The Biology and Conservation of Gorgonian-Associated Pygmy Seahorses

Pygmy seahorses are a little known, morphologically and ecologically distinct group of syngnathid fish. Reaching a maximum standard length of 26.9 and 24 mm respectively, Bargibant’s (Hippocampus bargibanti) and Denise’s pygmy seahorses (H. denise) are amongst the smallest of seahorses. They are also extreme habitat specialists living in an obligate association with certain gorgonian corals of tropical Southeast Asian reefs. The dissimilarities from their larger congeners, the extent of anthropogenic pressures within their geographical range and their listing by the IUCN Red List of Threatened Species as ‘Data Deficient’ highlight the need for targeted study. The aim of this thesis was to conduct the first biological examination of gorgonian-associated pygmy seahorses, elucidating the population structure, behaviour and potential threats to the species. The study site was a marine protected area created and administered by a SCUBA resort in southeast Sulawesi, Indonesia. Population size and structure of H. bargibanti and H. denise, were estimated at seven sites and five depth contours. Within the 7,000 m2 of reef surveyed by belt transect, the densities of both species were very low (H. bargibanti: 0.34 (± 0.16 S.E) 200 m-2, H. denise: 1.18 (± 0.20) 200 m-2). Their densities were amongst the lowest for an unexploited seahorse population. The more host specific and rarer, H. bargibanti, which lives exclusively in association with Muricella spp. gorgonians, was found on 20.0% of potential hosts but the density of hosts was low (0.72 (± 0.1) 200 m-2). The more common H. denise is reported to be comparatively cosmopolitan in host choice but within transects at this site was exclusively found inhabiting the relatively common Annella reticulata gorgonians (7.60 (± 0.8) 200 m-2), with 7.8% of gorgonian colonies of that species inhabited. Group sizes ranged between 1 and 6 individuals and sex ratios were in some cases skewed. The isolation of pygmy seahorses on a gorgonian host and the occurrence of variable group sizes were hypothesised to act as selective pressures on the ecology of gorgonian-associated pygmy seahorses. To investigate this, focal observations were conducted on the social and reproductive behaviour, space use and activity levels of 18 (9 Male: 9 Female) adult H. denise in four groups of different sex ratios and numbers of individuals. During 217 thirty-minute observation periods monogamy was observed to be the prevalent mating system but social polygamy and sequential polyandry were recorded in certain instances where sex ratios were skewed. Conventional sex roles were observed, with males competing for access to females. Small (150-1508 cm2) and strongly overlapping (by 2-5 individuals) home ranges, measured using minimum convex polygons, were observed and in no instances were seahorses observed away from their gorgonian host. Sites of concentrated use, or core areas, were observed and coincided with protected areas of the host. Nocturnal use core areas were usually shared by reproductive partners and were the site of social and reproductive behaviours. The ecology of H. denise appears to maximise reproductive output through mating system plasticity and early maturation of animals. Complementing the ecological studies I explored the role of gorgonian-associated pygmy seahorses in ecotourism through diver surveys and observations. Almost a third of survey respondents (30.8%) listed pygmy seahorses as one of the three underwater organisms they most wished to see during their stay at a dedicated SCUBA diving resort. The majority (88.9%) did see pygmy seahorses and of the 57.3% of respondents that used underwater photographic equipment 73.8% took images of them. Diver observations indicated that photographic activities led to the majority of contacts with the reef (72.2%) when in the vicinity of pygmy seahorses. Many of these contacts (29.2%) were with the focal pygmy seahorse’s gorgonian host directly. Use of a constant light source to highlight the animals and physical contact made with the gorgonian were found to be significant predictors of pygmy seahorse avoidance behaviours. Survey respondents were overwhelmingly in favour (91.8%) of Code of Conduct guidelines to minimise diver impacts on pygmy seahorses. Without management intervention pygmy seahorse-related ecotourism is likely to cause harm to both the seahorses and their gorgonian hosts. This research is the first of its kind into the ecology of pygmy seahorses: a group of biologically distinct and economically valuable species. My findings indicate that exceedingly low densities of H. bargibanti and H. denise, extreme habitat specialisation, high levels of habitat degradation coupled with pressure from dive tourism threaten seahorse populations. This study highlights the need for conservation and management of these species to prevent overexploitation and localised extinctions from loss of suitable gorgonian hosts.