REVISITING POHIKI POND WITH JOHN CHAN
In June of 1980, as part of the published proceedings of the Third Conference in Natural Sciences, a paper by John G. Chan and Rodney Fujii of the Department of Biology of the University of Hawaii at Hilo described the biology of a geothermally heated brackish water pool at Pohiki, Hawaii. Having had the opportunity to review the paper, I quickly accepted an invitation to accompany John in revisiting the pool, which is in the less visited eastern corner of the Big Island in a largely rural area.
The pool is unique in several aspects. In contrast to ponds in the open lava fields of the Kona coast, this pond is located in a heavily shaded, wooded area. It has one of the highest water temperatures and used for warm bathing by local residents and visitors who learn of it. There are no signs pointing to it and there is valid concern that a greater usage would have potentially disastrous results.
Ponds in East Hawaii are known to also contain both opae-ula (Halocaridina rubra) and Metataeus lohena, the larger predator shrimp, but in different ratios. In West Hawaii, there may be one Metabataeus for every 100 opae-ula. But in 1980, the initial report estimated the ratio at one Metabataeus for every 13 opae-ula. In visiting the pond, one objective was to collect both species to create an exhibit featuring a preponderance of Metabataeus.
The irregularly shaped pool is about 100' inland from a stony shoreline and about 24' long by about 15' wide and about 3' to 4' deep. The water is clear with reported salinity of 4.7 to 7.8 parts per thousand that varies with the tide as the geothermally heated ground water mixes with colder seawater. Water temperatures range from about 91° F to 95° F (33° C to 35° C) with air temperatures about 73° F to 82° F (23° C to 28° C).
|The pool is in a largely natural depression but it is obvious that some ancient person laborously removed or walled up the loose or collapsed stones and brought in wave smoothed pebbles to create a comfortable stony pool floor. Although large rocks along some edges would appear to be attractive seats, bathers stood or floated in the center as they report receiving tiny irritating bites when they sat on the rocks.|
The 1980 report includes extensive Metabaetaeus observations of predator activities lying in shaded crevices to ambush the smaller opae-ula. The report stated that when populations of the two species are confined to a small aquarium, there was a gradual decrease in the number of opae-ula. It also stated that Metabetaeus prefer to cannabalize their own kind over opae-ula when the two species are the same size.
Reported opae-ula sizes ranged from about 7 to 10mm with most in the 8 to 9mm range. Metabateus were mostly 10 to 13mm. These are relatively small compared to those grown at Fuku-Bonsai and their sizes and population is likely due to the limited amount of food available. In the shaded location, the algae likely has much lower nutritional value compared to the algae that grows in sunny ponds. The Metabateus was less than half their normal size. Why are they proportionally more numerous?
|John Chan reports that a major food source is decaying leaves falling into the ponds and that it is likely the primary food for juvenile Metabaetaeus. Larger Metabaetaeus populations may be due to better adapation to this food source and larger Metabaetaeus populations have been reported when trees are near to a pond.|
In the earlier study, the population of the pond was roughly estimated to be between 10,000 and 20,000 animals. The population is very difficult to determine as very few are found on the pool bottom or swimming in the water column. Most are in the rock rubble and in the dark rock overhangs along the side of the pool.
Enroute home, we discussed various aspects of the pond. Compared to the earlier 1980 reporting period, John thought the population was much smaller. He dropped me off with half of the day's catch and I just couldn't wait to examine them closely. After studying them for a while it was possible to guess which ones were opae-ula and which were metabataeus.
John may be "retired" but he's busier than ever! A few days later he was off on another plane and it may be a while before he has a chance to come back and do something with his half of the day's catch. I look forward to his critiques and participation in this on-going research project! Mahalo John!
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