A. Kamstra

Performance and optimisation of trickling filters on eel farms

The design of trickling filters used on commercial eel farms differs considerably with respect to dimensions, hydraulic and substrate loads and filter medium applied. In this paper, a model, developed for ammonium removal in a pilot-scale trickling filter, has been validated for a range of full-scale trickling filters, incorporating the above mentioned aspects. The performance of trickling filters was assessed at 14 commercial eel farms in The Netherlands. Analysis of the data obtained has shown that the model sufficiently explains and predicts the ammonium removal rate of a wide range of commercial trickling filters. The model is used to optimise performance of existing biofilters in terms of ammonium levels achieved and pumping costs. Observations have also been made on nitrite in recirculation systems. Concentrations of nitrite and the ratio between nitrite and ammonium vary widely in and between systems. The nitrite oxidation capacity in biofilms seems to be variable and sensitive to environmental disturbances.

The effect of size grading on individual growth in eel, Anguilla anguilla, measured by individual marking

Abstract Effects of size grading on growth in European eel were measured using individually marked eels. The mark was applied by burning and was tested in a separate aquarium experiment. No significant effect of marking on growth and survival could be detected. The return of marked individuals in the grading experiment was 97% on average. Size grading did not have a significant effect on total biomass output or size-frequency distribution. Individual growth rates were not affected by size separation and showed a distribution close to normality.

Estimating breeding values with molecular relatedness and reconstructed pedigrees in natural mating populations of common sole, Solea solea.

Captive populations where natural mating in groups is used to obtain offspring typically yield unbalanced population structures with highly skewed parental contributions and unknown pedigrees. Consequently, for genetic parameter estimation, relationships need to be reconstructed or estimated using DNA marker data. With missing parents and natural mating groups, commonly used pedigree reconstruction methods are not accurate and lead to loss of data. Relatedness estimators, however, infer relationships between all animals sampled. In this study, we compared a pedigree relatedness method and a relatedness estimator (“molecular relatedness”) method using accuracy of estimated breeding values. A commercial data set of common sole, Solea solea, with 51 parents and 1953 offspring (“full data set”) was used. Due to missing parents, for 1338 offspring, a pedigree could be reconstructed with 10 microsatellite markers (“reduced data set”). Cross-validation of both methods using the reduced data set showed an accuracy of estimated breeding values of 0.54 with pedigree reconstruction and 0.55 with molecular relatedness. Accuracy of estimated breeding values increased to 0.60 when applying molecular relatedness to the full data set. Our results indicate that pedigree reconstruction and molecular relatedness predict breeding values equally well in a population with skewed contributions to families. This is probably due to the presence of few large full-sib families. However, unlike methods with pedigree reconstruction, molecular relatedness methods ensure availability of all genotyped selection candidates, which results in higher accuracy of breeding value estimation.