Prince Edward Islands - Key Species
Orcas (Orcinus orca)
At least one pod of orcas (Orcinus orca) is considered resident at the Prince Edward Islands and up to 28 individuals have been seen in a year (Keith et al. 2001; Pistorius et al. 2002). Populations peak October to December, decrease in January, and have a small increase in late April to early May after which orca sightings during most years, decrease to almost nothing. They are known to consume seals and penguins, and also to have a high impact on long-line fishing taking fish off lines during hauling.
Picture © Genevieve Jones
Seals (Arctocephalus tropicalis)
Elephant seals and fur seals are both found at the islands. Elephant seals (Mirounga leonina) have been in decline since monitoring at the islands began in the 1950s and the population has been reduced by more than 60% (Bester and Hofmeyr 2005). The reasons behind the decline are uncertain though many hypotheses have been put forward, including competition with fisheries, intra-specific competition as well as climate change. In contrast, following exploitation the Fur Seal populations (the more numerous Sub-Antarctic Fur Seal, Arctocephalus tropicalis, and the less common Antarctic Fur Seal, Arctocephalus gazella) have recovered to what is thought to be pre-exploitation levels. Increasing from a few hundred individuals in the early surveys in the 1950s, through to around 150 000 today, the fur seals now dominate the PEI seal biomass (Hofmeyr et al. 2006).
Picture © Genevieve Jones
Penguins (Aptenodyptes patagonicus)
This group has the largest biomass of the land based top predators (seals and seabirds) and is made up of 4 species of penguins which vary in size, diet, and foraging range and exhibit different population trends (Adams et al. 1985; Adams 1987; Adams and Wilson 1987; Brown 1987; Crawford et al. 2003a; Crawford et al. 2003b; Crawford et al. 2003c). The King penguins (Aptenodytes patagonicus) are the largest and forage the furthest, having a predominantly small pelagic fish diet. The Gentoo penguin (Pygoscelis papua) also have a diet dominated by fish, but this species is restricted in its foraging range to near-shore waters. Both are found at the islands year round. The remaining two species, the Macaroni (Eudyptes chrysolophus) and Southern Rockhopper (Eudyptes chrysocome filholi) penguins, are summer breeders and are intermediary in their foraging range (between 100 and 300 km from the shore). Both have a predominantly crustacean diet. In terms of conservation status, according to Birdlife International (www.birdlife.org, Downloaded 27/09/2011), the Gentoo Penguin is classified as Near Threatened, both the Macaroni and Southern Rockhopper penguins as Vulnerable, while the King penguins are Least Concern. The population of King penguins at the islands makes up approximately 13% of the worldwide population with the remaining three making relatively small contributions to the world populations (Macaroni 4%, Southern Rockhopper 5% and Gentoo 0.5%).
Picture © Genevieve Jones
Albatross (Diomedea exulans)
There are five albatross species considered resident at the islands: the Wandering (Diomedea exulans), Grey-headed (Thalassarche chrysostoma), Indian Yellow-nosed (T. carteri) and Light-mantled (Phoebetria palpebrata) & Dark-mantled Sooty (P. fusca). Together the albatross populations account for a small percentage of the avian biomass (<3%), but they are considered important in terms of their conservation status. Birdlife International (www.birdlife.org) classifies both the Wandering and Grey-headed Albatross as Vulnerable, the Indian Yellow-nosed and Dark-mantled Sooty as Endangered, while the and Light-mantled Sooty is classified as Near Threatened. Population dynamics of albatross species (e.g. Wandering and Grey-headed) have been monitored with long term trends observed (Nel et al. 2002a) showing declines in the early 2000s which have been linked to the increase in tuna long lining as well as large scale Illegal Unregulated and Unreported (IUU) long-line fishing for Patagonian Toothfish (Nel et al. 2002b; Nel et al. 2002c). Mitigation measures have however reduced the by catch of birds significantly (CCAMLR 2010).
Picture © Genevieve Jones
Patagonian Toothfish (Dissostichus eleginoides)
The Patagonian Toothfish (Dissostichus eleginoides) is the sole target of the ongoing fishery at the PEIs. This Southern Ocean resource was discovered in 1970s & 1980s and exploitation at the PEIs is believed to have started in the early 1990s. Large Illegal, Unregulated and Unreported (IUU) catch early on in the fishery (estimated at ~ 20 000 t for the 1996/97 season, Brandao et al. 2002), resulted in the collapse of the fishery. Although a licensed fishery is ongoing today, the catch is currently less than 100 t per year.
Picture © Yuri Hooker, Fishbase
Nototheniid fish (Lepidonothen squamifrons)
The Nototheniids are considered the most successful migrants to the waters of the sub-Antarctic islands of the Indian Ocean both in terms of diversity and abundance. At the PEIs they occupy a number of habitats including inshore (Gobionotothen marionensis and G. acuta), the continental slope (the Painted Notie, Lepidonotohen larseni), and deep water (the three larger demersal species, the Grey Rockcod Lepidonotothen squamifrons, the Black Rockcod Notothenia coriiceps, and the Marbled Rockcod N. rossii). In the pelagic domain, the smaller species Paranotothenia magellanica is found, along with the dominant large pelagic Patagonian Toothfish for which the fishery exists. The only other species which has been identified as being of potential commercial value is the Grey Rockcod.
Picture © Wilhelms S., Fishbase
Myctophids
Myctophids are the most widely distributed and abundant pelagic fish in the Southern Ocean and at the PEIs 17 of the 35 species of small pelagic fish belong to this family. The key role of the myctophids in the system is clearly demonstrated through the diet of many of the top predators; myctophids are the principal prey item for the Fur Seals and King penguins, which together make up approximately 70% of the land based top predator biomass (Adams and Brown 1989; Klages and Bester 1998; Makhado 2002; Ryan and Bester 2008). Myctophids also form an important part of the diet of fish species and other nekton (cephalopods) (Lipinski and Linkowski 1988; Rodhouse et al. 1996; Pakhomov et al. 2006).
Picture © Evgeny Pakhomov
Benthic decapods (Nauticaris marionis)
The islands have an endemic benthic decapod (Nauticaris marionis), which has been shown to be an important diet component to some of the land based top predators, particularly those that feed inshore. It has the second highest crustacean biomass and it has been the subject of many studies as it is considered an important link between the benthic production and the higher vertebrates (Pakhomov et al. 1999; Pakhomov et al. 2000; Vumazonke et al. 2003; Pakhomov et al. 2004). It is widely distributed around Marion & PE, occurring mainly within the 200m depth contours (Perissinotto and McQuaid 1990).
Picture © Evgeny Pakhomov