Intensive production of Artemia franciscana cysts in outdoor ponds
Creators: name: Tyler, J P
Publisher: name: University Of Tasmania
Year: 2023

description: Despite an identified need world wide for reliable supplies of high quality Artemia cysts for aquaculture, the commercial supply of cysts remains exclusively sourced from unreliable and low productivity extensive systems producing cysts of variable quality. An intensive cyst production system for producing high quality cysts in large outdoor ponds was developed and tested at a commercial saltfield in the North West of Western Australia. To assist in the development of this system numerous experiments were conducted to optimise algal production, identify food requirements of Artemia, and spare algal inputs through the use of wheat po/lard. In addition, management strategies were assessed for reducing intraspecific competition in order to increase cyst output. Culture conditions for three species of naturally occurring brine microalgae (fetraselmis suecica, Dunaliella viridis and Dunaliella salina suitable as food for Artemia franciscana, were investigated (Chapter 3). T.suecica grew fastest in seawater (density 1.025 glmL) and was completely inhibited in concentrated brines which are optimal for A.franciscana culture (&gt;1.10 glmL, Chapter 6). D.salina grew best at brine densities &gt;1.10 glmL (Chapter 3.3) and D.viridis in less dense brine (&lt;1.10 glmL). A concentration of 0.01 g/L of soluble fertiliser was found to be sufficient for both Dunaliella species in outdoor culture. T.suecica had lower productivity (cellslmUday) than the two Dunaliella species by about two orders of magnitude. Four brands of commercially available soluble fertilisers were tested and found to be similar in their effects on microalgal productivity. The optimum physical requirements of both species of Dunaliella were similar. Vigorous aeration and peak light intensities above 7, OOO lux at noon were found to be necessary to sustain these cultures. Peak D.viridis populations (&gt;3 million cells per ml) with a sustainable daily harvest rate &gt;40% of culture volume were achieved. The major problem encountered with outdoor Dunaliella culture was grazing pressure from contaminating A.franciscana and the brine ciliate Fabrea salina. The most effective control procedure for these organisms was found to be; initial sterilisation of culture brine with at least 10 ppm of active chlorine, inoculation with zooplankton-free microalgae culture, continuous screening for A.franciscana, and maintenance of optimal conditions for microalgae culture. This brine often contained F.salina which would quickly denude a microalgae pond if allowed to colonise that pond. However, if the brine was added to the A. franciscana pond first, the brine shrimp would effectively eliminate the ciliates before they reached the algal pond. The growth responses and conversion efficiencies (dry weight food/dry weight Artemia), of both solitary and communal A.franciscana, for various amounts of D.viridis and D.salina, were determined in laboratory experiments (Chapter 4.1). Food level had a marked effect on both growth rates and efficiency at all stages from lnstar 1 through to lnstar 16 adults. A minimum of between 200,000 and 500,000 Dunaliella cells per animal per day (0.014 g - 0.035 g dry weight) was required for long term survival of A.franciscana and one million cells per day to enable development through to lnstar 16. Maximum growth rate was achieved with four million cells per animal per day. Growth efficiency decreased with increased feeding level. A feeding rate of between one million and two million cells per animal per day was considered to be an appropriate balance between growth and efficiency enabling full animal development with growth efficiency of about 25%. Under highly competitive conditions with uncontrolled A.franciscana population growth, the population stabilised at a level where each individual received about 500, OOO cells per day. Microalgal cultures are relatively dilute on a dry matter basis, however, this is not a limitation with inputs of dry feeds. Finely ground wheat po/lard flour supplemented with some D.salina culture was an adequate food for laboratory cultured A.franciscana (Chapter 4.2). Apparent growth efficiencies for wheat po/lard were only about 5% of those achieved with a D.salina diet but it was relatively inexpensive. Pollard on its own was also found to be a suitable food for A.franciscana in 1,000 L outdoor tanks, however, supplementation with D.salina (5% of the total food on a dry weight basis) greatly enhanced the biomass of A.franciscana compared to treatments without supplementation. Cyst production relies not just on biomass but on reproductive output and specifically on Artemia adopting an oviparous reproductive strategy. A minimum food ration of two million D.salina cells per individual A.franciscana per day was required for high reproductive output. This was difficult to sustain in a communal population due to population increase and resultant increase in intraspecific competition. A number of strategies to limit population expansion were trialed. Selective screening was the one practical technique that was effective in limiting population expansion and maximising reproductive output at a peak of about 25 offspring per female per three day reproductive period. A mesh size of 1 mm or smaller was required to retain physically small broodstock (&lt;1 cm length) in a system with high flow rates. A screen size as large as 2 mm would retain larger adults (&gt;1 cm length) ifthe flow rates were very low (&lt;500 Um2 /hour). The variety of factors that could conceivably affect the mode of reproduction were investigated (Chapter 5). Laboratory cultured animals tended to be ovoviviparous and animals in outdoor culture oviparous but the switching mechanism was not identified despite extensive experimentation. Food level, salinity, salinity-shock, light intensity and oxygen stress in the presence of iron were all tested and no clear patterns emerged in either laboratory or outdoor trials. Reproductive mode in the 0. 08 ha experimental pond was primarily oviparous. The above data were used to design an operational large-scale facility for the production of A.franciscana cysts (Chapter 6). Major practical problems encountered in the development of this facility included unreliability of electrical equipment in a saltfield environment, inadequate sealing of screens separating the D.salina pond from the A.franciscana pond and, sustaining the population of D.salina for long periods. These problems were all largely overcome and cyst production levels in excess of 1 kg per day were achieved from a 0. 08 ha pond for several weeks.