Microdistribution and feeding dynamics of Coilia nasus (Engraulidae) larvae and juveniles in relation to the estuarine turbidity maximum of the macrotidal Chikugo River estuary, Ariake Sea, Japan

To evaluate the nursery value of the estuarine turbidity maximum (ETM) for the anadromous engraulid fish Coilia nasus, physical and biological samples were collected along the macrotidal Chikugo River estuary, southwestern Japan, from June 2005 to January 2006. Generally, eggs were distributed upstream of the ETM, whereas larvae were distributed in the ETM. The first cohort was washed out of the estuary by a large flood in early July. The second cohort appeared after the flood, gradually extending its distributional area through the estuary and growing to 100mm in standard length (SL) by December. Larvae and juveniles (20-100mm SL) fed selectively on copepods, specifically large calanoid species, whereas more developed juveniles fed on mysids. Prey organisms in guts were significantly more abundant and present in greater weights in the ETM (salinity <10) than in the lower estuary (salinity >10). The better feeding conditions were primarily attributable to consistently high densities of two large calanoid species, Sinocalanus sinensis and Pseudodiaptomus inopinus, in the ETM. In contrast, the poor feeding conditions in the lower estuary were likely due to a seasonal decrease in copepod density and the dominance of small copepods such as Oithona spp. Given that better feeding conditions contribute positively to the growth and survival of larvae and juveniles, we conclude that the ETM has a significantly higher nursery value for C.nasus compared with other habitats in the Chikugo River estuary.