The archipelago of St Kilda is also listed as an NSA, alongside many other conservation designations. Flora and fauna Much of the archipelago is a protected habitat, including both the islands and the surrounding waters. There are 53 Sites of Special Scientific Interest of which the largest are Loch an Duin, North Uist () and North Harris (). South Uist is considered the best place in the UK for the aquatic plant Slender Naiad, which is a European Protected Species. There has been considerable controversy over hedgehogs on the Uists. Hedgehogs are not native to the islands but were introduced in the 1970s to reduce garden pests. Their spread posed a threat to the eggs of ground-nesting wading birds. In 2003 Scottish Natural Heritage undertook culls of hedgehogs in the area, but these were halted in 2007; trapped animals are now relocated to the mainland. Nationally important populations of breeding waders are present in the Outer Hebrides, including common redshank, dunlin, lapwing and ringed plover. The islands also provide a habitat for other important species such as corncrake, hen harrier, golden eagle and otter. Offshore, basking shark and various species of whale and dolphin can often be seen, and the remoter islands' seabird populations are of international significance. St Kilda has 60,000 northern gannets, amounting to 24% of the world population; 49,000 breeding pairs of Leach's petrel, up to 90% of the European population; and 136,000 pairs of puffin and 67,000 northern fulmar pairs, about 30% and 13% of the respective UK totals. Mingulay is an important breeding ground for razorbills, with 9,514 pairs, 6.3% of the European population. The bumblebee Bombus jonellus var. hebridensis is endemic to the Hebrides and there are local variants of the dark green fritillary and green-veined white butterflies. The St Kilda wren is a subspecies of wren whose range is confined to the islands whose name it bears. Population The islands' total population was 26,502 at the 2001 census, and the 2011 figure was 27,684. During the same period Scottish island populations as a whole grew by 4% to 103,702. The largest settlement in the Outer Hebrides is Stornoway on Lewis, which has a population of about 8,100. The population estimate for 2019 was 26,720 according to a Comhairle nan Eilean Siar report which added that "the population of the Outer Hebrides is ageing" and that "young adults [...] leave the islands for further