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 density of Trochus niloticus, recruitment, growth rates of juveniles, and characteristics of the microhabitat, on an intertidal reef flat at Orpheus Island between 1993 and 1995. Individuals between 1.5 and 62 mm shell 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, respectively. Juveniles were found in groups of 2-4 individuals m(-2) more frequently than would occur by chance, but denser aggregations were rare. As juveniles increased in size, there was a change in microhabitat: from small rubble (< 10 mm) to larger rocks and coral bench and from shallow pools (< 10 mm deep) to deeper pools. The size-frequency distribution was bimodal during parts of the year, suggesting seasonality in recruitment. Growth rates ranged between 2.3 and 2.6 mm SW month(-1). In relation to the seeding of cultured juveniles for population enhancement, the results indicate that seeding densities should be < 5 individuals m(-2). Shallow areas with abundant small rubble may represent favourable conditions for individuals of < 15 mm SW, but deeper areas with larger rocks are better for individuals of greater than or equal to 15 mm SW.