Development of a Sustainable Polyculture Seaweeds and Fish on Molokai

Development of a Sustainable Polyculture Seaweeds and Fish on Molokai

Summary

The second year’s work has been toward refining a two-phase polyculture system designed for producing seaweeds and marine fish on the island of Molokai. The system is operated by a nonprofit organization involved in aiding native Hawaiians. Research focused on developing protocols for the use of nutrients in effluent from fish culture to support the small-scale commercial production of the red alga Gracilaria parvispora, known locally as “ogo.” The fertilization protocols that we developed have been adopted by the producer, resulting in increased production and decreased labor. Additional research has demonstrated the importance of nutrient levels, water motion, and their interaction in the production of seaweed in this system. A series of workshops was held on Molokai to demonstrate the technology and to provide training for local aquaculture workers.

Objectives/Performance Targets

1. Determine optimal stocking densities for fish and seaweeds in an integrated, tank-based system for supplying fresh seaweed to local markets
2. Determine nitrogen budgets for these systems for use in farm management to reduce reliance on artificial fertilizers, increase seaweed production, and prevent eutrophication of the local seashore water
3. Increase farm revenues through diversified and increased production
4. Encourage, through direct demonstration and dissemination of technical information, the development of integrated aquaculture systems appropriate for small-scale coastal farms on Molokai and other Hawaiian islands.

Accomplishments/Milestones

The project team established a protocol for waste nutrients from fish culture to support the commercial production of the seaweed Gracilaria parvispora in Hawaii. Originally we planned to culture the seaweed in land-based tanks, but we found that production was better in a cage system with the tanks used for a fertilization period. We have determined seaweed growth rates, in response to nutrients and water motion. Seaweed growth was limited by nitrogen, but the growth of well-fertilized seaweeds is limited by water motion. We developed polyculture systems in which the seaweed is fertilized in shrimp effluent ditches or in fish tanks and then grown in a nutrient-poor, lagoon environment. Recent experiments along these lines were conducted to improve production of the seaweed from spores.

This year we have started work with fish in the system. We determined ammonia excretion rates of the fish used in the system and have set up growth trials with milkfish. Work on the fish was delayed because of difficulty in obtaining juveniles from the local hatchery. However, we developed methods of collecting milkfish fry from the wild and have transferred them to tanks for growth trials. Preliminary observations of fish growth indicated that milkfish grew much faster in the system than mullet, so the remaining trials will focus on milkfish.

Additional training workshops on the culture of seaweeds and fish were presented at the facilities of Ke Kua’aina Hanauna Hau on Molokai. The workshops combined lectures and demonstrations by scientists from the University of Arizona with hands-on field experience at the project site. This year the attendees were participants seeking training for potential employment with local aquaculture enterprises.

Impacts and Contributions/Outcomes

The protocols developed for a two-phase polyculture system have been successfully employed in the commercial production of seaweeds and fish on Molokai. The seaweed is the red alga Gracilaria parvispora, and the fish being raised are milkfish Chanos chanos. The adopted protocols have resulted in increased production and decreased labor requirements. The project has also generated data useful for selecting sites and arranging cages for seaweed production. Additional information has been generated that will aid in designing a nursery phase in the production system so that the seaweeds can be propagated via spores.

The details of the system are being published as the data are generated and analyzed. The data produced through the research have been presented at international conferences in San Diego, Japan, and Brazil. The project has resulted in two scientific publications so far. In addition, one graduate student at the University of Arizona has completed a thesis based on work from the project. Two additional graduate students from the University of Arizona received aquaculture training from their participation in the project and have co-authored papers.

The local workshops to demonstrate and explain the technical details of the polyculture system were well received. The information from the demonstration project has also generated interest among other growers and has stimulated the development of cooperative arrangements among several small-scale fish, shrimp, and seaweed producers on Molokai.

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