AQUACULTURE

High density tadpole rearing: Tadpoles were reared at high densities, with a high protein diet to satiation, and frequent water changes. High density rearing resulted in the very rapid growth of tadpoles to quality metamorphs. This protocol could be applied to the raising of tadpoles for reestablishment programs or consumption. Metamorphs could also be reared in a smaller area, in less time and at less cost.

Stock: The endangered Australian hylid the Green and Golden Bell Frog (GGBF, Litoria aurea) grows to about 100 mm snout vent length. GGBFs are aquatic and feed voraciously, predating on other frogs, small snakes, spiders, insects and snails etc. In their life history GGBFs share many traits with commercially raised Ranid frogs such as the Bullfrog (Rana catesbeiana) and the Iberian frog (R. perezi). This particularly applies to tadpoles.

Spawning: Adult frogs were kept in a glass tank (120 x 60 x 70 cm), with a pool for one third, and a raised dryish area with plants and logs for the other two thirds. A reptosun fluorescent provided balanced light, and mild aeration kept the pool fresh. A pair spawned naturally (5500 eggs). GGBF eggs are laid in loose strands which sink after about half an hour. The jelly mass was removed with an aquarium net and placed in a 20 L bucket with light aeration. Almost all the ooctes developed into larvae after about a day. The hatchlings were kept in the bucket for a week then placed in trays and fed grass infurusia. Grass infurusia was generated in a 20 L aerated bucket and fed with a moderate amount of grass clippings. The temperature of all tadpole rearing was 21 degrees centergrade.

Densities: The tadpoles were kept in trays (45 x 30 x 4 cm) at four densities with two replicate trays per treatment the highest being 160 L. Water was changed every 12 hours and ground trout feed was fed liberally.

Densities: This tray shows the discontinued density of 300 tadpoles per liter at 40 days old. This density was discontinued because mortality due to the accumulation of metabolites between water changes. Water changes would have to have been made more frequently than at 12 hours intervals for the tadpoles to survive at the feed rate.

Large scale production: Larger trays (45 x 80 x 10 cm) were trailed with 500 tadpoles per tray but at lowered feed levels. This system worked in terms of survival. However, two feedings per 12 hours and more frequent water changes would increase the growth rate and reduce the period of metamorphosis. For practical production a shelving system was built from 18 mm water resistant construction plywood. The shelves and supports were made to fit the large trays. A shelving system three trays wide and five trays high could easily produce 3000+ metamorphs a month.

Measuring tadpoles: Netted tadpoles were measured for weight as a group at the time of water change. There was no stress or damage to tadpoles in a moist tray, and at this time they can be examined or measured. Immediately water was added the tadpoles would begin feeding. These tadpoles are about 75 mm in total length.

Scoliosis: In the low density trays same tadpoles developed a bending of the backbone. All but the most extreme examples such as this tadpole developed as normal juveniles. The reason for scoliosis in tadpoles is unknown but in this case was not due to nutritional deficiencies, but perhaps due insufficient movement.

Measuring metamorphs: The metamorphs ( front limb emergence) were measured from the snout to a line between the back of the thighs (snout vent length), and the tail was measured from this this point to the end of the tail (tail length). The SVL was about 27 mm and tail length 65 m, weight 3 g.

Metamorphs rearing: The metamorphs were then placed in a rearing box. This consisted as a 120 x 60 x 60 cm nylon box with hinged lids at the top. Ventilation was provided through fly wire covered hole in the lids. The box was sloped forward with the back higher portion covered with a mound of gravel. The lower front of the box was filled with water. As the metamorphs developed they would move onto the gravel. Food was suppled as 10 mm mealworms.

Measuring juveniles: This juvenile is about 37 mm long. Another study measured the growth rate of these juveniles from this stage on at three temperatures 15, 22 and 28 degrees centregrade (in review).