鳀鱼仔鱼饥饿试验及不可逆点的确定

Experimental starvation on Engraulis japonicus larvae and definition of the point of no return

2000年6月26日8-10时在35°00′N、121°00′E周围海域利用大型浮游生物网采集了大量鳀鱼的天然受精卵,11时受精卵开始孵化,12时收集初孵仔鱼进行饥饿试验。试验结果表明:在培养水温为23.0～24.8℃条件下,鳀鱼仔鱼的混合营养期仅1d多时间;仔鱼的不可逆点发生在孵化后第6天。鳀鱼后期仔鱼发育阶段都具有胸角这一形态特征,其胸角不能作为鉴别健康仔鱼和饥饿仔鱼的依据。这一发现补充和完善了鱼类学上关于仔鱼胸角的理论。

A great many of natural anchovy (Engraulis japonicus) fertilized eggs were collected with a zooplankton trawl near the 35°00′N, 121°00′E sea waters 8 to 10 a. m. on June 26, 2000. The fertilized eggs began to hatch at 11 a.m. and the newly hatched larvae were collected for starvation experiment at 12 a.m. As observed, at the water temperature of 23.0-24.8℃, anchovy larvae started to feed in the 2nd day after hatching, their yolk sacs faded away in the 3rd day, and their mixed feeding stage was merely a bit more than 24 hours. The mean growth rate of larvae body length was 0.44 mm·d^(-1) in the feeding test. The initial feeding rates in the starvation test were increased day by day from the 2nd day to the 4th day after hatching, reaching 60.00% in the 4th day, hereafter decreased gradually to 33.33% in the 6th day. Until the 8th day, all the anchovy larvae died. Obviously, the PNR (the point of no retum) appeared at the 6th day after hatching. Therefore, the anchovy larvae were just able to survive for 2days after reaching the PNR. In the 4th day after hatching, both fed and unfed anchovy larvae began to present the body feature of the pectoral angle. But, the pectoral angles of the unfed and the PNR stage larvae were more evident and sharper than those of the fed larvae. According to our observation on the larvae collected from the spawning ground, the pectoral angles of the larvae with a total length ranging from 3.8 to 7.8 mm were visible. However, the pectoral angles of the larvae with a total length ranging from 8.4 to 9.4 mm waned. When the ontogeny entered the juvenile stage, the pectoral angles of the larvae with a total length of 10.8 mm vanished. Since the anchovy larvae in post-larvae stage were characterized with the pectoral angles, the pectoral angle could not be regarded as the basis to distinguish healthy or starving larvae. This conclusion may supplement and perfect the theory about larvae pectoral angle in ichthyology.