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The Kerguelen Tern Sterna virgata is one of the world's most restricted terns, occurring only on Subantarctic islands in the southern Indian Ocean. It satisfies the criterion for regionally Endangered, according to D (population of less than 250 mature individuals). Dedicated surveys, including in coastal areas seldom visited on Marion Island, and summer surveys during peak breeding at Prince Edward Island, may reveal a healthier population. In addition to its small population size, the species satisfies the criterion for regionally Vulnerable under D2 (population with a very restricted AoO, and number of locations (typically less than 5), such that it is prone to the effects of human activities or stochastic events within a very short time period in an uncertain future, and is thus capable of becoming Critically Endangered or even Extinct in a very short time period).

Distribution

The Kerguelen Tern is one of the world's rarest terns, and is restricted to the Crozet, Kerguelen, Marion and Prince Edward island groups in the southern Indian Ocean (Gomez and Voisin 2002). Taking into account the large expanses of sea between the Kerguelen and Crozet islands (1 480 km), and the Crozet and Prince Edward Islands (925 km), as well as the sedentary nature of the species (Weimerskirch and Stahl 1988), exchanges between the Kerguelen Tern populations must be rather restricted (Gomez and Voisin 2002). Ryan and Bester (2008) make mention that it remains at the Prince Edward islands year-round. On Marion Island, birds are found along the west and north coasts between Kampkoppie and Goney Plain (Ryan 1987). Kerguelen Terns are seldom seen on the east coast of Marion Island (Ryan 1987a). All birds recorded by Ryan (1987) were seen along the south-eastern side of the island between McNish Bay and the coast opposite Boggel.

Habitat View more ecological information

Population

The global population has been estimated at 3 500-6 500 individuals, roughly equivalent to 2 300-4 300 mature individuals (BirdLife International 2014). Weimerskirch et al. (1989) placed the Kerguelen Island population of the subspecies S. v. virgata at c. 1 750 mature individuals while Weimerskirch and Stahl (1988) estimated there to be 300-400 mature individuals on Crozet. Ryan (1987) recorded 45 terns on Prince Edward Island and 11 at Marion Island. Twenty one years later the balance of the populations between the two islands has shifted, with Ryan and Bester (2008) estimating there to be c. 50 pairs on Marion Island and only five pairs on Prince Edward Island. Confidence in this regional population estimate is high.

Population trend

The global population trend is believed to be stable (BirdLife International 2014). The regional population on Marion Island has fluctuated, averaging 24 pairs over a 12 year period (Whittington et al. 2009). The population on Marion Island decreased from c. 50 pairs in 1974 and 1977 (Williams et al. 1979) to less than 10 pairs in 1979 (Brooke 1984), and 5-10 pairs in 1984 (Ryan 1987). Numbers increased between 1984 and 1996, presumably an effect of reduced predation pressure from introduced cats. Between 1996/1997 and 2008/2009 numbers fluctuated between 12 and 56 pairs, without any apparent trend.

In contrast, the population on Prince Edward Island de­creased from an estimated 25-30 pairs in 1984 (Ryan 1987) to two pairs in 2001 (Ryan et al. 2003), and to a single pair in 2008 (Whittington et al. 2009). No pairs were observed during the 2011 summer survey at Prince Edward Island (RM Wanless pers. obs.), which was after peak breeding but still suggests the downward trend continued to local extinction; this remains to be confirmed. The increase at Marion Island over the past 20 years is attributed to reduced predation pressure due to the removal of cats (Ryan et al. 2003). Collectively, the regional population on Prince Edward and Marion islands is considered stable, although both populations have undergone marked fluctuations. Confidence in this regional trend estimate is medium.

Threats

The breeding populations of both Kerguelen and Antarctic S. vittata terns at île aux Cochons (Crozet Archipelago) were exterminated by feral cats. Terns were not recorded in the diet of cats at Marion Island (van Aarde 1980, van Rensburg 1985) although this may have been due to the small number of terns present in the early 1980s making discovery of their remains in cat diet studies unlikely. Assuming limited exchange between the three island groups, the Kerguelen Tern population at the Prince Edward Islands may be too small to maintain genetic diversity.

Conservation

Underway

There are no species-specific conservation measures underway. The Prince Edward Islands are listed as a Ramsar Wetland of International Importance, and as a Special Nature Reserve under the Environment Conservation Act (No 73 of 1989) and managed under the Prince Edward Islands Management Plan 1996. A variety of general conservation measures are in place at the Prince Edward Islands.

Proposed

Establishing the reasons for decreases in the regional population is a high priority. The extreme vulnerability of small populations to random events makes the entire regional population inherently at risk and this species should be accorded the highest conservation priority of all seabirds breeding at the Prince Edward Islands. It should be noted that any activities at Prince Edward Island necessarily involve extreme caution and minimal disturbance, requirements that add unusual constraints to research and conservation work. Other appropriate conservation measures may become apparent once some basic research into the Kerguelen Tern's ecology and population dynamics at Marion Island are better understood. Some initial potential research questions are provided below.

Research

* The geographic isolation and small population size may have led the Prince Edward Island birds to genetically diverge away from the populations of the Crozet and Kerguelen islands. An investigation into the phylogenetic relationships between these populations is warranted.

* Investigating dispersal and gene flow between island pairs/groups within the Crozet and Kerguelen archipelagos that are similar distances apart to Marion and Prince Edward islands will be informative. Sharing information about demographics, meta-population dynamics and population genetics would likely indicate avenues for productive, relevant research into factors contributing to decreases at the Prince Edward Islands.

* Understanding the cause for the collapse/local extinction at Prince Edward Island is a priority, although this may be impossible if the population there is indeed extinct.

* Foraging ecology and links between Kerguelen Tern and other inshore foraging species (including the Antarctic Tern and Kelp Gull Larus dominicanus) should be investigated at Marion Island as a means to understanding possible causes for the low numbers of birds at Prince Edward Island.

* Determine whether any of reproductive success, juvenile survival, recruitment rates, adult survival or other demographic parameters are preventing the population from expanding, or if food availability, nesting requirements or territory size are limiting factors.

* Regular checks on the sex ratios of chicks produced at Marion Island would give an indication of possible drift towards dangerously skewed sex ratios. In addition, banding fledglings of known sex would allow estimates of sex- and stage-specific survival rates and would facilitate population modelling and scenario planning to assess extinction risks.