EFFECTS OF DISCHARGE AND DENSITY ON MIGRATION OF THE ENDEMIC SNAIL (NERITINA GRANOSA SOWERBY) IN A DIVERTED MAUI STREAM.

T.M. Fernandes¹, K.R. Jennings¹, M.D. McIntosh², M.E. Benbow², and A.J. Burky¹. ¹Department of Biology, University of Dayton, Dayton, OH 45469-2320, ²Department of Entomology, Michigan State University, E. Lansing, MI 48824-1115

Capture-tag-recapture studies determined effects of discharge and density on migration of an amphidromous snail endemic to Hawaii. Snails from 3 - 5 mm shell length were collected, tagged, and released at a site upstream and downstream of a diversion in Iao Stream, a stream normally void of naturally occurring populations due to the diversion. A first experiment examined migration differences above and below the diversion (198 tagged and released at each site), while a second experiment tested for the effect of increased snail density (adding 1,962 untagged with 198 tagged) at the upstream site. Average discharge above the diversion was > 2 orders of magnitude higher than the below diversion site. Initial dispersion patterns revealed 42% downstream movement at the site below the diversion, with 1-5% at the above diversion site, indicating loss of rheotactic movement associated with reduced flow. There was a significant effect of discharge and density on average upstream migration with the fastest, intermediate, and slowest rates (m d^-1) found in the upstream group with density treatment (1.3), the untreated above diversion group (0.7), and the below diversion group (0.2), respectively. These data indicate that migration rates are positively correlated with density and discharge.