We have only sporadic knowledge of how the Kermadec Islands marine environment varies with the seasons. Due to the logistic difficulties of getting to the islands and spending time here, expeditions typically only visit for a few weeks at a time, and have not witnessed underwater seasonal transitions. Nonetheless, with each expedition we gather another piece of the puzzle.
It seems the algae we are seeing around us may be a seasonal treat. Tom and I (Libby) have only visited outside of this period of time. However, our colleague, Clinton Duffy from the Department of Conservation recalls seeing the same lush algae fields when he visited in November 2004. Lengthening days, warming water and settled weather over this period of the year may lead to this phenomenon every year.
What does the ‘algae season’ mean for the underwater residents of Raoul Island? Tom has noted hoards of juvenile fishes using the algae for shelter from the larger mouths on the reef. Several scientists in the past, including myself, have also remarked on the high diversity and abundance of urchins around Raoul – which was thought surprising given the lack of algae. The seasonal growth of algae we are witnessing at present may help explain the presence of these urchins.
In our surveys we have found six different species of urchin. In particular there are three dominant species: Centrosephanus rodgersii, Heliocidarus tuberculata, and Tripneustes gratilla. These three species vary in abundance from place to place, but are often intermixed, without a clear indication that one species dominates a certain type of reef or a certain depth. This has intrigued us, so we thought we might try to gain some insight into the ‘turf wars’ of the urchins. Urchins don’t move fast, so to capture their interaction we have set up cameras to record them from above (thanks to Arie Spyksma kindly loaning us his quadpods!). Their movement patterns are both comical and informative. We have established that T. gratilla moves and likely grazes during the day, whereas the others do not. And, last night, we established that once the light has faded, the other two species do move, and they move incredibly rapidly! The plot thickens. Will one species outcompete the others in time and become the dominant urchin species of the Kermadec Islands, like our own mainland kina?
Last night was also filled with activity for the Natural History New Zealand film crew. They were out diving Howard’s rock in the middle of the night, catching up on what the fishes get up to in the hours of darkness. By all accounts, life on the reef does not slow down in the dark (or in a raging current) – gropers were seen socialising and hunting, kingfish feeding, and whales were still frolicking alongside the divers and boat. We’re looking forward to seeing their full accounts in the next series of Our Big Blue Backyard.