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.

KEYWORDS:

• Climate change
• growth
• Hippopus hippopus
• recruitment
• spatial distribution
• Tridacna gigas
• Tridacna derasa
• Tridacna squamosa

Acknowledgements

I thank the Passions of Paradise for trips to Michaelmas Cay, the Undersea Explorer (no longer operating) for trips to Rachel Carson Reef and to Lizard Island, and to the Lizard Island Research Station for accommodation during surveys. I thank Dr. Andrew Lewis for major field work with me in the 2007–2009 and 2017 surveys and photos of clams. I thank Dr Dave Williamson and Mr. Niklas Taylor for some assistance with the surveys and to Dr. Dani Ceccarelli for helpful advice with preparing the graphs. Thanks to Dr. Leon Zann, Dr. Christine Crawford and Mr. Warwick Nash for their helpful comments on drafts of the paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).