NEWS

Climate impacts on native and non-native fishes in Hawaiian streams

Like many visitors, invasive fishes seem to enjoy the life of our tropical landscapes. But what might happen if climate changes the conditions of their stream habitats?

Leigh Engel, a Pacific Islands Climate Adaptation Science Center (PI-CASC) Graduate Scholar, and University of Hawaiʻi at Mānoa master’s student, decided to find out. Her thesis centers around how climate-induced higher temperatures and extreme events, like flash floods, might affect native and non-native fishes in Hawaiian streams to see if the invasive species will still have an edge under more challenging conditions.

Raised in a household of women in rural upstate New York, Engel took inspiration from her mother, who was not only a biologist but also a forester, forest firefighter, and wildlife rehabilitator. “Growing up, I learned that wild things, places, and people all had rights, and there was a strong camaraderie with the marginalized or vulnerable,” said Engel.

After finishing her undergraduate degree, she pursued her interests as a fisheries biologist, first working for the Louisiana Department of Wildlife and Fisheries and then for the Sitka Tribe of Alaska. These experiences provided contrasting approaches, one guided by Western science and the latter in traditional ecological knowledge and Indigenous management systems. Then in 2020, Engel attended the Indigenous Aquaculture Collaborative Network (IAC) in Oʻahu, which was a turning point for her.

“It was an opportunity to gain perspective and responsibility in service. I was inspired to return to school to develop additional skills in analysis, experimental design, and current problems and local/community-based solutions to be a better program manager, coordinator, collaborator, and ally,” she said. “I also hoped to develop and integrate multiple forms of knowledge, incorporating the ‘two-eyed perspective.’”

Now Engel works as a PI-CASC Graduate Scholar with Yinphan Tsang, an associate professor of the Natural Resources and Environmental Management Department at UH Mānoa, and Tim Grabowski, the Unit Leader for the U.S. Geological Survey Hawaiʻi Cooperative Fishery Research Unit and an adjunct associate professor in the Marine Science Department at the University of Hawaiʻi at Hilo. Engel’s project is titled “Increasing temperatures enhances swimming performance of invasive species in Hawaiʻi.” Her research investigates the swimming performance of three of the most common invasive fish species in Hawaiʻi; hybridized mollies from the three Poecilia species introduced to Oʻahu streams (Poecilia salvatoris/mexicana/sphenops), Green Swordtail (Xiphophorus hellerii), and Blackchin Tilapia (Sarotherodon melanotheron). The work focuses on how quickly they can switch from aerobic to anaerobic activity, their maximum sustained speed before becoming exhausted, and how those traits may change with different environmental temperatures. Ultimately, Engel’s research aims to determine a biologically meaningful definition of extreme flow events that can be used for developing effective management and conservation strategies, such as more efficient removal efforts and metrics for modeling the spread and persistence of these invasive species.

Engel captured the invasive fish species from Mānoa, Pālolo, and Makiki drainages using A-frame push nets, some targeted and captured through ‘Iolani School’s educational and outreach program. After acclimating the fish to one of three temperature treatments, they underwent swim trials to see how temperature affects performance in these different invasives.

Preliminary results showed that all three species demonstrated improved swimming performance at higher temperatures, and the swordtail exhibited significantly higher swimming speeds at elevated temperatures than other species. This could suggest that under climate change scenarios, these invasive species will likely continue to spread and persist in Hawaiian environments since they can tolerate and even thrive during increased temperatures and extreme flow events.

Engel’s research supports existing efforts and is one small part of the more significant challenge of determining which species will succeed in tomorrow’s changing climate. While this study focuses on the traits of these three species, the analysis may extend to other species and overlay this with habitat assessments.

Engel is in the final semester of her master’s program. In the future, she plans to continue contributing to interdisciplinary marine science efforts that integrate outreach, community engagement, and education to uplift and support Pacific Island communities and bridge local and traditional ecological knowledge.

“I’m appreciative of the PI-CASC Graduate Scholars program for their support, as well as local stream experts Cory Yap and Andi Charuk with Pa‘ēpa‘ē o Waikolu for their time, dedication, local and place-based knowledge, as well as Dr. Jacob Johansen, the Fish Resilience Lab, and Moku o Loʻe for expertise in swim performance and holding space,” she said.