Arndt, Carmen (2013) Testing the suitability of harpacticoid copepods as food for marine fish larvae. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 113 pp.

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Abstract

While capture fisheries have been stagnating since the 1990’s, aquaculture production has increased steadily from being negligible in the 1980’s to sharing now 40% of the world fish supply. This increase entails a growing demand of fingerlings which results in an intensification in rearing fish larvae. Intensive rearing systems imply exogenous feeding which in turn led to a research activity to find food for fish larvae which is nutritious and can be produced cost effectively, since food is the highest cost factor in aquaculture production. Brachionus and Artemia are nowadays the normally used prey items for fish larvae. They can be produced in high densities with relatively low labour costs. However, they have to be enriched, e.g. with certain fatty acids, to fulfil the nutritional requirements of fish larvae and are still outperformed by copepods which are the natural food of marine fish larvae. Fish larvae fed with copepods show higher survival and growth rates as well as fewer malpigmentation and malformations. For this reason, this study focused on the rearing of copepods, especially harpacticoids as food for marine fish larvae. Harpacticoids can be reared in higher densities than calanoids and are more robust to salinity and temperature changes, which promise an easier and a more cost effective production. A copepod screening revealed Tachidius discipes as a possible new species as food for marine fish larvae. In the first chapter the effect of algal species on the growth performance, reproduction and fatty acid composition of T. discipes was investigated and compared to Tisbe sp. Additionally, the food saturation density was determined to allow an economical feeding procedure of the copepods. The two algal species Rhodomonas sp. and Phaeodactylum tricornutum revealed the best performance of both copepod species meeting the recommended ratio of essential fatty acids. T. discipes can compete with Tisbe sp. in terms of development time and reproduction when feeding on a nutritious food source, but Tisbe sp. had a higher fatty acid desaturation capacity and/or is a more opportunistic species which can compensate an inadequate food source by switching to other sources such as bacteria compared to T. discipes. Nevertheless, T. discipes was evaluated as a new food source for Baltic herring larvae (Clupea harengus) and compared with Brachionus plicatilis (Chapter 2). A surprisingly low performance of the herring larvae fed with T. discipes led to an in-vitro trypsin digestibility test of several prey types used in aquaculture. This study revealed a lower digestibility of T. discipes than Tisbe sp. The most digestible prey item was Artemia sp. Subsequently, a 2D-video analysis was conducted to investigate if the benthic living mode of harpacticoid copepods is posing a problem for pelagic fish larvae in terms of feeding success and energy demand (Chapter 3). Benthic copepods are obviously detected by pelagic fish larvae, but the larvae had a lower feeding success when feeding on copepods compared to B. plicatilis, especially at first feeding. However, this improved with ontogeny. Providing harpacticoid copepods via a floating sieve improved the feeding success and lowered presumably the energetic expenditure of fish larvae. In conclusion, harpacticoid copepods can be a valuable food source for marine fish larvae, but their digestibility is a critical point, which should be considered when evaluating new species and conducting further feeding experiments. Furthermore, although harpacticoid copepods are cultured in relatively high densities, they do not reach the densities obtained with B. plicatilis. Consequently, copepods will be a food supplement rather than the sole food source.

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