Management effects on water quality, sediments and fish production in extensive fish ponds in the Dombes region, France

In aquaculture, management practices such as supplementary feeding or fertilisation of water are generally considered to improve fish yield in ponds or shallow lakes. Nevertheless, in semi-natural systems where many ponds or lakes are situated in a cultural landscape, this is much less evident for certain fish farmers because fish production systems are often quite extensive, and fish production is only one economic activity among others for these fish farmers. In this paper we analyse the influence of different management practices on fish yield and nutrient status of fish ponds’ water and sediments, and we have an additional regard on potential implications of this in the perspective of the European Water Framework Directive. This directive demands that artificial water bodies such as fish ponds have to attain a good ecological potential in 2015, and thus to adapt water body management to achieve this.In total, 83 fish ponds were studied from 2007 to 2009 in the Dombes region, France. This region is characterised by 1100 nutrient rich fish ponds located in a heterogeneous agricultural landscape with cropping, animal husbandry and forestry. Different water parameters (PO43−, NO3−, total P, total N, NH4+, chlorophyll-a) were analysed from April to October in each year. Sediments were sampled in March and October and analysed for available P, total N, organic matter and Ca concentration. Data about pond management practices such as fertilisation of pond water, supplementary feeding as well as fertilisation and liming of pond grounds when they are emptied and let dried out during a year, and harvested fish were collected by interviewing pond owners and pond managers.The main results found are that the combination of the annual management practices supplementary feeding and fertilisation, increased significantly the fish yields. When combining the annual with the non-annual management practices fertilisation of pond grounds and liming of pond grounds during a year when ponds are emptied, highest yield were obtained. Using only the non-annual practices, yields could be positively influenced. Lowest yields were found when no management practice was applied. Significant, but contrasting effects of pond management practices on water or sediment parameters were only found for available P of sediments and NO3− for the management practices supplementary feeding, fertilisation of water, or liming of the pond ground. Whereas available P of sediments showed higher values with the three practices, NO3− in the water showed lower values. Although only few significant differences were found, means of parameters showed a certain trend as they were in many cases, besides for total N and NO3−, higher with the management practice.Our results show that there is a limited effect of pond management practices on the chemical status of the pond water and sediments. This also suggests a limited potential to change management practices to respond to the demand of the European Water Framework Directive for good water quality and ecological potential.