Morphology and microchemistry of abnormal otoliths in the ayu, Plecoglossus altivelis

Abstract

The sagittal otolith morphology and microchemistry of reared juvenile ayu, Plecoglossus altivelis, were examined to describe the occurrence and microchemical characteristics of the abnormal otoliths in this species. Juvenile ayu (N = 31) were collected in June 2004 at three different locations, Wakayama, Kumamoto, and Biwa Lake in Japan, where they were being reared in freshwater aquaculture ponds after having been collected in the wild as larvae. Otolith abnormality was found in the sagittae of 26% (N = 8) of the individuals examined, of which five fish had abnormal otoliths only on one side, while the otolith on the other side was normal. Abnormal otoliths were more transparent and crystalline in appearance with irregular shapes compared to normal ones that were more opaque and less irregular. Abnormal otoliths were divided into two types, semi-abnormal (Type 1) with a normal part in the center, and fully-abnormal (Type 2) that were completely crystalline in appearance. The line transects and whole otolith concentration maps showed that the contents of Sr, Na and K were lower in the abnormal otolith regions compared to the normal ones, while those of Ca and S were almost constant in both. The appearance and microchemical properties of the abnormal ayu otoliths were similar to the abnormal otoliths in other species in which vaterite replaces the aragonite. Abnormal formation of otoliths occurred in ayu from Biwa Lake (30%) and Kumamoto (45%), while the Wakayama samples had no abnormality. The microchemical analyses of the normal and abnormal otoliths indicated that some abnormal otoliths had formed before the fish were captured and transferred to the hatchery, so the possible causes of otolith abnormality in ayu are discussed.