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The regional population of Caspian Tern Sterna caspia is estimated to be less than 1 000 mature individuals. In addition, the species has a restricted number of breeding locations leaving it prone to the effects of human activities or stochastic events within a short time period. Accordingly, the species is assessed as regionally Vulnerable.
Despite being distributed globally, the species is thinly spread throughout the Holarctic, Australasian, Oriental and Afrotropical regions (Voous 1960). Within the region, it is concentrated at estuaries and sheltered bays along the coastline and at large, permanent inland waterbodies (Brooke 1984). Twenty-eight historical breeding localities are known although the species currently breeds at only ten sites, two of which are Important Bird Areas, namely iSimangaliso Wetland Park (IBA SA058) and West Coast National Park and Saldanha Bay Islands (IBA SA105). The movement of a ringed bird from Algoa Bay, Eastern Cape to Lake St Lucia suggests that the southern African breeding populations may not be isolated from each other (Martin 1991).
The global population is estimated to number c. 240 000-420 000 mature individuals (Dodman and Diagana 2006). The regional population is a small portion of this. In 2011 in Western Cape, 69 pairs bred on private property near Velddrif; eight pairs bred at Caspian Island in the southern portion of Langebaan Lagoon; one pair bred on each of Jutten, Meeuw and Schaapen islands in Saldanha Bay; and one pair bred on Robben Island offshore from Cape Town (Crawford et al. 2012). Further east, several pairs breed at De Mond Nature Reserve near Aniston. Crawford et al. (2009) reported c. 50 pairs at the Swartkops River estuary in Eastern Cape and c. 170 pairs at False Bay in Lake St Lucia, KwaZulu-Natal. The regional breeding population is in the order of 300-316 pairs (Kemper et al. 2007, Crawford et al. 2012). The confidence in this population estimate is high.
The global population trend is increasing, although some populations are decreasing, stable, or have unknown trends (Dodman and Diagana 2006). Between 1980-1991, Cooper et al. (1992) estimated the regional population to be in the order of 1 500 birds with about 1 000 of those being mature individuals. It must be noted that there is sometimes extreme fluctuations in the number of birds breeding at a locality; for example, counts at Lake St Lucia, KwaZulu-Natal between 1975 and 1985 ranged from as low as six to a maximum of 833 birds (Ryan et al. 1986). The current population estimate of 600-650 mature individuals represents a reduction of 30-35% over three generations. Confidence in this regional trend estimate is medium.
The primary threats to this species are during the breeding period when it is highly susceptible to human disturbance including through egg collecting and predation by domestic dogs (Martin and Randall 1987), and even potentially through avitourism. Extreme weather events such as heavy rainfall, droughts and heat waves can also impact on the breeding success of this species (du Toit et al. 2003). Underhill (2000) pointed out that rising water levels may flood nests, while falling water levels grant access to terrestrial predators. Kelp Gulls Larus dominicanus are also notable nest predators. Lesser threats which deserve more investigation include the bio-accumulation of heavy metals, pesticides, and other chemical pollutants (to which this species may be particularly prone because it eats relatively large fish); this may lead to eggshell thinning and other impacts on breeding success (Underhill 2000).
Jutten, Schaapen, Meeuw and Caspian islands are all accorded Ramsar site status and, together with West Coast National Park, fall within the National Park network. Lake St Lucia is included under the iSimangaliso Wetland Park, a World Heritage Site as well as a Ramsar site. The species is listed on Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals and under the African Eurasian Waterbird Agreement. The species is included in CWAC surveys undertaken under the auspices of the Animal Demography Unit of the University of Cape Town. There are no other current species-specific conservation measures underway.
The legal protection of breeding sites at locations outside formally protected areas is proposed. At some breeding sites, particularly those at artificial waterbodies, sensitive management or judicious alteration could increase breeding success, for example by maintaining optimal water levels, raising islands to avoid flooding, or by excluding predators (Cooper et al. 1992). Habitat management and monitoring are further management recommendations (du Toit et al. 2003). A full study of what conservation measures will be most successful should also be conducted. Disturbance at breeding and roosting sites should be limited. The primary factor limiting populations in other parts of the world is lack of suitable nest sites, and the possibility of creating artificial nesting opportunities (such as was done for Lesser Flamingo Phoeniconaias minor at Kamfers Dam, Kimberley) should be investigated.
* Data on long-term population trends should be amassed through regular surveys of breeding sites.
* The impact of gull predation on chicks and eggs should be quantified.
* An investigation into measures to possibly improve breeding success at existing sites, or creation of additional breeding sites, should be considered.
* The movements of this species are little known in the region; key post-breeding, moulting and foraging areas should be identified and protected.