Copepods as live food organisms in the larval rearing of halibut larvae ( Hippoglossus hippoglossus L.) with special emphasis on the nutritional value

Abstract

In the natural food web, zooplankton constitutes a major part of the diet for marine fish larvae and it is generally believed that copepods can meet the nutritional requirements of fish larvae. In this study, different copepod species were analysed for total lipid content, fatty acid and protein content and used in first feeding experiments with halibut larvae ( Hippoglossus hippoglossus L.) together with enriched nauplii and juvenile stages of Artemia franciscana. Special attention was paid to the DHA content in both the live food organisms and the fish larvae. Copepodid and adult stages of the marine copepods Temora longicornis and Eurytemora sp. had a total lipid content varying between 7% and 14% of dry weight (DW). Copepodid stages I, II and III of Calanus finmarchicus had a lipid content varying between 10% and 19% of DW, increasing to 22–25% in copepodid stages IV, V and the adult stages. The predominant fatty acids of all copepods were docosahexaenoic acid (DHA; 22:6 n−3), eicosapentaenoic acid (EPA; 20:5 n−3) and the saturated fatty acid 16:0. In T. longicornis and Eurytemora sp. DHA constituted 26–42%, EPA 15–24% and 16:0 8–12% of total fatty acids. In C. finmarchicus, the DHA content ranged between 21% and 32.5%, whereas the content of EPA and 16:0 was 15–21% and 9–15% of total fatty acids, respectively. The sum of n−3 HUFA was highest in T. longicornis and Eurytemora sp. (55–62% of total fatty acids) and lowest in C. finmarchicus (38–47%). The loss rate (% day −1) of the total lipid and fatty acid content during starving conditions was 18 and 24% day −1 in T. longicornis, and 12 and 16% day −1 in Eurytemora sp., respectively. In both copepods, the relative content of DHA (% of total fatty acids) increased with time during starvation, reaching 43% in T. longicornis and 49% in Eurytemora sp. The quantitative content of DHA decreased steadily although at a lower rate than that of other fatty acids. The protein content of various copepods varied between 52.4% and 57.6% of dry weight (DW), and was significantly higher than in A. franciscana (41% in newly hatched nauplii and 34% after 24-h enrichment). There was a close relationship between the percent DHA content in 28–36-day-old halibut larvae and the live food organisms they were fed on. High percent DHA content in the live food organism resulted in a high percent DHA content in the fish larvae and vice versa. The relatively high DHA content in the larvae was significantly reduced after 8 days when given enriched A. franciscana. For the rest of the feeding period, the DHA content decreased steadily when fed on A. franciscana, whereas it remained at the initial high level in larvae-fed copepods.