Assessing the Impact of Future Climate on Hawai‘i’s Aquatic Ecosystems

The stream systems of Hawai‘i are unique and home to many rare species, including five native fish and five native shellfish. These native species have amphidromous life cycles, meaning that they spend part of their lives in the ocean and part in freshwater streams. Streamflow serves as a vital natural pathway, connecting saltwater and freshwater habitats so that these animals can migrate between them and carry out critical life stages (e.g., development, reproduction). Over the last 20 years, the amount of rainfall in Hawai‘i has decreased, and climate models predict that this trend will continue. It is uncertain how reduced rainfall will affect streamflow and, consequently, the native stream species that depend on it. This study will advance our understanding of what climate change means for streamflow and native stream species in Hawai‘i. Such information can be used to inform conservation management decision making. PI: Yin-Phan Tsang

Biodiversity of Freshwater and Estuarine Communities of Pearl Harbor

Estuaries are where freshwater from the land meets with ocean water, typically in discrete basins that have partial surface connectivity with the open ocean and are subject to tidal flux. Approximately 50 semi-enclosed waterbodies are considered estuaries throughout the main Hawaiian Islands. The mix of nutrient and water input from inland makes estuaries in Hawaii productive aquatic systems and serve as important nursery and breeding habitat for freshwater and marine species. Currently, there is no standardized way to survey these important habitats at stream mouth and estuary in Hawaii. We are conducting a series of biological surveys along with eDNA sampling to establish the current baseline inventory of freshwater, estuarine, and marine organisms. The eDNA sampling method for biodiversity of streams and estuaries has not been done anywhere is Hawaii. The paired biological survey and eDNA sampling results will be compared and provide unprecedented information to determine a cost-efficient method for understanding the biodiversity at stream and estuaries areas. When possible, the gathered data will be compared with previous surveys to document the changes in biodiversity at these habitats. PI: Yin-Phan Tsang

Characterizing Natural Barriers to Non-Native Stream Fauna in Hawaii

Waterfalls, caused by the abrupt changes of elevation in streams, are natural barriers that influence the distribution and dispersion of aquatic species. The resulting habitat fragmentation has contributed to species specialization as well as barriers that inhibit passage of non-native species upstream. In Hawaiʻi, it is assumed that non-native species are unable to pass waterfall barriers, yet they are present above some waterfalls, possibly facilitated by human introduction. In this study, we used a landscape approach to identify likely human introductions and examine the ability of non-native stream fauna to bypass waterfalls. We identified the human activities associated with the high likelihood of species introduction. We found that when a local catchment has a population density > 4.24 people/km² or road length density is > 0.01 km/km², the presence of non-native species in the stream is likely a result of human introduction. After filtering human facilitated introduction, we also assessed the potential waterfall climbing ability of 14 non-native taxa. We found that 12 out of the 14 taxa were absent upstream of waterfalls, indicative of their inability to traverse waterfalls. Only two species, Tahitian Prawn Macrobrachium lar and American Bullfrog Rana catesbiana, seem able to pass waterfalls. This study highlights the role that people play in facilitating species introductions in otherwise inaccessible habitats. Without human interference, waterfalls can be considered effective barriers to non-native species and can be instrumental in supporting non-native species eradication and control strategies.PI: Yin-Phan Tsang

Assessing Stream Restoration Effects, Kahana, Oahu

Streams provide valuable ecosystem services and stream health should be a priority. However, threats such as the invasive shrub, hau (Hibiscus tiliaceus) have led to the degradation of streams. Kahana Stream on Oʻahu is one example. Restoration efforts can be implemented to reestablish favorable hydrological conditions for ecosystem processes. The intent of this study was to assess the impact of hau removal on flow regime and aquatic community composition, where the analysis was based on pre and post restoration data. Pre-restoration biological data was gathered from 1969-1970 and 2000, while pre-restoration hydrological data was collected in 2016. Post restoration hydrological and biological data were collected from 2017-2018. The primary objectives were to evaluate impacts of hau removal on water quality and flow regime (point-velocity method) by performing surveys in the proposed restoration area and to establish a baseline sampling campaign for post-restoration effects to be quantified. The secondary objective was to assess the impact of hau removal on aquatic community composition by conducting surveys in the upper reaches of Kahana Stream (point-quadrat method) and within Kahana Estuary (seine method) to replicate historic surveys. Results from this study showed a slight improvement in stream velocity, where the mean velocity of the stream increased by .02 m/s in the restoration area after hau was removed. Species composition was significantly different from historical findings, but the benefits of restoration may take more time to respond to improved conditions. Findings from this study suggests that the removal of invasive species may improve stream functions. PI: Yin-Phan Tsang