Population response to connectivity restoration of high elevation tropical stream reaches in Hawai'i

Abstract

Tropical island streams are heavily impacted by water withdrawals to the detriment of native ecosystems. Freshwater fauna in the tropics often exhibit diadromous life-history strategies, requiring hydrologic connectivity between upstream and nearshore habitats. During some or most of the year, this connectivity is generally lost at points of water withdrawal, typically constructed as a low-head dam diversion. In 2010, instream flow standards (i.e., minimum environmental flows) were established for streams in East Maui, Hawai'i, which led to the partial restoration of flow in certain streams. In 2016, flow in several of these streams was then fully restored. We surveyed 10 streams before (2012) and after (2017) varying degrees of flow restoration (none, partial, full) to explore the short-term consequences of reestablishing natural streamflow for the migratory fauna of high-elevation tropical streams. Using visual point-quadrat surveys, we quantified density and habitat usage of the endemic atyid shrimp (Atyoida bisulcata) and sampled A. bisulcata populations using seine-netting to assess size frequency, sex ratio, percent ovigerous (egg bearing) females, and percent juveniles. In streams with full flow restoration, shrimp density increased in 2017 compared to 2012, while mean body size declined, suggesting increased recruitment following restoration. By contrast, streams that did not experience any flow restoration had similar shrimp densities and body sizes across years. Further, the male: female ratio declined from 18:1 in 2012 to 6.5:1 in 2017, suggesting an increase in reproductive potential of the population. Our results demonstrate that freshwater ecosystems upstream of diversion structures may respond rapidly to the restoration of hydrologic connectivity via increased recruitment of amphidromous species.