A population study of giant clams (Tridacninae) on the Great Barrier Reef over three-decades

ABSTRACT Four giant clam species were monitored for population abundance, recruitment, growth and spatial distribution over three decades. Natural, undisturbed populations were mapped at five sites on the Great Barrier Reef from surveys in 1982–1986, 1987–1992, 2007–2009 and 2017. Populations of all species declined at three sites due to low recruitment and death of old clams. At two sites a significant juvenile recruitment cohort of Tridacna gigas and T. derasa followed a mass mortality event which returned the population to the pre-mass mortality abundance. At one site 55.5% of the cohort survived after five years. Population abundance of T. squamosa and Hippopus hippopus increased slightly at the 2017 survey at these two sites. Tridacna gigas and T. derasa live long enough to reproduce successfully in some years, as evidenced by recruitment. Where significant recruitment occurred, high-aggregation reproductive centres may remain functional for many decades. Individual growth rate from the recruitment surveys averaged six cm yr−1 to five years. Longer-term growth rates averaged nearly two cm yr−1 with the highest rate being over four cm yr−1. Climate change will have some deleterious effects on giant clam populations.