Just before sunset on the night of the new moon (27 September 2011) Mike Emslie, Lyle Vail and Anne Hoggett visited a known spawning aggregation site for coral trout not far from the research station.
We were fortunate to observe a sizeable aggregation of the fish (Plectropomus leopardus) there engaged in spawning behaviour. Mike - a seasoned fish researcher - counted 39 individuals.
Males change colour to indicate spawning readiness - they become almost white with a distinct black outline, especially on the tail. The females, much smaller than the males, cluster at the site with many sitting on the sandy bottom and others swimming slowly just above. The males rove along about 50 metres of reef edge, trying to spawn with as many females as possible while keeping out rival males. Courting behaviour involves the male approaching an individual female on his side while doing a shimmy dance, inviting her to join him in a spawning rush.
As sunset approached, courting behaviour intensified and we watched as a single dominant male chased away all other contenders and shimmied countless times at females. We saw two successful spawning runs. When a female succumbs to the shimmy dance she swims very rapidly towards the surface with the male. At the apex of the rush, both fish emit a puff of spawn then swim back towards the bottom - where the male immediately shimmies up to another female.
What a treat to witness this behaviour right on the doorstep of the research station. The eggs fertilised that night will have formed pelagic larvae that will spend two to three weeks at sea before returning to the reef to settle. Recruitment of larval fish to the reef is a critical stage of a fish's life and the subject of much study at Lizard Island Research Station. The busy summer research season is about to begin.