Native predatory fish help control invasive species in Hawaiian fishpond

Traditional fishponds, called loko iʻa, were once common across coastal areas of the Hawaiian Islands and provided a sustainable and pragmatic solution to food security concerns for local communities for centuries. Fishpond management typically focused on the cultivation of mullet, moi, and awa which are plant-eating fish that thrived in fishpond environments. These desirable fishes would be eaten by fish like jacks and barracuda, which needed to be periodically removed from the fishpond. After invasive fish species were introduced to Hawaiian waters, they made their way into fishponds and altered fishpond marine food webs.

UH Mānoa's Hawaiʻi Institute of Marine Biology (HIMB) Associate Research Professor and study co-author Erik Franklin said the prominence of the invasive mullet in the predatory fish diets was unexpected.

"Our study showed that the introduction of invasive Australian mullet altered the diets of the native predatory fish in the fishpond which really expanded our understanding of the food web interactions in these systems," said Franklin. "The jacks and barracuda seemed to preferentially feed on the invasive mullet rather than native mullet and moi. This would help control the population of the invasive species which competes for resources with the native fish."

Importance for Hawaiʻi

Hawaiian fishponds can be a tool to increase food security by producing seafood for local consumption. Over 80% of food consumed in Hawaiʻi is imported at a cost of nearly $3 billion annually. Economic analyses suggest that a small replacement of imported foods would generate substantial sales, earnings, state tax and employment for the seafood sector. In 2019, over 10 million tourists visited the Hawaiian Islands and placed an immense demand on food resources well beyond the needs of the 1.4 million residents. Prior studies have found that tourists to Hawaiʻi from the continental U.S. are willing to pay more for locally-sourced foods while on vacation in the islands to help the state become a more sustainable tourism destination. A network of restored fishponds could help meet the demands for locally-produced foods.

"The restoration of functional Hawaiian fishponds represents a promising opportunity to increase local seafood production but there is still much to learn about the ecology and dynamics of fishpond communities," said lead author and UH Mānoa Marine Biology Graduate Program graduate student Anela Akiona, a Kamehameha Schools alumna. "A functional fishpond would contribute to enhanced island food security that can supplement catch from fisheries and other aquaculture operations."

Franklin said the research resulted in analyses of the abundance and prey of predatory fishes in the Heʻeia fishpond. The study used a tag-recapture experiment, genetic barcoding, stable isotope analysis, and statistical methods to assess the number of jacks and barracuda in Heeia fishpond and their diet composition.

The research team included Franklin, Akiona, and Professors Rob Toonen from HIMB and Brian Popp from the Department of Earth Sciences, Post-doctoral Researcher Margaret Siple from UCSB, and Keliʻi Kotubetey and Hiʻilei Kawelo from Paepae O Heʻeia. A group of volunteer fishers from the community contributed to the tag-recapture experiment. Volunteer interns from the Laulima A ʻIke Pono program contributed to the collection and preparation of samples for stable isotope analysis. Akiona was supported by a Hauʻoli Mau Loa Graduate Fellowship at UH Mānoa.