on growth. The final weightSGR and DWG of hybrid snakehead juveniles increase with stocking densities from 25ind/m~3 to 100 ind/m~3, reach the highest at the stocking density 100 ind/m~3, decreasedwhen stocking density were the 200ind/m~3. Feeding rate decreased with increase instocking densities, but not significantly. Feed conversion rate increased along withstocking density increasing. There was significant difference between low stockingdensity and high stocking density. Stocking density had no significant effects on cortisol,organ weight index and muscle component. The survival rate was 100 % in all stocking density groups.4. The study was conducted with three sizes (2.21, 2.72, 4.10 cm) of larval hybridsnakehead in different treatments under full diet and no diet. The results showed that therewas almost no cannibalism happening when food supply was enough. Cannibalism of thehybrid snakehead larvae was most serious when total length reached 2.21 cm after 4 dfood deprivation. No obvious cannibalism of fish at larger body size (2.72 cm and 4.10cm) was observed. Size differences when the body length ratio of smaller fish to largerones increased to 0.49:1 may lead to serious cannibalism. It is suggested that insufficientdiet and size differences is a main reason causing the cannibalism among hybridsnakehead larvae; high stocking density can induce and stimulate the cannibalism.
|
PDF Full Text Request