Mission Blog: September 23, 2009
Opihi surveying at Puhahonu
By Christopher Bird, PhD, Scientist
Frosting on Puhahonu (Photo: NOAA)
The protection of marine life in the Papahānaumokuākea National Marine Monument affords the unique opportunity to observe fisheries species that are isolated and completely protected from harvest. This allows us to gauge what impact humans are having on the targeted species in the main Hawaiian Islands.
Puhahonu, or Gardner Pinnacles, is the last and most remote outpost for opihi in the Hawaiian Islands. Opihi are limpets, or snails with a cone shaped shell, that inhabit the wave-washed rocky shores of Hawaii. Opihi are harvested for subsistence, commercially, and recreationally. All three species (makaiauli, alinalina, and koele) are endemic to the Hawaiian Archipelago, but we had only seen makaiauli and alinalina on Mokumanamana and Nihoa. Our goals for Puhahonu were to identify which opihi species are present, how abundant those species are, and measure their average size.
Opihi Photo Quad (Photo: NOAA)
As we speed towards Puhahonu on the jet boat, it becomes readily apparent that this is a small island - approximately the size of Moku iki off the coast of Lanikai. This pinnacle, composed of solid bluish basaltic rock is what remains of a massive island which was once the size of the Big Island. There is a little bit of swell from the North, so we decide to circle the island in an attempt to find some good locations to access the intertidal zone. Puhahonu is frosted by seabird guano, which gives way to bare basalt where the icing has been washed away by large Northwest swells. Nearer the water, there is a halo of densely packed whitish-tan opihi ringing the island, immediately above the sharpest boundary of crustose coralline algae (pink rock) I have seen in the Hawaiian Islands. Below this halo of makaiauli is normally where you would find alinalina and haukeuke (the helmet urchin), but they are conspicuously absent. Instead of well manicured bright pinkish-peach color, there is a brownish fuzz of algae covering the crusts, presumably due to the obvious lack of grazers.
Upon the location of a suitable access point, Scott maneuvers us close to the shore where Matt Ramsey and I jump into the water. The waves never look as powerful from afar as when you are swimming in them. As we try to latch onto the island, the waves move us down the shore and there is a powerful backwash that pulls us back into the water. Within a couple minutes, Matt and I find good hand-holds and make our way out of the water and into the intertidal zone.
Zonation (Photo: NOAA)
We have already satisfied goal one, the only species of opihi residing on Puhahonu is makaiauli. Now we conduct our survey of opihi density and size. Every single opihi is tannish-white, most have eroded shells where the ridges are no longer visible, and most are very tall and pointy relative to those in the main Hawaiian Islands. We run 14 transects from land into the water, starting at the first opihi and ending at the last opihi. The shore is very steep, but there is little danger because there is 25 feet of soft water below us if we lose our perch. We record and measure each opihi lying on the transects, occasionally dodging waves.
There is a good range in opihi sizes from 3 to 7 cm long, with most being about 5cm, which is larger than average in the main Hawaiian Islands. Interestingly, there are very few other intertidal snails on Puhahonu, save for the tube snails inhabiting the crustose coralline algae. Overall, there is a very low diversity of animals on this rock in the middle of the ocean, with a'ama crabs being the other abundant animal.
Puhahonu (Photo: NOAA)
Upon the completion of our survey, Matt and I swim back to the jet boat. We had identified another access point, but two monk seals have hauled themselves out in the very same spot and have no intentions of leaving. It will have to wait for another trip, because we are headed to Maro Reef in a couple hours. While we have to analyze the data, it seems that the opihi on Puhahonu are bigger and more abundant than on the main Hawaiian Islands. However, this tiny remote island does not harbor the diversity of intertidal life we are used to seeing on the larger Hawaiian islands.