Vermicompost for reduction of vegetable waste - and a possible means to produce fish feed in aquaponic systems?

Sammanfattning: Aquaponics is a food production system consisting of a consecutive cultivation of plants and aquatic animals, fish or shellfish, in recirculating water. The plants feed on the nutritious effluents from the fish tank and bacteria converts ammonia from the fish tank into nitrate that is absorbed by the plants. The plants receive nutrition and the fish gain purified water. The environmental impact of the production of fish meal and fish oil used in conventional fish feed includes a large consumption of fossil fuels with subsequent carbon dioxide emission. The aquaponic system can be made environmentally sustainable and self-supporting if supplemented with a vermicompost/vermiculture for the production of worm protein as a fish feed. Vegetable waste from the hydroponic part of the system can be reused as worm feed in the compost and the protein rich worms can be harvested, dried and grinded and brought back to the aquaculture as fish feed. As a first step in this direction, I have investigated the effect of recycling of vegetable waste in vermicomposts on the growth and number of earthworms (this was evaluated after harvesting). The hypotheses were that earthworms can be cultivated in the vegetable waste and that by adding manure to the vermicompost it is possible to obtain a nutrient content adequate to support a continuous worm harvest, in other words create a vermiculture. Eisenia fetida and Dendrobaena veneta were grown in 20 L bins containing peat mixed with either poultry or cattle manure. The proportions were 70% peat and 30 % manure. Discarded plant parts from lettuce cultivation was added to the vermicomposts (35 g per week) and water was supplied with 0.8 L per week. The bins were placed in a climate controlled greenhouse chamber with a constant temperature of 25.0°C. The experiment was two factorial with the factors worm species (two types) and types of manure (two types) resulting in four treatments. Five replicates of each treatment were used. The experiment lasted four months and was assessed once at the end of the experiment. Temperatures within the composts were constant throughout the entire experiment (25.0°C). Value of pH ranged between 4.6-5.3 in the composts, with no significant difference between treatments. Moisture content varied between 62-94 % in the compost substrate, with no significant difference between treatments. C/N ratios in the substrates were relatively high at the start (C/N: 37-45) and dropped in all treatments to C/N: 33-40 during the experiment. The results showed that the number of worms increased only in the E. fetida with poultry manure treatment. Number of worms were significantly higher in the E. fetida treatments compared to the D. veneta treatments but the worm weights had decreased substantially in all treatments. No significant difference in number of worms or worm weight was found between the two types of manure. The results did not support the hypotheses but may indicate that choice of earthworm species in the vermicomposts might have greater influence on the outcome in terms of worm production than the choice of manure added to the vermicomposts.