Comparison of larval distribution in two clupeoid fishes (Ilisha elongata and Sardinella zunasi) in the inner estuaries of Ariake Sound, Shimabara Bay, Japan

: In early life history, the clupeoid fishes, Ilisha elongata (Pristigasteridae) and Sardinella zunasi (Clupei-dae) have similar spawning seasons and nurseries from spring to summer in the Ariake Sound, Shimabara Bay, Japan. Surveys conducted using larva nets and a beam trawl in July showed that the horizontal egg and larva distributions of I. elongata and S. zunasi both overlapped considerably in the inner estuaries; however, I. elongata settled in the estuaries while S. zunasi retained a pelagic distribution. A comparison of the vertical distributions of the two species in the estuary showed that I. elongata larvae were dispersed from the surface to near-bottom layers during the flood tide, and moved downwards to avoid being swept out of the estuary after the high tide, while S. zunasi larvae remained spread throughout the water column in all tidal phases. The findings suggest the existence of niche segregation during the pe lagic larval period in the two species. Thus, by examining differences in larval mouth sizes, which are much larger in I. elongata than in S. zunasi, it appears that prey preference varies between the two species, prey specialization occurs, and there is no niche competition between the two species.