Oogenesis in the common Japanese conger Conger myriaster

ABSTRACT:    We investigated the process and characteristics of oogenesis in the common Japanese conger Conger myriaster . Young fish caught in November 1996 were reared for use in this experiment. Fish were sampled monthly from December 1997 to August 1998. Some were injected with human chorionic gonadotropin to stimulate ovarian maturation from May to August 1998. Oocytes from the chromatin nucleolus stage to the secondary yolk globule stage were obtained from non-hormone-treated fish; those of more advanced stages were obtained from hormone-treated ones. We divided oocyte development into eight stages from the chromatin nucleolus stage to the maturation stage. The yolk vesicle stage was not separated because yolk vesicles began to appear just after appearance of yolk globules. Oocyte, oil droplet, yolk globule and nucleus diameters all increased concomitant with oocyte development. Oil droplet and yolk globule diameters increased remarkably at the maturation stage. However, zona radiata thickness peaked at the secondary yolk globule stage, decreasing gradually thereafter. Increased gonadosomatic index was related to oocyte development as found in European and Japanese eels receiving hormone treatment to mature. The present study is the first report describing oogenesis characteristics in congrid eels. It indicates that oogenesis is almost identical to that of other anguillid eels.     

Relationship between feed input and sediment quality of the fish cage farms

ABSTRACT: A study was conducted to examine the variation in fish farm sediment quality with varying organic carbon input in the form of feed. In order to elucidate the seasonal variation in farm sediment quality, relation between seasonal organic carbon input, temperature and sediment quality was studied. The sediment ignition loss, redox potential and acid volatile sulfide sulfur (AVS-S) content showed a correlation with annual organic carbon input. The seasonal AVS-S content showed a significant correlation with the seasonal organic carbon input as well as temperature. Multiple regression analysis revealed that both of these factors explicated 80% of the seasonal variation in AVS-S content. Although redox conditions of the sediment could also be correlated with both the organic carbon input and temperature, the latter seemed to have a greater control. Thus, the farm sediment quality could be directly related to the annual organic carbon input to the fish farm under similar hydrographic conditions. Acid volatile sulfide sulfur content exhibited a significant correlation with both the annual as well as the seasonal organic carbon input. Therefore, AVS-S content proved to be a fairly simple and better indicator for monitoring the fish farm environment for effective farm management and sustainable aquaculture. Importance of size of pellets as a factor affecting the feed input-sediment quality relation is also demonstrated in the present study.     

Development of embryos and larvae in the common Japanese conger Conger myriaster

Development of embryos and larvae in the common Japanese conger Conger myriaster was observed after artificial fertilization. Eggs were obtained from females matured artificially by hormone injections and milt was obtained from males matured naturally. Fertilized eggs were kept in seawater at 12–14°C. The first cleavage occurred at 4 h, epiboly began at 24 h, the embryonic body was formed at 38 h and hatching occurred at 84 h after insemination. Newly hatched larvae were approximately 2.5 mm (total length) and similar to those of Anguilla japonica in terms of external features. The mouth and anus opened on the 7th day after hatching. Pigments began to appear at the tip of the tail on the 10th day. The total length of the larvae reached approximately 8 mm on the 11th day. Eye pigmentation began on the 14th day. One larva lived for 19 days without food.