POPULATION STUDIES OF JUVENILE TROCHUS-NILOTICUS ON A REEF FLAT ON THE NORTH-EASTERN QUEENSLAND COAST, AUSTRALIA

Transects and quadrats were used to sample the distribution and densit y of Trochus niloticus, recruitment, growth rates of juveniles, and ch aracteristics of the microhabitat, on an intertidal reef flat at Orphe us Island between 1993 and 1995. Individuals between 1.5 and 62 mm she ll width (SW) were found in all parts of the reef flat. Their density averaged 0.178 and 0.115 individuals m(-2) in 1993 and 1994, respectiv ely. Juveniles were found in groups of 2-4 individuals m(-2) more freq uently than would occur by chance, but denser aggregations were rare. As juveniles increased in size, there was a change in microhabitat: fr om small rubble (< 10 mm) to larger rocks and coral bench and from sha llow pools (< 10 mm deep) to deeper pools. The size-frequency distribu tion was bimodal during parts of the year, suggesting seasonality in r ecruitment. Growth rates ranged between 2.3 and 2.6 mm SW month(-1). I n relation to the seeding of cultured juveniles for population enhance ment, the results indicate that seeding densities should be < 5 indivi duals m(-2). Shallow areas with abundant small rubble may represent fa vourable conditions for individuals of < 15 mm SW, but deeper areas wi th larger rocks are better for individuals of greater than or equal to 15 mm SW.