Seahorse Density Research Articles

Home range behaviour of the monogamous Australian seahorse, Hippocampus whitei

We provide a quantitative account of local movements in the monogamous Australian species Hippocampus whitei, as a rare report of home range size in fishes living in seagrass habitats. Our study took place in shallow Posidonia seagrass beds in Port Jackson (Sydney Harbour), principally during January to March. Daily monitoring of individual seahorses during underwater observations revealed that both sexes maintained small and apparently undefended home ranges for several breeding cycles at least. Female home ranges were significantly larger than males, when analysed by both the minimum convex polygon and grid cell methods. Home range size was not correlated with either body size or seahorse density. Presumably, home ranges were small in H. whitei because camouflage (to avoid predation and to capture prey), mate fidelity and parental brooding meant they accrued little benefit (and potentially considerable cost) from moving more extensively. Sex differences in home range size may arise from constraints associated with male pregnancy. These fish are among the most sedentary of vertebrates, with relatively small home ranges equalled only by coral reef species. In terms of their conservation, relatively small protected areas may be sufficient to support breeding populations of H. whitei although that limited movement may result in considerable delays in the recolonisation of depleted areas.

First Field Studies of an Endangered South African seahorse, Hippocampus capensis

South Africa's endemic Knysna seahorse, Hippocampus capensis Boulenger 1900, is a rare example of a marine fish listed as Endangered by the IUCN because of its limited range and habitat vulnerability. It is restricted to four estuaries on the southern coast of South Africa. This study reports on its biology in the Knysna and Swartvlei estuaries, both of which are experiencing heavy coastal development. We found that H. capensis was distributed heterogeneously throughout the Knysna Estuary, with a mean density of 0.0089 m−2 and an estimated total population of 89 000 seahorses (95% confidence interval: 30 000–148 000). H. capensis was found most frequently in low density vegetation stands (≤20% cover) and grasping Zostera capensis. Seahorse density was not otherwise correlated with habitat type or depth. The size of the area in which any particular seahorse was resighted did not differ between males and females. Adult sex ratios were skewed in most transects, with more males than females, but were even on a 10 by 10 m focal study grid. Only three juveniles were sighted during the study. Both sexes were reproductively active but no greeting or courtship behaviours were observed. Males on the focal study grid were longer than females, and had shorter heads and longer tails, but were similar in colouration and skin filamentation. The level of threat to H. capensis and our limited knowledge of its biology mean that further scientific study is urgently needed to assist in developing sound management practices.