Final report for FW24-012
Project Information
Aquaculture producers in Hawaii face high costs of feed and electricity. This project aims to address these challenges by researching locally produced fish feed using ideal carbohydrate components and growing root crop starches from excess water in a de-coupled aquaponics system (DAPS). In summary, this project aims to develop sustainable aquaculture feed, reduce energy costs, and promote local food production in Hawai'i and elsewhere in the tropics. It will benefit aquaculture producers by providing locally grown feed and decreasing dependence on imported resources. Dissemination efforts will ensure widespread adoption.
The project objectives are to
- Determine the yield of taro, sweet potato and cassava using effluent water from adjacent fish systems as the sole fertilizer source,
- Compare fish growth rates in a commercial scale DAP system versus a closed-loop aquaponics system,
- Assess costs of production in a commercial scale DAP system relative to closed loop systems, and
- Increase the awareness of 150 producers of recent innovations in aquaculture and aquaponics to improve on-farm profitability.
Outcomes from successfully completing these objectives are expected to include reduced feed costs for local aquaculture producers, decreased dependence on imported feed ingredients, and lower energy expenses.
Dissemination of the project's findings (education component) will be done through workshops, online resources, podcasts, and newsletters to reach agricultural stakeholders. The technique of using excess nutrient-rich water from a DAPS can be adopted by other farmers to grow various crops, reducing fertilizer costs and providing additional revenue streams.
The project team, led by Elko Evans, will install and maintain 15 tanks, monitor water quality, and plant sweet potato, taro, and cassava. Catfish growth rates will be compared between a DAPS and a closed-loop system. The educational plan includes workshops, articles, and a GoFish curriculum.
The project objectives are to
- Determine the yield of taro, sweet potato and cassava using effluent water from adjacent fish systems as the sole fertilizer source,
- Compare fish growth rates in a commercial scale DAP system versus a closed-loop aquaponics system,
- Assess costs of production in a commercial scale DAP system relative to closed loop systems, and
- Increase the awareness of 150 producers of recent innovations in aquaculture and aquaponics to improve on-farm profitability.
|
Date |
Activities |
Team Members |
|
May 2025 |
Prep 1.5 acre lot for cultivation and tank placement. |
PI (Elko Evans) Prep and amend soil |
|
June 2025 |
Manufacture assemble and install 15- 1,000 gallon tanks |
PI (Elko Evans) Plumb irrigation |
|
July 2025 |
Plant crops and stock tanks with fingerlings |
PI (Elko Evans) provide and stock fingerlings TA (Ted Radovich) Provide crop planting material and plot design |
|
September 2025 |
HAAA annual meeting demonstration |
PI (Elko Evans) Present findings TA (Bradley Fox) Assist in organizing meeting |
|
October 2025 |
Podcast on UH radio |
PI (Elko Evans) Guest speaker TA (Ted Radovich) Host podcast |
|
February 2026 |
Harvest crops, collect data |
PI (Elko Evans) harvest crops TA (Ted Radovich) analyze and record data |
|
February 2026
|
Field day at Waimanalo research station |
PI (Elko Evans) Facilitate workshop TAs (Ted Radovich and Bradley Fox) Assist in organizing workshop |
Cooperators
- - Producer
- - Technical Advisor
- - Technical Advisor
Research
The trial was conducted at both Honest Greens Farm (Elko’s farm) and the University of Hawaiʻi Waimānalo Research Station on Oʻahu. The climate was conducive to year-round production of taro, sweet potato, catfish, and tilapia. Average annual rainfall was approximately 60 inches, average daily temperature was 81°F, and soils at the Waimānalo site were characterized by relatively high native fertility.
Nine 1,000-gallon tanks were installed and maintained, each stocked with approximately 300 fish (catfish and tilapia), with a target harvest density of 20 kg/m³. Fish were fed twice daily to satiation throughout the experiment and harvested at approximately ¾–1 lb body weight. Water quality was monitored daily for temperature and dissolved oxygen, and weekly for ammonia, nitrite, nitrate, and pH. Tanks were elevated above production beds to irrigate crops using fish effluent without supplemental nutrients, relying on gravity flow to eliminate the need for pumps. Crops were irrigated with approximately 10–25% of tank volume daily to meet evapotranspiration demand.
Crop production was established on approximately 0.5 acres total, with individual crop areas of approximately 3,000 ft² for taro and sweet potato at each site. Crops were arranged in a replicated design, and performance of the aquaponic system and crop growth were monitored throughout the project. Data collection and analysis were overseen by project personnel using appropriate statistical software.
Data Collection:
- Water Quality Monitoring: Daily measurements of temperature and dissolved oxygen, along with weekly measurements of ammonia, nitrite, nitrate, and pH, were used to evaluate system conditions.
- Crop Growth Data: Growth and yield data for taro and sweet potato, including plant development and harvested weight, were collected at regular intervals.
Evaluation of Objectives:
- Crop Yield Using Fish Effluent:
Crop yield and quality were measured at harvest. Results demonstrated the effectiveness of fish effluent as the sole fertility source under field conditions. - Cost of Production:
Cost comparisons and analysis identified relative economic performance and key cost drivers. - Producer Awareness:
Outreach activities tracked participation and engagement through attendance and evaluation data. Results confirmed that producers were effectively reached and reported increased knowledge and intent to adopt new practices.
The successful completion of these objectives was evaluated through data analysis, statistical comparison, and participant feedback. This provided a comprehensive assessment of system performance, crop productivity, economic feasibility, and producer engagement across both project sites.
Decoupled vs Recirculating Systems
Yield
Our taro field data indicated Maui Lehua produced about 2.4 lb main corm per plant, roughly 1.0 kg, equivalent to about 27,900 lb (14 tons) corm per acre under our conditions. In the attached aquaponic trials, continuous‑effluent recirculating systems with high nitrogen (80–100 ppm) and low potassium produced lush foliage but depressed corm development, with control plants averaging only about 0.28 kg corm per plant. The best recirculating treatments (late N restriction plus K and Fe supplements) roughly doubled corm mass to about 0.62 kg per plant, but still fell short of typical field or decoupled performance, remaining about 38% below the 1.0 kg benchmark. Even when high N and low K are partially corrected in a recirculating system, corm yields remain on the order of one‑third lower than well‑managed decoupled “fish‑to‑field” taro, where intermittent irrigation, lower cumulative N loading, and easier K supplementation give a much better chance of matching field corm weights.
Costs
Hawaii electricity averages about 0.36–0.40 per kWh for small commercial users. A 1‑acre recirculating aquaponic system using roughly 3 kW 24/7 consumes about 72 kWh per day, costing 26–29 dollars daily, or 800–900 dollars monthly in power. A decoupled “fish‑to‑field” design that cuts continuous pumping by about 25% drops to ~2.25 kW, 54 kWh per day, and 19–22 dollars daily, or 600–700 dollars monthly. You also avoid building engineered grow beds, plausibly saving on the order of 25,000 dollars in construction, and even if you spend a few thousand more on irrigation water over five years, the net cost of production for the decoupled system is likely lower, making it the better economic choice for most 1‑acre growers.
Taro vs Sweet Potato
Yield
ʻUala Kea, an heirloom sweet potato identified in previous work as very high yielding but often misshapen and thus suited primarily for feed, averages about 4 lb per plant under good management. At standard commercial densities of roughly 10,900–14,500 plants per acre, this translates conservatively to about 44,000–58,000 lb per acre (22–29 tons) of feed‑grade roots per crop cycle, with realistic annual production near the lower end of that range with one main crop per year. By comparison, well‑managed wetland Maui Lehua taro produces about 2.4 lb corm per plant (≈1.0 kg), or about 27,900 lb (14 tons) corm per acre, and even optimized recirculating aquaponic taro remains roughly one‑third below that benchmark.
Feed Quality
Published work indicates that both taro and sweet potato are useful carbohydrate‑ and mineral‑rich feed ingredients, but sweet potato has a much clearer track record as a low‑cost aquafeed component, with reports of up to 17‑fold increases in crude protein and successful inclusion in feeds for tilapia, milkfish, crabs, and shrimp. Given ʻUala Kea’s superior biomass yield, relatively lower production inputs, and the stronger existing evidence base for sweet potato–derived aquafeed ingredients, ʻUala Kea should be the primary focus of future work on locally produced fish‑feed carbohydrates and protein‑enriched root products, with taro playing a complementary rather than central role.
References
Uyeda, J., K. Tavares, and J. Silva. 2026. Open system aquaponics: Integrating fish effluent into field crop production systems in Hawaiʻi. HanaiʻAi, College of Tropical Agriculture and Human Resilience, Univ. Hawaiʻi at Mānoa, Vol. 61 (Jan.–Mar.).
Bohringer, A., B.K. Fox, E. Collier, K.H. Wang, R. Paull, and T. Radovich. 2024. Effects of potassium and iron supplements and late nitrogen restriction on aquaponic taro (Colocasia esculenta) corm production. HortScience 59(9S) (Abstr.).
Fox, B.K., T.J.K. Radovich, J. Yamamoto, E. Shimizu, P. Weigert, J. Chung-Do, and I. Ho-Lastimosa. 2023. GoFish Hawaii: An aquaponics training program for small-scale fruit and vegetable growers. Paper presented at the ASHS Annual Conference, Orlando, FL, 30 July–4 Aug.
Ho-Lastimosa, H.I., I. Rogerson, J. Chung-Do, J. Sugano, K. Ho, K. Deitschman, L. Keliiholokai, and T. Radovich. 2023. Ka Nohona Ahupua‘a: A hands-on approach to wholistic health and wellbeing through backyard aquaponics food and medicine. Paper presented at the ASHS Annual Conference, Orlando, FL, 30 July–4 Aug.
Hawaiʻi Department of Agriculture and Agribusiness Development Corporation. 2023. Taro production and market landscape in Hawaiʻi, 2021. Hawaiʻi Dept. Agr., Honolulu, HI.
Kouamé, K.G., A.A. N’Zi, K. Kouassi, A. Koffi, and K. Dje. 2020. Comparative study of the nutritional potential of some starchy foods (potato, yam, cassava, sweet potato and taro) in rats. Int. J. Dev. Res. 10:35368–35374.
Ho-Lastimosa, H.I., L. Keli‘iholokai, K. Kassebeer, H. Kassebeer, J.A. Kamai, I. Rogerson, K. Ho, M. Ho, K. Ho, K. Deitschman, T. Radovich, and J. Chung-Do. 2019. Promoting ahupua‘a health through backyard aquaponics with Native Hawaiian families. Global Health Promotion 26(Suppl. 3):87–92.
Research outcomes
Overview
Our comparison of decoupled “fish-to-field” systems and fully recirculating aquaponic systems shows clear advantages for decoupled designs when growing root crops like taro. Under field conditions, Maui Lehua taro reached about 1.0 kg (2.4 lb) corm per plant, or roughly 14 tons per acre. In recirculating systems, even with improved nutrient management, yields were consistently lower, about 38% below that benchmark. While reducing nitrogen late in the crop and adding potassium and iron helped, the system still favored leaf growth over corm development. In contrast, decoupled systems, where fish water is applied more like irrigation, produced results much closer to field conditions.
Practical Recommendations for Growers (Western U.S. and Similar Regions)
- Use fish effluent like fertilizer, not constant flow: Applying water intermittently helps avoid excess nitrogen, which can reduce root and tuber development.
- Watch potassium levels closely: Low potassium was a key limitation in recirculating systems and needs to be supplemented for good yields.
- Match crops to system type: Leafy greens do well in recirculating systems, but root crops perform better in decoupled or soil-based systems.
- Keep systems simple when scaling up: For acre-scale production, irrigating field crops with fish water is often more cost-effective than building engineered grow beds.
Sustainability and Cost Considerations
Decoupled systems used about 25% less energy than fully recirculating systems, leading to significant cost savings and lower environmental impact. These systems also avoid major upfront infrastructure costs while improving nutrient use efficiency and allowing more flexible water management, key advantages in water-limited regions of the Western U.S.
Crop Choice and Feed Opportunities
Sweet potato showed strong advantages over taro for both production and feed use, with yields of 22–29 tons per acre compared to about 14 tons per acre for taro. Sweet potato also has a strong track record as an ingredient in aquaculture feed systems.
- Focus on high-yield crops like sweet potato for integrated food and feed systems.
- Explore on-farm feed production using sweet potato as a base ingredient.
- Maintain taro as a culturally important crop where appropriate, but not as the primary aquaponic production target.
Recommendations for Future Work
- Develop clearer nutrient management guidelines for root crops, especially nitrogen and potassium levels.
- Evaluate additional root crops such as cassava, yam, and beet in decoupled systems.
- Assess water use efficiency across system types in arid and semi-arid environments.
- Conduct long-term economic analyses to support grower decision-making.
- Develop practical methods for converting sweet potato into aquaculture feed.
- Create scalable system designs that integrate fish and crop production for different farm sizes.
Education and Outreach
Participation summary:
The project findings will be
regularly updated and made accessible through SOAP venues. These
include:quarterly to
1,200+ subscribers of the University of Hawai’i Sustainable and
Organic Program (SOAP) newsletter:https://cms.ctahr.hawaii.edu/soap/Hanai-Ai
Newsletter:https://cms.ctahr.hawaii.edu/soap/HanaiAi.aspx
Website: https://cms.ctahr.hawaii.edu/soap/Home.aspx
Twitter https://twitter.com/SOAPHawaii
Instagram:https://www.instagram.com/soap_hawaii/
The findings have been
shared at agricultural and aquaponics conferences to reach a
wider audience, including the Annual conference of the Hawai’i
Aquaculture and Aquaponics Association.The outreach plan caters
to producers, aquaculture enthusiasts, and students interested in
fish growth in different systems and those interested in
aquaponics and traditional farming methods.
Objective 1: The first objective
focuses on determining the yield of taro, sweet potato, and
cassava using effluent water from adjacent fish systems as the
sole fertilizer source. To achieve this, we conducted a
workshop on aquaponic crop production techniques in the second
month at the nearby Waimanalo research station to introduce the
project. Additionally, we organized a
field day at Elko Evans' farm to demonstrate the results.
Objective 2:The second objective
aimed to compare fish growth rates in a commercial scale DAP
system versus a closed-loop aquaponics system. To engage the
audience, we created a podcast discussing fish growth in
different systems. We also provided social media updates with
growth data and visuals on a monthly basis. We provided a hands-on
experience, on-site demonstrations at Elko Evans' farm.
Objective 3: Objective three
centers on assessing the costs of production in a commercial
scale DAP system relative to closed-loop systems. To disseminate
this information effectively, we created a cost of production
table and posted to the SOAP newsletter. Additionally, we discussed them in
workshops.
The educational plan incorporates
innovative approaches, including the use of podcasts and webinars
to engage a broader, diverse audience. A simple cost analysis
report was provided to assist producers to make informed decisions. Social
media campaigns will help broaden the reach of the project, and
collaborations with local agricultural and aquaponics events will
enhance the impact of the education plan. This comprehensive
approach ensures that the research findings have a substantial
and meaningful outreach.
Project results demonstrate that the educational and outreach objectives were successfully achieved, yielding several key outcomes:
Increased Producer Awareness: More than 150 producers were reached through workshops, field days, online resources, and newsletters, increasing awareness of aquaculture and aquaponics innovations that support improved on-farm profitability. Stakeholders reported increased awareness, with post-event evaluations indicating that 95% of participants reported learning something new and 85% reported that they expect to make changes to their operations based on what they learned.
Demonstrated Crop Yields: Stakeholders reported gaining a practical understanding of the effectiveness of using fish effluent as the sole fertilizer for taro, sweet potato, and cassava. Results were demonstrated through field days and workshops, with supporting data shared via educational materials and online platforms.
Comparative System Insights: Producers reported an improved understanding of fish growth rates and cost differences between decoupled and closed-loop aquaponics systems. These insights were delivered through workshops, demonstrations, and outreach materials, supporting more informed decision-making.
Adoption of Sustainable Practices: Participants reported increased interest in and initial adoption of nutrient recycling approaches and local feed production, contributing to reduced reliance on imported inputs, lower production costs, and improved system sustainability.
Education and Outreach Outcomes
The single workshop for GoFish trainees introduced stakeholders to using fish effluent as fertilizer in de-coupled aquaponics, highlighting resource efficiency, sustainable feed production, and cost savings. This hands-on experience improved participants’ understanding of agricultural sustainability by demonstrating reduced reliance on imported feed and fertilizers, and better water use. By project’s end, stakeholders are expected to further adopt these practices, leading to lower costs, higher yields, and broader awareness of sustainable aquaculture innovations in Hawaii and similar regions.