Overwinter mortality of young-of-the-year Ezo abalone in relation to seawater temperature on the North Pacific coast of Japan

Abstract

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.