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Life history characteristics of the native Hawaiian stream snail Neritina granosa (hihiwai)

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Title: Life history characteristics of the native Hawaiian stream snail Neritina granosa (hihiwai)
Authors: Brasher, Anne M.
LC Subject Headings: Freshwater invertebrates -- Hawaii -- Molokai.
Snails -- Monitoring -- Hawaii -- Molokai.
Issue Date: Nov 1997
Publisher: Cooperative National Park Resources Studies Unit, University of Hawaii at Manoa, Department of Botany
Citation: Brasher AM. 1997. Life history characteristics of the native Hawaiian stream snail Neritina granosa (hihiwai). Honolulu (HI): Cooperative National Park Resources Studies Unit, University of Hawaii at Manoa, Department of Botany. PCSU Technical Report, 114.
Series/Report no.: Technical Report
Abstract: Waikolu Stream is notable for supporting a large population of the relatively uncommon native Hawaiian stream snail, hihiwai (Neritina granosa). Because hihiwai are easier to identify, collect and measure than gobies (typically used in stream evaluation surveys in Hawai'i), they are especially amenable to monitoring programs, and the habitat requirements of hihiwai make them a good indicator of water quality. The methods presented in this report provide techniques for monitoring population density and distribution of hihiwai, and the important life stages of reproduction and recruitment (post-larvae returning from the ocean to the stream). During this study, sampling stations were surveyed every other month for a period of two years. This baseline data can be used for comparison with future surveys to monitor the condition of Waikolu Stream and to evaluate the impact of any future changes to the stream system. The number of hihiwai found at each station during the baseline surveys showed wide variation, but the mean number of snails was consistent between seasons and over the two-year monitoring period. Snail densities recorded in and above the areas affected by water diversion were dramatically lower than in the lower stream reaches. Areas of the stream between the lower and upper dam were often completely dry during sampling periods. Even when water was allowed to flow through this area, snails were rarely present. In addition to not providing suitable habitat, a completely dry stream prevents both downstream dispersal by larvae and upstream migration by post-larvae (spat) and juveniles. In the very highest elevation stations, no snails less than 25 mm were observed. This is most likely a remnant/isolated group of snails; the few snails that made it past the diverted area during a time when water was allowed to flow have settled here. Restoration of continuous water flows would presumably allow for recolonization of this area. Egg production showed both seasonal and yearly variation. Peaks in reproduction occurred during the late fall, late spring and summer. Reproduction was higher the first year than the second and both years showed little egg production in January. Recruitment occurs throughout the year, with the largest event in May and a second major recruitment in November. The majority of all adult snails tagged during the first study were found within 30 meters upstream or downstream of release, suggesting that once snails settle out, they stay in that area. This illustrates the importance of continually flowing water, since once the hihiwai settle they are unlikely to begin an additional upstream migration, even if there is flow in a previously dry area. The younger, smaller hihiwai tagged in the second study showed much more rapid upstream movement than snails in the first. Hihiwai in the second tagging study also grew more rapidly than the older, larger snails in the first tagging study, and during the second study the 11 mm and 12 mm snails grew more rapidly than the 13 mm snails. The population monitoring data suggest that hihiwai grow rapidly when returning to the stream (reaching 9 mm in a few months). If snails are then growing 1 to 3 mm a year after they reach 9 mm, with growth slowing as they age, as suggested by both of the tagging studies, hihiwai life span could be estimated to be 6 to 10 years. The tagging studies show that snails do not move very fast or very far once they have settled out, making it difficult for them to pass barriers of dry stream during either floods or brief periods of increased water flows. During periods when water was present in the stream area between Stations 2640 and 3217, fish moved immediately into these areas (personal observation), while snails rarely did. It appears that prolonged periods of continuous flow are necessary for hihiwai to be able to migrate through periodically dewatered sections of a stream.
Description: Reports were scanned in black and white at a resolution of 600 dots per inch and were converted to text using Adobe Paper Capture Plug-in.
Sponsor: National Park Service Cooperative Agreement CA 8006 2 9004
Appears in Collections:The PCSU and HPI-CESU Technical Reports 1974 - current

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