Nematode community response to fish-farm impact in the western Mediterranean.

A previous investigation on fish-farm biodeposition effects on benthos, carried out in the Gaeta Gulf (northwestern Mediterranean Sea), revealed a strong impact on meiofaunal assemblages. This study implements these findings by examining in detail the nematode assemblage and its response to organic enrichment from the start of a fish farm activity to the conclusion of the fish rearing cycle. Density, community structure and individual size were utilised for univariate (genus, trophic diversity and abundance patterns) and multivariate analyses (MDS) in order to identify the best descriptors of impact and the response of the nematode assemblages. Nematodes displayed significantly reduced densities, diversity and richness in sediments beneath fish farms. The impact of biodeposition was evident not only from structural community parameters but also in terms of functional indices. Forty-five days after starting fish farming, an increase of the nematode individual biomass was observed. MDS ordination pointed out the presence of two different nematode communities in disturbed sediments and in the control site. These results were substantiated by the analysis of the temporal changes of k-dominance curves, the maturity index and, to a lesser extent, by the index of trophic diversity. Some nematode genera were highly sensitive to biodeposition (Setosabatieria, Latronema and Elzalia) and disappeared almost completely in farm sediments, whereas other genera largely increased their dominance (Sabatieria, Dorylaimopsis and Oxystomina). This study indicates that nematodes are very sensitive to this kind of environmental disturbance. The use of simple tools, such as the k-dominance analysis and maturity index, are recommended for monitoring of aquaculture impact.