Duckweed as fishmeal-protein replacement for omnivorous and carnivorous fish fry

Summary

Duckweed are fast growing floating aquatic plants with the potential of higher protein productions per unit time and area than most terrestrial plants including soy beans. Liquid animal manures and slurries can efficiently be recycled with high N and P uptake efficiencies of duckweed. As protein feedstuff, duckweed has successfully been used for a variety of animals, including pigs, poultry and fish. Here, we present the results of three feeding studies utilizing graded duckweed (Spirodela polyrhiza) concentrations as fishmeal-protein replacement for fry of common carp (Cyprinus carpio), Eurasian perch (Perca fluviatilis) and rainbow trout (Oncorhynchus mykiss), all of them important aquaculture species. Two different duckweed types, dried and fermented, were utilized to replace up to 45% of fishmeal-protein in carp and up to 35% in trout and perch. Fish were kept in a recirculating system in four replicates per treatment and hand fed for four weeks. Once a week the fish were weighed to adapt the feed allowance. Evaluated effects included changes of whole body chemical composition, growth performance, feed, protein and lipid utilization. Results were analysed by one-way ANOVA and subsequent Tukey HSD post-hoc test. Duckweed has been well utilized by carps and surprisingly also by rainbow trout with replacement levels up to 45% in dried and 30% in fermented duckweed for carp fry and 24% of dried and fermented duckweed for trout fry without significant reductions in performance.
Perch fry, however, showed even at lowest duckweed concentrations of either dried or fermented duckweed significant reductions compared to control fish with stronger effects for fermented duckweed. Our results show, that for the omnivorous carp and for the carnivorous rainbow trout a certain replacement of fishmeal-protein is possible even for fry, the most sensitive life-stage. For Eurasian perch fry, however, duckweed is not a suitable source of protein.