Effects of different substrata on the growth rate of early juvenile Trochus niloticus (Mollusca: Gastropoda)

Juvenile trochus, Trochus niloticus, were reared for 72 days in three different substrata (fibreglass plate, shell grit and pieces of coral) which were prepared in a similar way and inoculated with the same algal species Nitzschia sp. Feeding ad libitum, the best growth rate (101.603 +/- 9.851 mu m/day) was recorded for juveniles growing on shell grits (SG), slightly lower growth (99.949 +/- 6.702 mu m/day) was recorded for those growing on coral pieces (CP) and significantly poorer growth (81.489 +/- 10.646 mu m/day) was noted on juveniles reared on fibreglass plates (FRP). The growth rate of juveniles using FRP as substrata was significantly lower (P < 0.05) from those juveniles grown on either SG or CP. Differences were also recorded in the survival rates of juveniles grown on the three different substrata. Survival rate was highest (66 +/- 7.416%) for the juveniles grown on SG, slightly lower (65 +/- 6.124%) for those grown on CP and significantly lower (45 +/- 17.678%) for those grown on FRP substrate. The survival rate of juveniles grown on FRP as substrata was significantly lower (P < 0.05) from those juveniles grown on SG and CP. Based on the results of this study, the hatchery system developed at the Northern Territory University has chosen an ap propriate substrate for the biological filter of the closed recirculating system.