Growth of the longline-cultured sea squirt Halocynthia roretzi in a temperate bay of Korea: Biochemical composition and physiological energetics

Abstract

The broad temperature range on the temperate coast of Korea has led fishermen to develop a unique and specialized procedure for the longline culture of ascidians. In Korea, warming of the coastal sea in winter has accelerated over the past few decades. After warmer winters, the rising temperatures of earlier springs have precluded the rearing of the ascidian spat in the culturing area and have imposed spring spat cultivation in colder-water nursery grounds. To examine the seasonal dynamics of energy reserves and the physiological strategies to optimize energy balance for the growth in the cultured sea squirt Halocynthia roretzi, its gross biochemical composition and physiological energetics were investigated monthly over two culturing periods from July 2005 to January 2006 and July 2013 to January 2014 in Geoje-Hansan Bay on the south coast of Korea. No indicators of the growth performance of sea squirts showed differences between the two culture practices (2005–2006 and 2013–2014) carried out using spat reared in different localities, i.e., the grow-out area and the colder nursery grounds. The seasonal patterns in accumulation and utilization of biochemical constituents in the sea squirt tissues were similar between the two periods. During the culturing period in the grow-out area, sea squirts retained physiological functions across the temperature range. Food energy acquisition and metabolic cost were positively related to water temperatures in the field conditions, probably due to the low and narrow range of suspended particulate matter (SPM) concentrations. A less-clear seasonal variability in food consumption rates yielded a seasonal discrepancy in scope for growth (SFG; i.e., negative during summer vs positive during autumn–winter). Consequently, the tissue weight and protein reserves of sea squirts varied concomitantly with the seasonal changes in SFG, supporting a fast growth during autumn–winter. Our results suggest that the spat reared in colder nursery grounds are suitable for cultivating sea squirts in the traditional grow-out area and support their sustainable culturing performance as an adaptation strategy to the winter–spring warming conditions that are unique to coastal seas. © 2019