The effect of acute salinity change on white spot syndrome (WSS) outbreaks in Fenneropenaeus chinensis

In order to observe the effect of salinity on disease resistance and white spot syndrome virus (WSSV) proliferation in Fenneropenaeus chinensis, shrimps with latent WSSV were subjected to two acute salinity changes from the original salinity of 22 ppt to 18 and 14 ppt in an hour, respectively. The total haemocyte count (THC) of the challenged group showed no evident change under salinity adjustments, but the phenoloxidase (PO) index declined significantly (P<0.05) corresponding to continuing acute salinity changes from the 24th to the 72nd hour. According to the WSSV load detected by quantitative real-time PCR method, it was found that WSSV carried by the challenged group and control group were significantly different (P<0.05); acute salinity change from 22 to 14 ppt led to the WSSV carried in the challenged group being significantly higher (P<0.05) than that of those surviving in 22 ppt, but salinity change from 22 to 18 ppt had no such effect. At the end of the 72-h experiment, the challenged group subjected to salinity change from 22 to 14 ppt had nearly 3 times the WSSV load as the control group with no salinity change. Therefore, salinity changes over a particular range could result in a decrease of immunocompetence and obvious WSSV proliferation in the shrimps, leading to white spot syndrome developing from a latent infection to an acute outbreak. (C) 2005 Elsevier B.V All rights reserved.

In situ study on effect of food competition on diet shifts and growth of silver and bighead carps in large biomanipulation fish pens in Meiliang Bay, Lake Taihu

Silver and bighead carps were cultured in large fish pens to reduce the risks of cyanobacterial bloom outbreaks in Meiliang Bay, Lake Tauhu in 2004 and 2005. Diet compositions and growth rates of the carps were studied from April to November each year. Both carp species fed mainly on zooplankton (> 50% in diet) in 2004 when competition was low, but selected more phytoplankton in 2005 when competition was high. Silver carp had a broader diet breadth than did bighead carp. Higher densities and fewer food resources increased diet breadths but decreased the diet overlap in both types of carps. It can be predicted that silver and bighead carps would be released from diet competition and shift to feed mainly on zooplankton at low densities, decreasing the efficiency of controlling cyanobacterial blooms. Conclusively, when silver and bighead carps are used to control cyanobacterial blooms, a sufficiently high stocking density is very important for a successful practice.

长江口上升流区营养盐动力学

近年来，东中国海赤潮灾情严重，且74.7%的赤潮集中在 30°30′～32°00′N、122°15′～123°10′E的“赤潮高发区”。在研究该区赤潮成因时，长江口沿岸上升流的影响越来越受到人们的关注，并被一些专家观测和研究。但目前为止，针对该区营养盐动力学特征及其对叶绿素a影响的研究较少，且不系统。 本文根据2004年四个季度月的调查资料，系统地探讨了长江口上升流区营养盐动力学特征；估算了上升流的营养盐通量，并和陆源输入通量进行了比较。初步探讨了上升流对该区营养盐结构和浮游植物生长的影响。为深入研究长江口富营养化和赤潮形成机制提供了参考。 结果表明春季在122°20′～123°00′E，31°00′～32°00′N以北海域存在低温、高盐、低溶解氧的沿岸上升流。它不但可把底层高含量磷酸盐输送到10m层以上海区，而且还为上层海区输入了相对低含量硝酸盐和硅酸盐，从而改善了上层营养盐结构，使得营养盐比值接近Redfield Ratios，同时还改善了上层的透明度；从而有利于浮游植物的繁殖。夏季上升流受到强大的长江冲淡水压制，表现不如春季明显，主体水团出现在122°20′～123°00′E，31°15′～31°50′N海区10m层以下。 在秋、冬季，上升流现象被更强的对流现象所掩盖，表现为台湾暖流表层水的入侵。表、底层水域不仅温、盐度分布十分接近，而且营养盐结构差异也较小。冬季台湾暖流水中的磷酸盐含量远比秋季高，与春、夏季上升流水团中磷酸盐含量接近。硝酸盐和硅酸盐含量比秋季稍高，比春、夏季上升流水团中的含量稍低。 叶绿素a季节性分布表明，在春、夏季的10m层以下水域，叶绿素a受到透明度限制，含量相差不大；而在表层和10m层之间，春季叶绿素a的含量远高于夏季，说明春季的营养盐结构和自然条件更有利于浮游植物的繁荣生长。在秋季台湾暖流水影响的区域，表层叶绿素a含量较夏季稍低。而冬季该区叶绿素a含量则是最低的。 对长江口上升流水团春季营养盐通量的计算结果表明，上升流水团中磷酸盐输送通量远远高于长江径流输入，是其径流通量的两倍以上，可能会成为影响该海区磷酸盐分布以及浮游植物生长的一个值得关注的因素。 关键词：上升流，营养盐动力学，营养盐结构，叶绿素a，长江冲淡水

Studies on the transmission of WSSV (white spot syndrome virus) in juvenile Marsupenaeus japonicus via marine microalgae

We studied the possible role that marine microalgae may play during the outbreaks of WSS (white spot syndrome). In order to elucidate the possibility of marine microalgae carrying WSSV (white spot syndrome virus), six marine microallgae (Isochr.vsis galbana, Skeletonema costatum, Chlorella sp., Heterosigma akashiwo, Scrippsiella trochoidea, Dunaliella salina) were co-cultured with adult Marsupenaeus japollicus infected with WSSV and were assayed daily by nested-PCR to study whether they could carry WSSV. Further experiments were conducted to investigate whether the virus carried by microalgae could re-infect juvenile M. japonicus. Results showed that all of the experimental microalgae, except H. akashiwo could carry WSSV, and among them, Chlorella sp. and S. trochoidea had the strongest WSSV-carrying ability. Unlike other invertebrate carriers of WSSV, the WSSV detections in microalgae, which were positive after I and 3 days, were negative after 10 days of incubation. WSSV detection results in juvenile M. japonicus showed that the juvenile shrimp were re-infected by co-cultured Chlorella sp., although the juvenile M. japonicus carried so small an amount of WSSV that it could only be detected by nested-PCR. The results of this experiment suggest that microalgae might be one possible horizontal transmission pathway for WSSV. Further research, however, is required to better understand the factors behind the different carrying abilities and virus-carrying mechanisms of different microalgae. (c) 2007 Elsevier Inc. All rights reserved.