Flow Ecology of Invasive Suckermouth Catfish in Urbanized Ridge-to-Reef Systems on O'ahu, Hawai'i

Timothy Brian Grabowski^1*Yinphan Tsang²Danielle Bartz³Cory Yap²Jeffrey Falke⁴J. Ryan Bellmore⁵Jason Fellman⁶

• ¹Hawaii Cooperative Fishery Research Unit, US Geological Survey Ecosystems, Reston, United States
• ²University of Hawai'i at Manoa Department of Natural Resources and Environmental Management, Honolulu, United States
• ³Marine Biology Graduate Program, University of Hawai'i at Manoa, Honolulu, United States
• ⁴Nevada Cooperative Fish and Wildlife Research Unit, US Geological Survey Ecosystems, Reston, United States
• ⁵Pacific Northwest Research Station, US Department of Agriculture Forest Service, Washington, United States
• ⁶Program on the Environment and Alaska Coastal Rainforest Center, University of Alaska Southeast, Juneau, United States

Introduction: The effects of flow regimes on the ability of invasive species to establish and maintain populations in the ridge to reef (R2R) systems common to oceanic islands is not well understood. The hydrology of the relatively short, high-gradient, and flashy R2R streams of oceanic islands may be extremely different from that of the continental watersheds invasive species originate from and thus may exert a stronger influence on their ecology. Our objective was to evaluate the effects of annual variability in flow conditions on the growth and recruitment of invasive armored Suckermouth Catfish Hypostomus c.f. watwata in Hawaiian streams. Methods: Suckermouth Catfish were captured from three streams of the Ala Wai Watershed on O'ahu. We then measured, weighed, and extracted the lapilli from each fish. We used back-calculated lengths at age to estimate the effects of interannual variability in flow on growth and recruitment. Results: Individuals ranged from 0 – 16 years old and grew rapidly in their first three years after which growth slowed substantially. The growth of Suckermouth Catfish was positively influenced by flow conditions indicative of wetter years and more stable flow and negatively influenced by flow conditions indicative of drier years and more variable flows. However, the specific annual flow metrics most strongly influencing growth varied by stream. Similarly, recruitment was positively influenced by higher winter flows with lower daily variability. Discussion: The observed effects of flow on Suckermouth Catfish growth and recruitment suggests that they are not particularly well suited for the flow conditions characteristic to R2R systems and that anthropogenic alterations to the hydrology and physical in-stream habitats may have enabled the species to be more successful on O'ahu than it would have been otherwise.