Optimising the culture environment for early juvenile pot-bellied seahorses Hippocampus abdominalis Leeson, 1827 (Teleostei : Syngnathidae)

The general aim of this study was to address questions on early juvenile Hippocampus abdominalis husbandry posed by research and commercial ventures. Experiments assessed the effect of tank colour, temperature, salinity, stocking density, photoperiod and substrate on survival, growth and Artemia ingestion of cultured juveniles. Seahorses are visual feeders; experiments tested the effects of eight background colours (black, blue, green red, orange, yellow, white and clear) and three photoperiods on prey intake and growth. Ten minute observations and growth trials indicated no significant differences among treatments. Seahorses improved growth in 16:08 (L: D) compared to constant light and 08:16 (L:D). Seahorses under continuous light did not improve growth despite continuous feeding opportunity. A subsequent study on adults indicated that H. abdominalis produces elevated levels of plasma melatonin during the scotophase and low levels in the photophase. The aquarium trade in seahorses is primarily focused on tropical species. To assess the adaptability of H. abdominalis to tropical conditions, the effect of four temperatures (17, 20, 23 and 26 °C) on juvenile survival and growth was investigated in two 6-week experiments. Seahorse growth was higher at 20 and 23 °C than 17 °C while 100% mortality occurred at 26 °C. The availability of space in seahorse culture depends on the availability of the attachment substrate in addition to free tank space used during swimming and forgaing. The effect of different stocking-densities and substrate preferences on H. abdominalis was examined. Four stocking densities (45, 30, 15 and 5 juveniles 3 `1^-1`) were tested on newborns over six weeks. A second experiment aimed to remove the mortality effect experienced during the first experiment using 21-day-old fish to test three stocking-densities (25, 15 and 5 juveniles 3 `1^-1`). There were no significant differences between treatments in both trials. Juveniles were provided with three choices in substrate diameter (0.17, 0.55, 0.90 mm) and mesh-density (5, 10 and 24 mm in bar-length, giving high, medium and low mesh density). Newborns and 28-day-old seahorses displayed preference for larger diameters and low mesh-density. Commercial facilities culturing H. abdominalis occasionally experience reduced salinities during seasonal rainfall runoff. To determinate the adaptability of this species to low salinities, juveniles were direct- and gradual-transferred from 32 ppt to salinities do iii to 5 ppt. Juveniles grew and survived in a range of 32 to 15 ppt while 5 ppt produced 100 % mortality. From the study, the environmental conditions which potentially promote optimal juvenile growth and survival are a temperature of 20 °C, a salinity of 20 ppt, a stocking density of 5 seahorses r and a photoperiod of 16:08 (L:D). The information from this study identifies the baseline for future research protocols and provides scientific information on the husbandry of If. abdominalis juveniles that can be applied to commercial culture.