Publication Type: Journal Article
Year of Publication: 2003
Authors: Bell, E.M., Lockyear JF, McPherson JM, Marsden AD, Vincent ACJ
Journal: Environmental Biology of Fishes
Keywords: abundance, behaviour, Distribution, Knysna Estuary, population structure
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.