Improvement of gonad quality of the sea urchin Mesocentrotus nudus fed the kelp Saccharina japonica during offshore cage culture

In northern Japan, Saccharina kelps are natural diets for sea urchins and are known to enhance sea urchin gonad size. However, little is known about the effects of the kelp diet on gonad quality and consistency among individuals. In the present study, Mesocentrotus nudus adults collected from a barren were cultured in a suspended cage from late February to early June 2014 in Shizugawa Bay, Miyagi Prefecture. They were fed stipes of Undaria pinnatifida for 56 d followed by fresh Saccharina japonica kelp for 42 d. At the end of the culture period, gonad indices, gonad hardness, gonad color (L* (lightness), a* (redness), b* (yellowness)), free amino acid (FAA) contents in the gonads, and sensory qualities of the cultured sea urchins (EC) were measured and compared to those of the natural population from the barren (EB) and those from the Eisenia bicyclis kelp bed (EK) in the bay. Gonad index and L* values of EC urchins were higher than those of urchins from the barren at the start of culture (SCB). Total color (L*, a*, b*) of the gonads of EC and EK urchins were similar. Gonad hardness of EC and EK urchins was similar and lower than that of EB urchins, and the coefficients of variation of gonad index, color, and hardness were lower than those from the barren (SCB and EB). Sweet tasting alanine contents of the gonads of EC urchins were markedly higher than those of the gonads from the other urchin specimens. Bitter tasting FAA contents of the EC urchin gonads were higher than those of EK and EB urchins, although arginine and lysine contents were lower than those of EB urchins. Sensory scores of overall taste of EC urchins were higher than those of EB urchins and similar to EK urchins, possibly due to the increased alanine content. These results suggest that gonad size, color, hardness, taste, and consistency among individuals of M. nudus collected from barrens can be improved to reach levels similar to those of urchins from the E. bicyclis kelp bed by cage culture with short-term feeding of Saccharina kelps. Changes in gonad qualities seemed to be mainly due to S. japonica as the food source because hard U. pinnatifida stipes were rarely eaten and had markedly low protein content. Extension of cage culture duration with S. japonica feeding from spring to summer would increase gonad size and further improve gonad quality.