Effect of stocking density and gender segregation in the seahorse Hippocampus abdominalis

The aims of this investigation were to determine the effects of stocking density and gender segregation on growth and survival of the seahorse Hippocampus abdominalis. Growth and survival of juvenile seahorses (72 mm in standard length and 0.5 g in wet weight) was tested at 1, 2, and 5 juveniles/l and monitored for 60 days. Growth parameters (standard length, weight, and Condition Factor) generally increased as stocking density decreased. There was no difference in survival between 1 and 2/l (mean±1 SE survival=100% and 95.8±2.7%, respectively), but survival in the 5/l treatment was significantly reduced (mean survival=78.3±3.7%). With increasing stocking density, a greater incidence of physical interference between juveniles (i.e. tail-grasping and wrestling) was observed. The effect of gender segregation was tested with sexually mature seahorses in tanks where courtship displays were possible, but egg transfer prevented by limiting tank vertical height. There were no significant differences in growth (standard length, wet weight, Condition Factor, and mean Specific Growth Rate) in males and females either gender segregated or kept together for 90 days. Overall, comparison between males and females revealed no significant differences in growth, apart from the normal difference in wet weight, which begins at the onset of sexual maturity with male brood pouch development. Survival was 100% across all treatments. In addition to courtship displays occurring between males and females kept together, same-sex displays were also observed between males and females and females which were gender segregated, although to a lesser extent, and not always with the full range of courtship behaviours exhibited.