A Simple Experiment in Polyculture: Red Sea Bream (Pagrus major) and Ulvales (Ulva pertusa)

Abstract

Mixing fauna and flora in culture cages is a fundamental approach for attaining homeostasis of ecosystems in aquaculture farms. We conducted an experiment to explore the benefits of culturing fish as fauna and seaweed as flora within single, open water, aquaculture cages. The aim of this study was to examine the effect of polyculture on the oxygen and carbon dioxide concentration of the water in the polyculture cage and on the growth, survival, feeding efficiency, and condition factor of the fish in the polyculture cage (Cage A, which contained red sea bream {[}Pagrus major] and a green alga {[}Ulva pertusa]) versus these measures in a monoculture cage (Cage B, which contained red sea bream only). The experiment was conducted from August 1992 through February 1993. The two cages each were 7 m x 7 m x 7 m. The red sea bream used in this experiment were three years old, had an average body weight of 654 g at the beginning of the experiment, and were the product of selective improvement for more than 25 years. Each cage contained 1750 fish. The alga was a variety of U. pertusa that had been improved by selection every two weeks for two years. The U. pertusa was Cultured in floating net bags at the surface of Cage A and harvested every two weeks. The best-growing plants in the harvested group were used to re-inoculate Cage A. The remaining algal fronds were dried, crushed, formulated into pellets with standard red sea bream fish food, and fed back to the fish in Cage A. The fish in the control cage were fed the standard red sea bream fish food. The partial pressure of dissolved oxygen (pO(2)) and of carbon dioxide (pCO(2)) of the water within the cages and the growth of the fish (wet weight, standard length) were measured. Average pO(2) was significantly higher (9.0%) and average pCO(2) was significantly lower (5.2%) in the polyculture cage than in the control cage. Nearly all fish in both cages survived throughout the experiment. The average growth, average feeding efficiency, and average condition factor of the fish in the polyculture cage were, respectively, 3.2% higher, 6.0%, higher, and 2.2% higher than those factors were in the fish in the control cage. In addition, the polycultured fish were redder in color (and, therefore, were more marketable) than were fish in the control cage. This simple project suggests the potential benefits of this particular type of polyculture in Japanese marine waters and, from a broader perspective, of algal/fish polyculture in general.