Effect of three photoperiod regimes on the growth and mortality of the Japanese abalone Haliotis discus hannai Ino

Feeding activity of the Japanese abalone Haliotis discus hannai is clearly diurnal, with ail increase in feeding between the onset of darkness and midnight. The feeding rate increases slowly in the afternoon, approaching sunset and becomes constant between Sunset and midnight. The abalone remain within shaded refuges during the day, and only emerge from them at night. The present study evaluated some effects of varying the photoperiod on a group of these abalone to determine its potential effects on their feeding behavior. Three groups of juvenile abalone with an initial mean length near 14 mm were maintained in raceway cage culture systems with a continuous flow of fresh seawater and abundant food where each group was exposed to different photoperiods. Growth of the abalone in these systems was monitored for a period of 106 days. Treatment A, considered "natural," included 12 h of light and 12 h of darkness. In Treatment 13, the abalone were maintained in complete darkness, and Treatment C included four h of light and 20 h of darkness. The results showed that juveniles cultured in total darkness demonstrated some retardation in growth early in the experiment compared with abalone in a natural light regimen, and those with a partial light regimen. After 43 days, the abalone in the partial light regimen reached and surpassed sizes of the abalone maintained in the natural light regimen and in total darkness, although their feeding activity and growth patterns were erratic compared with abalone in the normal light and total darkness regimes. The results of the experiment were inconclusive as to which light regimen was most favorable when considering the growth patterns and comparative mortality rates within the systems.