Copepods as live feed nutrition for fish larvae – a natural choice for aquaculture?
| Autor: | Thomas Allan Rayner, 駱亞倫 |
|---|---|
| Rok vydání: | 2018 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 106 As aquaculture continues to grow globally as a relevant protein source, more attention is given towards improving live feed nutrition. While much effort has been put into improving nutrition in traditional live feeds (rotifers and Artemia) through enrichments techniques, interest in the use of copepods exists among aquaculture researchers. This is mainly due to copepods being a typical component in marine fish larval diet in nature, and natural copepods tend to have a nutritional profile that is more optimal for fish larval survival and growth without the need for enrichment techniques. The most discussed topic regarding nutrition in fish larvae is fatty acids, which has become a determining factor for nutritional quality of live feed in aquaculture. Free amino acids (FAA) are also deemed important, although fatty acids are the primary topic when discussing nutritional quality in live feed. This is due to a handful of essential fatty acids for fish larvae, and the ratios between them, have been identified. The aim of this thesis was to investigate how natural copepods produced under extensive and open production methods could provide a steady supply of quality live feed. The understudied calanoid copepod Pseudodiaptomus annandalei became the main focus, as it is an important live feed subject in Taiwanese aquaculture. Taiwan is one of the few places that have made copepods a component of fin-fish rearing. A total of six studies were carried out. The first two (Study I and II) describe the production of zooplankton in a tropical pond system in Southern Taiwan. The first study focuses on describing the biochemical profile of the dominant zooplankton in the ponds, while the second described the pond productivity and quality over an annual production season. The ponds were open earth-dug ponds of around one hectare in size with a depth of one metre. These ponds were warm (20-32°C) and contained very large quantities of phytoplankton (indicated by chlorophyll a values above 70 µg L-1). The zooplankton assemblage was low in diversity with the copepod P. annandalei dominating the zooplankton assemblages for the whole annual production season. The abundance of copepods observed in the ponds was relatively low at 215 ± 208 ind. L-1 throughout the year. As P. annandalei was the main pond organism, a laboratory trail (Study III) was performed in order to determine whether or not copepod density affected their reproductive performance. In the third study, the number of ovigorous females was determined in samples with increasing densities. The highest abundance of ovigerous P. annandalei was observed at 385 ind. L-1. It is argued that P. annandalei may be limited by surface area rather than volume, as they tend to cling to surfaces similar to harpacticoids. The quality of fatty acids in pond grown P. annandalei was good with adequate amounts of C22:6 n-3 (DHA) observed in all copepod stages. In fact, changing quality of seston available to the copepods did not result in an undesirable fatty acid profile in P. annandalei. Further investigation into how DHA abundance was consistently high in P. annandalei was pursued. This resulted in Study IV on possible DHA synthesis in P. annandalei similar to what has been observed in harpacticoids and cyclopoids. Here, P. annandalei adults and nauplii were grown on a DHA-lacking algae (Tetraselmis chuii), and their growth and fatty acid profiles were analysed. The copepods managed to grow at a decent daily rate (0.31) without any significant mortality. Respective DHA contents at around 8 and 4 mg g-1 dry weight for P. annandalei adult and nauplii were observed in spite of no DHA being detected in the algae. It is therefore not surprising that pond-grown P. annandalei are so rich in DHA. For determining the quality of FAA in the copepods, an evaluation on the FAA requirements in fish larvae was conducted in Study V. Data on the FAA decline in fish embryos and larvae were acquired through the literature and regression models were applied. A Q10 coefficient of 2.61 ± 0.26 was estimated and applied as a correction factor in order to achieve a rate of decay of total FAAs common for all twelve fish species involved in the study. It is hereby possible to estimate the number of copepods needed as live feed to cover the FAA requirements of the fish larvae, given that the FAA profiles are similar. Changes in FAAs in lab-grown Acartia tonsa copepods during naupliar development were observed in Study VI. Total content of FAA in A. tonsa nauplii was around 17% of dry weight at the first development stage, and declined to 6% for later stages. This suggests degradation of residual FAAs from the embryonic stage. Glutamic acid was the dominant FAA in newly hatched nauplii. A mismatch in FAA profiles appears when comparing the FAA profile of the lab-grown A. tonsa nauplii with the common FAA profile for fish embryo in Study V. On the other hand, when comparing pond-grown P. annandalei nauplii (Study I) a more suitable match occurs. This may be due to the mono-diet given to A. tonsa as opposed to P. annandalei’s mixed pond diet. Overall, pond-grown P. annandalei have demonstrated that they can be considered quality live feed even when produced using simplistic production methods. |
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