Today was the culmination of months of planning and preparation. Acting on reports from previous expeditions of reef mantas at Egmont Athol we set off to tag them and – for my part – use an echosounder to map the distribution of their plankton prey.
With the help of the Pacific Marlin’s engineers, we installed our 120 kHz echosounder on the fast rescue craft (a rigid inflatable boat). We sailed from the Pacific Marlin at 08:00 and picked our way through a channel on the north side of the atoll in to the lagoon.
We searched along wind lanes, which are surface indications of upwelling and downwelling zones that aggregate plankton, and hence feeding zones for filter-feeding mantas. Our first sighting came after about an hour of searching: we saw in the distance a manta leaping clear of the water – breaching. This was an exciting announcement to us that the previous reports of regular manta presence were well-founded!
We headed towards the splash and soon saw white flashes near the surface caused by several mantas barrel rolling. This is a type of feeding behaviour that mantas use to keep tightly within dense aggregations of plankton. Over the remainder of the day we managed to tag 5 animals with position-recording tags. These tags will transmit the mantas’ positions over satellite, enabling scientists to track their movements. We observed the mantas in the water, and saw groups of up to 11 at a time.
The mantas appeared to be cruising up and down the outside of the reef, feeding from the very surface down to depths where they disappeared from view – well beyond 20 m down. We ran a series of echosounder survey lines from the reef flat out in to the deeper water and detected a strong sound scattering layer in the region where the mantas appeared to be feeding. This was probably comprised of zooplankton including copepods and euphausids, a common food of mantas. We left the area at around 17:00 tired, sunbeaten but satisfied that our efforts had paid off, and happy in the knowledge that we had been privileged to observe such a natural spectacle.
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