Monitoring the distribution and abundance of native gobies ('o'opu) in Waikolu and Pelekunu Streams on the island of Moloka'i
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1996-02
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Cooperative National Park Resources Studies Unit, University of Hawaii at Manoa, Department of Botany
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Abstract
With ever-increasing demands of water in Hawai'i, managers are faced with the need to balance water removal for use by humans with maintaining the viability of stream ecosystems. Thorough, quantitative data on 'o'opu distribution and abundance, collected in a rigorous scientific manner is necessary for sound management decisions. This study was designed to establish standardized methods for conducting surveys of native 'o'opu populations to monitor streams in Hawai'i. These methods may be used for long-term comprehensive monitoring projects as described here, or for one-day reconnaissance surveys to evaluate stream conditions, perhaps as a preliminary step in planning more extensive monitoring surveys or ecological studies. To collect baseline data on Waikolu Stream and Pelekunu Stream, a two-year study with quarterly 'o'opu surveys at randomly selected stations located from the mouth to the headwaters of each stream was conducted. This study focused on three species of 'o'opu: 'o'opu nakea, 'o'opu nopili and 'o'opu alamo'o. Abundance of each species and size of each goby was recorded at each sampling station. Results of these surveys provide information on natural variation in 'o'opu distribution and density over time, and can be compared with future monitoring surveys in these streams or to data collected in other streams across the state. Overall densities of 'o'opu nakea were much lower in Waikolu Stream than in Pelekunu Stream and no 'o'opu nakea were observed in the upper stations of Waikolu Stream, where the two dams and the reduction of flow in this section may have restricted upstream movement. 'O'opu nakea were observed at all elevations surveyed
in Pelekunu Stream, suggesting that in streams where the gradient is relatively low, and there are no large waterfalls, 'o'opu nakea may move quite far upstream. In both Waikolu and Pelekunu Streams, densities of 'o'opu nopili were highest near the mouth, decreasing with upstream distance. While 'o'opu nopili are more abundant at stations near the mouth in Waikolu Stream than in Pelekunu Stream, at all other comparable stations, nopili are more abundant in Pelekunu Stream. The greater abundance of 'o'opu nopili at stations near the mouth in Waikolu Stream is due in part to the large number of hinana (post-larval gobies returning to the stream from the ocean) in that area. However, over time the abundance of hinana in Waikolu Stream did not translate into higher densities of adult 'o'opu nopili upstream. In Pelekunu Stream, 'o'opu alamo'o were most abundant at stations at higher elevations. This is the pattern typically observed in Hawaiian streams. However, 'o'opu alamo'o were more abundant in the lower reaches of Waikolu Stream, and less abundant in the upper reaches, especially above the diverted section. The lower number of 'o'opu alamo'o in the mid and upper reaches of Waikolu Stream was most likely a result of decreased flow and periodic dewatering of the stream section below the upper dam, reducing available habitat for the 'o'opu and inhibiting upstream migrations. Abundance of all three species of 'o'opu, in both Waikolu and Pelekunu Streams, remained consistent between seasons and over the two-year monitoring period. The three species showed considerably more overlap in longitudinal distribution than has previously been reported for Hawaiian streams. This may be due to the relatively low gradient of the stream channel and lack of major waterfalls in the lower and mid reaches, allowing all three species to continue upstream migration, and eliminating the upper reaches as a refuge for 'o'opu alamo'o. Recruitment appears to occur, at least at some low level, throughout the year. Very few 'o'opu nakea recruits were recorded during the two-year monitoring period although large numbers of post-larval 'o'opu nakea were present near the mouth of Pelekunu Stream in August 1992, and spawned-out nakea and eggs were found at the mouth of Waikolu Stream in November 1992 (both prior to the commencement of the monitoring study).
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Keywords
Fish populations -- Hawaii -- Molokai., Freshwater fishes -- Hawaii -- Molokai., Gobiidae -- Hawaii -- Molokai.
Citation
Brasher AM. 1996. Monitoring the distribution and abundance of native gobies ('o'opu) in Waikolu and Pelekunu Streams on the island of Moloka'i. Honolulu (HI): Cooperative National Park Resources Studies Unit, University of Hawaii at Manoa, Department of Botany. PCSU Technical Report, 113.
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