Tagging And Releasing Techniques Of Ctenopharyngodon Idellus Juveniles In Dongjiang River

The results of this paper showed that the grass carps(Ctenopharyngodon idellus) at body length of 4.5-6cm were too small to tag, because they were subjected to infection and died of diseases, while the larger grass carps at body length of 11-17cm were suitable to be tagged with elliptic plastic tags (8mm×5mm×1mm) with brass wires at the diameter of 0.14mm, which drilled through the interstice between the first and the second dorsal fin pterygiophore. The tag in front of the dorsal fin base had no influence on swimming, feeding and other behaviors for the tagged grass carp. Two methods (marked with a plastic tag and clipping a vental fin) were employed in the tagging experiment. The tagged grass carps needed to be recovered from tagging trauma for about 10 days in a pool, and the tag retention rate was 82.4% for the individuals marked with plastic tag and 100% for ones tagged with clipping ventral fin during the period of recovery culture. Then the tagged grass carps were reared in outdoor pond for 70 days, the survival ratio was 91.5% for the grass carps marked with plastic tag and 83.6% for ones tagged with clipping ventral fin, the tag retention rate was 79.9% for tagged grass carps and just 19.3% for clipped ventral fin individuals because of regeneration of the clipped ventral fins. It was about two weeks for the gass carps of feeding artificial feed acclimate to feeding grasses.The grass carps with average body length of 14.8cm and average body weight of 65.9g were released in Dongjiang River. The methods of tagging grass carp, tagging trauma treatment, feeding habits acclimatization were identical to those mentioned above. The total number of released grass carps was one hundred thousand, the number of marked grass carps was forty thousand. The percentage of tagged grass carps was 40%. The total number of recaptured grass carps during the period of more than a month after releasing was 881, there were 278 tagged grass carps, the percentage of tagged grass carps recaptured account for the total recapture grass carps was 31.6%, the recapture rate was 0.695%. The food residues in the digestive tube of recaptured grass carps were residues of common aquatic plants including hyacinth roots when compared with the aquatic plants in Linjiang section of Dongjiang River with dissection microscopy, the food residues also contained some algae. These results demonstrated that released grass carps were completely acclimated in the river environment for more than a month after releasing. The results also demonstrated that tagging and releasing techniques which were established in this study were effective, applicable and practical. The points of the techniques were as follows:(1) The tagged grass carps with a body length range of 11-17cm had higher survival ratio, this size grass carps were suitable to be tagged with plastic tags, the optimal site marked was the interstice between the first and the second dorsal fin pterygiophore. (2) The tagged grass carps needed a 10-day tagging trauma treatment. (3) Before released to the river the gass carps of feeding artificial feed needed 2 weeks to acclimate to feeding grasses. (4) The optimal releasing site was at the river section with abounding aqua-plants which could meet the needs of released grass carps.