Hippocampus capensis 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Actinopterygii Syngnathiformes Syngnathidae

Scientific Name: Hippocampus capensis Boulenger, 1900
Common Name(s):
English Knysna Seahorse, Cape Seahorse
Taxonomic Source(s): Lourie, S.A., Pollom, R.A. and Foster, S.J. 2016. A global revision of the seahorses Hippocampus Rafinesque 1810 (Actinopterygii: Syngnathiformes): Taxonomy and biogeography with recommendations for future research. Zootaxa 4146(1): 1-66.
Taxonomic Notes: Mkare et al. (2017) found that genetic admixture between estuaries was high, and recommend recognizing the subpopulations from the three estuaries as a single Evolutionarily Significant Unit (ESU).

Assessment Information [top]

Red List Category & Criteria: Endangered B1ab(iii,v)+2ab(iii,v) ver 3.1
Year Published: 2017
Date Assessed: 2017-03-09
Assessor(s): Pollom, R.
Reviewer(s): Ralph, G.
Contributor(s): Bell, E.M. & Czembor, C.A.

Hippocampus capensis is an estuarine seahorse species that inhabits river mouths in South Africa. This species is assessed as Endangered under criterion B1ab(iii,iv,v)+2ab(iii,iv,v) because it has an estimated extent of occurrence (EOO) of 300 km², an area of occupancy (AOO) of 27 km², and it is only found in three locations (and has previously been lost from at least one other location). Also, the species is experiencing continuing decline in quality of habitat and number of mature individuals. Additional long-term monitoring is needed to determine population size and trends for this species. Habitat protection and water management measures need to be implemented to secure the future of this species.

Previously published Red List assessments:

Geographic Range [top]

Range Description:Hippocampus capensis currently is found in the Knysna, Swartvlei, and Keurbooms estuaries in South Africa. Historical and anecdotal records indicate that this species could previously be found in Klein Brak, Groot Brak, Goukamma, Groot, Kromme, Kabeljous and Gamtoos estuaries (Bell et al. 2003, Lockyear et al. 2006), but surveys carried out between 2001 and 2003 could only locate individuals in the Knysna, Swartvlei, and Keurbooms estuaries (Lockyear et al. 2006).

The combined water surface of these estuaries is approximately 27 km².
Countries occurrence:
South Africa (Western Cape)
FAO Marine Fishing Areas:
Atlantic – southeast
Additional data:
Estimated area of occupancy (AOO) - km2:27Continuing decline in area of occupancy (AOO):Unknown
Extreme fluctuations in area of occupancy (AOO):UnknownEstimated extent of occurrence (EOO) - km2:300
Continuing decline in extent of occurrence (EOO):UnknownExtreme fluctuations in extent of occurrence (EOO):No
Number of Locations:3Continuing decline in number of locations:Yes
Extreme fluctuations in the number of locations:No
Lower depth limit (metres):20
Upper depth limit (metres):1
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Recent surveys and molecular studies on the species indicate ongoing declines in the number of mature individuals in each of the three inhabited estuaries, but these have not been quantified (Mkare et al. 2017). Declines are thought to be in addition to those described below. Further research is needed to determine the level of decline and how the population fluctuates between years. 

The total population estimate for Hippocampus capensis in the Knysna estuary (mean, with range representing 95% confidence intervals) was 89,000 seahorses (range 30,000–148,000) in 2000 (Bell et al. 2003) and 62,000 (range 41,000–82,000) in 2001. In 2002, the Swartvlei and Keurbooms supported 995,000 (range 390,000–1.7 million) and 836,000 (range 242,000–1.7 million) seahorses, respectively. In 2003, the Swartvlei estuary supported only 176,000 (range 83,000–280,000) seahorses, and no seahorses could be found in the Keurbooms estuary (Lockyear et al. 2006).

As of 2003, assuming the total population of H. capensis in the Knysna estuary remained relatively constant at 62,000 individuals between 2001 and 2003, the estimated total population in all estuaries was 238,000 seahorses (range 124,000-360,000), down from 1.9 million seahorses (range 675,000-3.5 million) between 2001 and mid-2002 (Lockyear et al. 2006). Juveniles typically constitute 5-20% (average 13%) of individuals sampled (Lockyear et al. 2006), resulting in an estimated mean population size of approximately 207,000 mature individuals as of 2003.

In 2005, the Knysna and Swartvlei population sizes appeared to be healthy (M. Cherry pers. comm. 2010), and in 2010, populations were reported anecdotally to be seen in abundance in many of the marinas in the Knysna estuary and as being present in the Swartvlei estuary (T. Meintjes pers. comm. 2010, P. Joubert pers. comm. 2010). Similar to the 2003 surveys, preliminary surveys of the Keurbooms estuary in 2011 found no seahorses, despite the availability of suitable habitat (Appleby 2011). However, anecdotal evidence indicates individuals may occasionally be seen in this estuary (T. Meintjes pers. comm. 2010; Appleby 2011).

Recently constructed artificial habitats (e.g., marinas/boat harbours) appear to act as protective habitat for H. capensis (P. Joubert pers. comm. 2010) and, as relatively large numbers of individuals are seen in these habitats (B. Allanson pers. comm. 2010), they may have a beneficial effect on population size.

It is unknown whether extreme fluctuations of population size occur for this taxon. In 2002, seahorse densities were high in the Swartvlei and Keurbooms estuaries but, in 2003, seahorses were absent from the Keurbooms estuary and the population size in the Swartvlei estuary had decreased by more than 80% (Lockyear et al. 2006). The population size in the Knysna estuary declined approximately 30% between 2000 and 2001 (Bell et al. 2003, Lockyear et al. 2006). Since 2003, individuals have been seen in abundance in the Knysna and Swartvlei estuaries, although population size has not been quantified recently. Populations may not yet have recovered in the Keurbooms estuary, according to preliminary 2011 surveys (Appleby 2011). These results suggest that populations may occasionally undergo extreme fluctuations in the smaller estuaries, but experience fluctuations to a lesser degree in the larger Knysna estuary. It is expected that fluctuations are not due to changes in life stages or dispersal, but rather that individuals are susceptible to being washed out of the smaller estuaries as a result of floods and increased stormwater run-off from developments (Marker 2003, Lockyear et al. 2006). With H. capensis’ poor swimming ability and reliance on plant holdfasts, strong currents could sweep animals and plants out into the sea and silt depositions could result in the smothering of habitat and the clogging of gills. Individuals remaining in small pockets of water in the drained estuaries after floods may be subject to mortality from increased water temperatures (Russell 1994) and predation (P. Joubert pers. comm). The larger Knysna estuary thus may act as an infrequent but continuous exporter of colonists to the smaller estuaries (Teske et al. 2003). The rates of recolonization are after large population declines are unknown. However, anecdotal evidence indicates that the populations in the Knysna, Swartvlei, and Keurbooms estuaries have increased in numbers since the 85% decline in population observed in 2003 (T. Meintjes pers. comm. 2010, P. Joubert pers. comm. 2010).

Overall, the wild population of H. capensis is inferred to have declined by at least 50% over the past 10 years. Pressures that led to earlier declines have not ceased and are likely increasing, and although unquantified recent declines have been documented.
Current Population Trend:Decreasing
Additional data:
Continuing decline of mature individuals:Yes
Extreme fluctuations:UnknownPopulation severely fragmented:Unknown
No. of subpopulations:3Continuing decline in subpopulations:Unknown
Extreme fluctuations in subpopulations:UnknownAll individuals in one subpopulation:No

Habitat and Ecology [top]

Habitat and Ecology:Hippocampus capensis are found at depths between 0.5-20 m (Toeffie 2000 in Bell et al. 2003) in association with submerged aquatic plants. Within Knysna, H. capensis are associated with five dominant aquatic plants: Zostera capensis, Caulerpa filiformis, Codium extricatum, Halophila ovalis and Ruppia cirrhosa (Teske et al. 2007), though they are most likely to be found grasping Zostera capensis eelgrass holdfasts (Bell et al. 2003). This species is also associated with Pyura stolonifera (a large ascidian) and sponges, but significantly more adult and juvenile seahorses are found at sites characterized by high vegetation cover (75%) than at sites with lower cover (Teske et al. 2007). Vegetated sites make up approx 11% of Knysna estuary (Teske et al. 2007), thus large areas of habitat in the Knysna estuarine system may be unsuitable for H. capensis because of the absence of submerged vegetation (Toeffie 2000 in Lockyear et al. 2006). H. capensis can also tolerate a wide range of environmental conditions (Lockyear et al. 2006), such as salinities ranging from 1-59 g/kg (Whitfield 1995). The species is also known to use artificial habitats extensively (Claassens 2016).

Juvenile H. capensis are often found in low frequencies during surveys, likely because they spend time in the plankton, at least initially, and older juveniles may use different habitat from the adults sampled (Bell et al. 2003). Juvenile length is strongly dependent on temperature and photoperiod, with higher temperatures and longer photoperiods resulting in juveniles being smaller in captivity (Lockyear et al. 1997).

Hippocampus capensis adults feed predominantly on small crustaceans which are sucked from submerged leaf surfaces or from the water column (Whitfield 1995).

This species is ovoviviparous, and the males brood the young in a pouch prior to giving live birth (Foster and Vincent 2004). Breeding occurs in the austral summer, when water temperatures approach 20°C and sexual maturity is attained in about one year at 65 mm standard length (Whitfield 1995). In captivity, H. capensis are diurnally active and have an elaborate courtship and mating ritual involving brood pouch inflation, tail grasping and ‘face-to-face’ positioning (Grange and Cretchley 1995).
Continuing decline in area, extent and/or quality of habitat:Yes
Generation Length (years):1-3
Movement patterns:Not a Migrant

Use and Trade [top]

Use and Trade: As this species is protected, it is not commercially traded or used for subsistence. Small numbers of specimens, however, were previously provided to researchers (McPherson and Vincent 2011). Historically, individuals were taken for the aquarium trade (Skelton 1987 in Whitfield 1995). The species may hold value on international markets similar to other seahorses (Vincent et al. 2011), but this has not been recorded.

Threats [top]

Major Threat(s): This species’ primary habitat, the Knysna estuary, is among the most heavily used water bodies in South Africa (Bell et al. 2003) and human settlements and associated industrial, domestic, and recreational activities are increasing around all estuaries where this species has been recorded (Mkare et al. 2017).

The area is under threat from urban expansion and land-use changes in the catchment; stormwater runoff has increased in volume, and peak flows carry suspended sediment into the estuary (strong currents sweep animals and plants out into the sea and silt depositions result in the smothering of habitat and the clogging of gills); rising population and developments to provide piped water threatens freshwater inflow; and waste water disposal brings in chemical and biological pollutants (Marker 2003). Development surrounding the estuary is known to deposit trace metals, hydrocarbons, pesticides and organic wastes into the estuary (Chmelik 1975 in Bell et al. 2003). Within the estuary, the cumulative impact of boats may significantly affect the seagrass habitats of this species (Lockyear et al. 2006). Pollution events or other disturbances which affect the submerged plant beds of these estuaries will have a direct and indirect impact on H. capensis populations (Skelton 1987 in Whitfield 1995), yet construction developments and pollution continue (Lockyear et al. 2006, Teske et al. 2007).

Hippocampus capensis also appears to be vulnerable to water temperature increases. In 1991, 3,000 dead seahorses were found along the shores of the Swartvlei estuary after heavy rainfall and flooding caused a breach in the estuarine mouth and partially drained the estuary, leaving many individuals trapped in small pockets of remaining water. Mortality was attributed to the sudden increase in water temperature following the reduction in water level (Russell 1994). When the water levels drop and individuals are restricted to smaller pools, they are also susceptible to high levels of predation (P. Joubert pers. comm. 2010). Furthermore, large-scale flooding may substantially alter the estuaries in which this species is found (C.A.P.E. Estuaries Management Programme 2010). The present rate at which construction developments and other human activities are increasing along the estuary is alarming; the resulting habitat degradation may make recovery of populations after a naturally occurring disaster, such as a freshwater flood, increasingly difficult (Teske et al. 2003).

Furthermore, recently observed specimens of H. capensis also appear to be infected with lesions; the disease appears most often in individuals using artificial habitats (P. Joubert pers. comm. 2010).

Conservation Actions [top]

Conservation Actions: Hippocampus capensis is protected by law in the Knysna estuary (Sea Fisheries Act of 1973) and both the Swartvlei and Knysna estuaries fall under the protection of the National Parks Board (Whitfield 1995).

A conservation breeding program was started in 1998 to help mitigate the natural and anthropogenic threats to this species (Galbusera et al. 2007). Offspring of this founding stock was transferred from the Zoological Society of London to the Royal Zoological Society of Antwerp (RZSA), currently the only zoo in Europe where this species is being bred in captivity (Galbusera et al. 2007).

This species is listed on Appendix II of CITES.

Citation: Pollom, R. 2017. Hippocampus capensis. The IUCN Red List of Threatened Species 2017: e.T10056A54903534. . Downloaded on 18 December 2017.
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