Trophic interactions and productivity of copepods as live feed from tropical Taiwanese outdoor aquaculture ponds

The present study describes three outdoor ponds for mass rearing of copepods in tropical southern Taiwan. The systems are designed for culturing and harvesting of copepods, which are used as live feed in the production of groupers in the region. However, the production of the most common copepod species Pseudodiaptomus annandalei (Sewell 1919) has, according to the pond managers, decreased over the last 10 years for no apparent reasons. In order to understand the limitations in terms of production, the present study was carried out. A one month comprehensive monitoring of abiotic factors, inorganic nutrients, phytoplankton and zooplankton as well as the spatial patchiness of zooplankton in the ponds was conducted. The ultimate aimswere to: (I) obtain a better understanding of the variability and the interactions among the trophic levels in the pelagic; (II) obtain a sampling practice to enable a correct description of the copepods ecology and a preliminary evaluation of the status of the pond management; and (III) provide advices for improved management leading towards a higher and more stable output of P. annandalei. Copepods were experiencing prey ad libitumsince the quantity of phytoplankton available in the ponds was high (value of chlorophyll a in average 97.7 +/- 10.9 mu g L-1) and dominated by diatoms, flagellates and pico-algae. The estimated abundance and biomass of adult copepods and copepodites of P. annandalei was on average 93 +/- 40 individuals L-1 and 214 +/- 98 mu g L-1, respectively. P. annandalei's specific growth rate in the ponds, thus the secondary productivity, was 0.89 +/- 0.1 d(-1) (average +/- S.D.). The average harvest (21 kg of copepods wet weight per pond every second to fifth day), was a relatively low quantity to satisfy the demand of the fish farmers, especially during high season. This harvest, in terms of biomass, corresponded to 20-40% of the copepod standing stock, which indicates a sustainable management when compared to the secondary productivity of the copepods. To further increase the copepod production of the ponds, better water quality must be achieved. Moreover, improved harvesting techniques are recommended. (C) 2015 Elsevier B.V. All rights reserved.