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The ontogenetic changes in the feeding habits of the abalone Haliotis discus hannai were elucidated for the first time in natural habitats using stable isotope analyses. Abalone individuals were grouped into three developmental stages: small juveniles (50 mm SL). The inferences of natural diets indicated that benthic diatoms, small red macroalgae and (or) juvenile brown macroalgae, and adult brown macroalgae were the primary food sources for small juveniles (6.5 ± 1.0 mm SL), large juveniles (23.7 ± 6.1 mm SL), and adults (81.8 ± 14.3 mm SL), respectively. The changes of δ13C in abalone were similar among three sampling stations and were explained by ontogenetic changes in feeding habits. The decrease of δ13C in abalone (≤20 mm SL) indicated a transition from diatom feeding to juvenile brown macroalgae and (or) small red macroalgae feeding, whereas the subsequent increase of δ13C (>20 mm SL) represented a feeding transition to adult brown macroalgae. These results prove the hypothesis of the ontogenetic changes in the feeding habits of the abalone H. discus hannai.

The Ezo abalone Haliotis discus hannai is the only species of abalone inhabiting the coastal waters affected by the cold Oyashio current in the Pacific waters of northern Honshu, Japan. Overwinter mortality of young-of-the-year (YOY) Ezo abalone was examined for 2000, 2001, and 2003 to 2006 year classes in Kadonohama Bay, northern Japan, to test the hypothesis that winter sea- water temperature controls recruitment fluctuation of this northernmost abalone and regulates their distribution limit. Eggs of Ezo abalone hatch between late summer and mid-autumn, and grow to a size of 2 to 8 mm in shell length by winter. Overwinter mortality rates varied among years (range: 0 to 97.4%), and there was a strong negative relationship between the instantaneous mortality rate and the winter temperature. High mortalities of YOY abalone were observed in February 2001 and March 2006, respectively. In both cases, the Oyashio current extended south to the study area, and the mean temperature fell to below 5°C. High mortalities were not observed in the other years when the mini- mum temperatures were higher than 6°C, as the Oyashio current did not extend to the study area. The differences in shell length between early winter and post-winter suggest that the low tempera- ture tolerance of YOY abalone may be size-dependent. Although not conclusive, the winter seawater temperature is an important factor affecting the survival of YOY and is the principal factor for the variation in the recruitment of Ezo abalone and the limit of their northward distribution.