Effect of greenwater culture on Greenback Flounder (Rhombosolea tapirina)

Brief description

This data represents research conducted as part of a PhD project on Greenback flounder (Rhombosolea tapirina). Larvae were reared in either green water (Tetraselmis suecica) or clear water tanks during short-term feeding trials to investigate the effect of larval culture history, live prey density, and turbidity level on feeding performance.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Methods are divided into 2 sections: larvae rearing and feeding experiments (for more detailed methodologies, see Supplemental Information): 1. Larval rearing Broodstock greenback flounder (R. tapirina) were housed at the School of Aquaculture Aquatic Centre in two recirculation systems. Each system comprised a 4000 l culture tank and a 300 l sump containing a trickle biofilter. Broodstock flounder, maintained at 10.5 °C and a photoperiod of 10:14 LD (lights on at 0700 h), were either allowed to court and spawn naturally (cohorts 1 and 3), in which case eggs were collected on a submerged 280 μm sieve attached to the outlet drain above the biofilter, or eggs and sperm were stripped from ovulated and spermiated broodstock and fertilised artificially (cohort 2). Eggs were collected and transferred to the larval rearing room, allowed to acclimate over 2-4 h to the water temperature of 12±0.5 °C, and transferred to a 100 l egg incubation upweller. Eggs were incubated with vigorous aeration and eggs hatched 4 (cohort 3) or 5 (cohort 1 and 2) days post-fertilisation. Immediately after hatching larvae were harvested, divided equally and stocked into two larval rearing recirculation systems, each comprising two 270 l rearing tanks and a 200 l sump containing a trickle biofilter. One recirculation system was maintained with green water (Tetraselmis suecica) at a turbidity ranging from 3-5 NTU throughout the larval rearing period and the other was clear water with turbidity <0.05 NTU. Larval rearing systems were maintained with low aeration and water flow of 1 l min-1, until 19 days post-hatching (dph) at which stage water flow was increased to 3 l min-1. Air conditioning in the larval rearing room maintained tank water temperatures. Salinity on experiment days was constant between treatments but varied across days from 32-35, pH ranged from 7.98-8.14 and water changes were made using 1 -m filtered seawater to maintain total ammonia levels below 0.2 mg l -1. Larvae were maintained at a photoperiod of 16:8 L:D and light intensity at the tank water surface ranged from 6.72-8.54 micromol s-1 m-2 for all cohorts. 2. Feeding experiments - general methods Photophase, air temperature and light intensity were controlled for feeding experiments. Each gut evacuation tank was provided with aeration and larvae were left undisturbed overnight to digest prey already consumed. The following morning 10 larvae from each gut evacuation tank were sampled as 'gut evacuation controls', placed on a frozen glass slide to induce hypothermic anaesthesia, and the digestive tract of each larva was examined under a compound microscope. Mastax/ trophi structures of rotifers were still evident in the digestive tract of larvae, thus, in subsequent feeding trials only rotifers that still contained both a lorica and mastax structure were counted. A further sample of approximately 30 larvae was taken from each tank for length determination. Turbidity for all experiments was provided by T. suecica and measured daily using a Hach 2100P portable turbidimeter. Turbidity measurements were made using water that did not contain rotifers or Artemia. Once gut evacuation assessment was completed, the remaining larvae were transferred to the prepared test aquaria and left undisturbed for 30 min in order to acclimate to the new environments. The number of larvae transferred to each aquarium differed according to larval age and the prey density used in each experiment. For prey density experiments (Experiments 1 and 2) three larvae were added to each aquarium to ensure that some prey remained even at the lowest prey density tested. Gut contents of larvae 9-20 dph were assessed by first pipetting larvae from each test aquarium onto a frozen glass slide, covering them with a cover slip to disclose the gut contents, then counting the number of prey consumed by each larva. Larvae were then measured (total length), their digestive tract removed, and the number of Artemia in the digestive tract counted. Experiment 1: the effect of rotifer density on the feeding performance of larvae from different culture environments. Larvae 12 and 18 dph from cohort 3 were fed the rotifer B. plicatilis at one of 6 different densities, 0.025, 0.05, 0.1, 0.5, 1 and 5 ml-1, for 60 min. There were 5 replicate aquaria per treatment each containing 3 larvae. Larvae were fed in the same environment as their original culture tank; i.e. green-water (5 NTU) or clear water (0 NTU). Experiment 2: the effect of Artemia density on the feeding performance of larvae from different culture environments. Larvae 25, 31 and 39 dph from cohort 3 were fed Artemia nauplii at 0.01, 0.025, 0.05, 0.1, 0.5, and 2.5 ml-1 for 10 min. There were 5 replicate aquaria per treatment each containing 3 larvae. Larvae were fed in the same environment as their original culture tank; i.e. green-water or clear water. Experiment 3: the effect of prior culture history on feeding performance of larvae fed rotifers at 1 ml-1 in a range of algal cell induced turbidities. Larvae 9, 16 and 20 dph from cohort 1 in a range of algal cell induced turbidities (0, 10, 20 and 40 NTU). Larvae were fed B. plicatilis at a density of 1 ml-1. There were three replicate aquaria per treatment, each containing 20 larvae. Prey ingestion and digestion rates increased with increasing age of larvae and so as to ensure the accurate counting of rotifers it was necessary to sequentially reduce the feeding period with increasing larval age. Experiment 4: the effect of prior culture history on feeding performance of larvae fed Artemia at 1 ml-1 in a range of algal cell induced turbidities. This examined the effect of culture history on the feeding performance of 31 and 38 dph larvae from cohort 1 in a range of turbidities from 0 to 40 NTU fed Artemia nauplii at a density of 1 ml-1. Experiment 5: the effect of prior culture history on feeding performance of larvae fed rotifers at a prey density of 0.1 ml-1 in a range of algal cell induced turbidities. This examined the effect of culture history on the feeding performance of 10, 14 and 18 dph larvae from cohort 2 in a range of turbidities (0, 5, 10, 15 and 20 NTU) fed B. plicatilis or B. rotundiformis (10 dph) at a lower density of 0.1 prey ml-1. At a prey density of 0.1 ml-1, higher feeding in a treatment could significantly reduce prey density and therefore affect subsequent feeding within that treatment. Thus, the number of larvae added to each aquarium was be reduced to 3 to ensure that the prey density remained above 0.08 ml-1 (80% of initial stocking density) for the duration of the feeding period. Accordingly, the number of replicates in each combination of culture history and turbidity treatment was increased to 5.

Notes

Credit
Cooperative Research Centre for Sustainable Aquaculture of Finfish

Credit
Australian Government CRC Program

Credit
Fisheries Research and Development Corporation (FRDC)

Purpose
To improve techniques for successful culture of greenback flounder larvae (Rhombosolea tapirina).