Mass removal fishing has been applied in Finnish lakes as a means of reducing eutrophication caused mainly by nutrient leaching from agriculture.
Composting in windrows was studied as a way of making use of the surplus fish and returning the nutrients to agricultural or horticultural use.
Fish compost is of potential interest for ecological farmers as a nitrogen-rich fertilizer.
Different bulking agents (peat, mixtures of peat and straw or peat and reed, Phragmites australis) and different fish concentrations were tested in passively aerated windrow composting on a practical scale.
Nutrient leaching losses were measured during composting and fertilizer effects were studied in a two-year field experiment on onion (Allium cepa) and potato (Solanum tuberosum).
The thermophilic process started within a few days, which prohibited malodours from decomposing fish.
After a composting period of four weeks, at which point fish were totally decomposed, the heaps overwintered without being turned.
Some leakage of nutrients, mostly nitrogen, from the composts could be observed, but not to an extent that harmful effects on groundwater could be expected.
The loss of nutrients showed a growing tendency with increasing fish concentration.
Fish compost showed no phytoinhibitory effects in the field trials.
The heavy metal contents were below the limits set for agricultural use, even for ecological farming in Finland.
The fish composts were characterized by high contents of soluble nitrogen, which was clearly connected to the yield effect of different compost variables.
Potassium contents were low, especially in composts containing only fish and peat.
Composting in windrows was concluded to be a noteworthy alternative to handling surplus fish and returning nutrients from eutrophicated lakes back to agriculture.
With suitable mixtures of fish and bulking agents it is possible to conserve the nutrients and produce an effective organic fertilizer of good structure, in which an important part of the nutrients is in soluble form.
To avoid nutrient leakage, the fish content in the compost mixture should not exceed certain limits which depend on the used materials and the composting conditions.
The lack of potassium observed in the fish can be