A biophysical profile of Tristan da Cunha -Sue Scott - 2017 A Biophysical Profile of the Tristan da Cunha Archipelago Sue Scott Commissioned and reviewed by The Pew Charitable Trusts 2017 1 A biophysical profile of Tristan da Cunha -Sue Scott - 2017 Contents 1. Key information on Tristan da Cunha 7 2. Geophysical setting 11 2.1. Isolated islands 11 2.2. Geology and topography 11 2.3. Climate 17 3. The ecology and biodiversity of the marine ecosystems of Tristan da Cunha 21 3.1. The pelagic ecosystem 21 3.1.1. Oceanography and ecosystem productivity 21 3.1.2. Plankton and pelagic life 25 3.1.3. Pelagic fishes 27 3.2. Marine biological surveys 29 3.3. The coastal environment 32 3.3.1. The seabed 32 3.3.2. Tides 34 3.3.3. Seashore 35 3.3.4. Intertidal bedrock and stable boulders 35 3.3.5. Rock pools and channels 39 3.3.6. Boulder beaches 42 3.4. Shallow subtidal to 40m depth 43 3.5. Deeper subtidal 45 3.6. Deep sea 48 3.7. Diversity and biogeography of benthic biota 53 3.7.1. Diversity and biogeography of fish communities 53 3.7.2. Diversity and biogeography of invertebrates 57 3.7.3. Diversity and biogeography of macroalgae 60 4. Marine mammals of Tristan da Cunha 67 4.1. Introduction 67 4.2. Seals (order Carnivora) 68 4.2.1. Eared Seals (family Otariidae) 68 4.2.2 True Seals (family Phocidae) 70 4.2.3. Rare vagrant seals 72 4.3. Cetaceans 72 4.3.1. Baleen Whales (suborder Mysticeti) 73 4.3.2 Toothed Whales (suborder Odontoceti) 75 5. Seabirds of Tristan da Cunha 83 5.1. Introduction 83 5.2. Population size, ecology and distribution of the seabirds of Tristan and Gough 85 5.2.1 Penguins (family Spheniscidae) 85 5.2.2. Albatrosses (Family Diomedeidae) 91 5.2.3. Petrels and Shearwaters (family Procellariidae) 99 5.2.4 Southern storm-petrels (Family Oceanitidae) 120 5.2.5 Skuas (family Stercorariidae) 126 2 A biophysical profile of Tristan da Cunha -Sue Scott - 2017 5.2.6. Gulls, Terns and Noddies (family Laridae) 128 5.2.7. Terrestrial birds 131 6. Exploitation of the marine resources of Tristan da Cunha 143 6.1. Historical exploitation of seals, whales and seabirds 143 6.2. Fishing 146 6.2.1 History of fishing 146 6.2.2. Current fisheries 153 6.2.3. Biology of the Tristan rock lobster Jasus tristani 162 7. Conservation and threats 169 7.1. Current protection measures 169 7.1.1. Designations and boundaries 169 7.1.2. Biodiversity action plans 170 7.1.3. International conventions 172 7.2. Current threats 173 7.2.1. Non-native species 173 7.2.2. Marine incidents and pollution 180 7.2.3. Longline fishing 181 7.2.4. IUU fishing 182 7.2.5. Disturbance 183 7.2.6. Plastics and other seaborne pollution 184 7.3. Future threats 185 7.3.1. Fisheries 185 7.3.2. Coastal development 186 7.3.3. Climate change 187 3 A biophysical profile of Tristan da Cunha -Sue Scott - 2017 Summary Biodiversity and ecology The Tristan archipelago is situated in temperate South Atlantic waters, approximately half way between South Africa and South America, thousands of kilometers from the nearest land. The territory consists of three northern ('top') islands, Tristan, Inaccessible and Nightingale, about 25km apart, together with Gough situated on its own around 350km to the south southeast. The islands are the tops of volcanoes that arose from hotspots beneath the seafloor, and range in age from less than 200,000 years (Tristan) to 18 million years (Nightingale). They are all small, with Tristan the largest at 12km across, and an area of 96km2. The Peak on Tristan rises to 2,060m above sea level, and slopes steeply to the seafloor at 2-3000m within a few km of the coast. The top islands have a warm temperate climate, while Gough is cool temperate. All the islands are exposed and windy, with no sheltered marine habitats. Tristan claims to be the most isolated inhabited island in the world, and has a small community of around 270 islanders, in a single settlement on Tristan. The community is largely self-supporting, with the main sources of income being rock lobster (crayfish) fishing, and the sale of stamps and other souvenirs. Tourism also brings in revenue, but potentially lucrative trade from passing cruise and expedition ships is severely limited by the remote location and often difficult landing conditions; the small harbor is often unusable because of swell. Wildlife conservation and scientific research is increasingly bringing funded projects to the islands, employing both outside biologists and islanders. There are no other islands in the temperate south Atlantic to act as stepping stones, so life that has colonized the islands is a collection of chance arrivals of spores and seeds borne on the wind, on ocean currents or carried by birds, while much of the shallow-water benthic marine life probably arrived on drifting kelp and other marine debris. In common with other small, isolated islands, biodiversity is generally low, but varies with group; sponges and seaweeds are relatively diverse, while echinoderms and mollusks are very impoverished. Subsequent evolution in isolation has resulted in a high proportion of endemics in some plant and animal groups. The 'top' islands of Tristan, Inaccessible and Nightingale lie on the north edge of the Subtropical Convergence (STC), where the warm waters of the South Atlantic gyre meet colder waters to the south. Seawater temperatures at the top islands average 14°C in winter and 19°C in summer. Gough, some 350km to the south southeast, is around 3°C colder year-round than the top islands, and is consequently more subantarctic in nature. In shallow water down to at least 40m, forests of giant and pale kelp thrive, forming a highly productive habitat for smaller seaweeds, abundant fish and numerous invertebrates including the Tristan rock lobster, the basis for an MSC certified fishery. Little is known of life in deeper water around the islands. While Tristan itself has been much changed over the years through clearing of natural vegetation, harvesting of mammals and birds, agriculture and introduction of alien species, the uninhabited islands still support an abundance of wildlife. They are of global importance for breeding seabirds, endemic land birds, and Subantarctic Fur Seals, and home to many endemic plants and invertebrates. Gough and Inaccessible islands together comprise a World Heritage Site, with boundaries out to the 12nm territorial limit. The islands are home to entire, or a large proportion of, the world breeding populations of Spectacled Petrels, Tristan Albatross, Atlantic Yellownosed Albatross and Northern Rockhopper Penguins, as well as seven endemic land birds. The islands also appear to be a stronghold for Shepherd's Beaked Whales, an otherwise little known whale species, and a nursery area for Southern Right Whales, although these are much reduced from former numbers by hunting. At least another nine cetacean species are regular visitors. 4 A biophysical profile of Tristan da Cunha -Sue Scott - 2017 Sensitivity As with many isolated islands, the Tristan islands are particularly vulnerable to the effects of invasive introduced species. House Mice on Gough are having a devastating effect on Tristan Albatross and Atlantic Petrel populations, by eating chicks and eggs, and their eradication is a priority. Rats on Tristan are similarly preventing the recovery of petrels, following the cessation of harvesting by humans. Inaccessible and Nightingale islands so far remain free of mice and rats, but are highly vulnerable to their introduction from Tristan, where both are present, and from shipwrecks. Invasive plants including New Zealand Flax and Procumbent Pearlwort are threatening natural vegetation, and introduced invertebrates have become agricultural pests on Tristan. In the marine environment, the South American Silver Bream (porgy), a fish introduced with a stranded oil rig in 2006, is now abundant around Tristan; its effect on native fish and rock lobster populations is as yet unknown. As well as historical impacts and ongoing mortality from introduced rodents, foraging seabirds are at risk from activities a long way from the islands, and outside territorial waters. Longlining in particular still kills many albatrosses, though simple deterrents have greatly reduced the mortality in more responsible fisheries in recent years. As evidenced by the stranding of an oil rig in 2006 and the wreck of the bulk carrier Oliva in 2011, the islands are at risk from marine incidents, and subsequent impacts from spilt oil and cargo, as well as introduced species. This risk has increased in recent years with more ships passing near the islands en route from South America to the Far East. Some means of flagging the islands as a sensitive area, and one to be avoided by commercial shipping, is required. Climate change could cause profound changes in the Tristan marine environment. Giant kelp, a keystone species in shallow water around all the islands, is already at the upper limit of its temperature tolerance at the top islands. Even slight warming of seawater here could result in large-scale disappearance of giant kelp, with knock-on effects on the numerous organisms occupying the kelp forests, including the rock lobsters on which the Tristan economy depends. IUU fishing is a real threat in a situation where policing such remote islands is difficult, and in a fishery with such a small total target area, any IUU fishing is significant. Reduction or collapse of the fishery could create pressure to exploit other marine resources. Other services provided by the fishing company, particularly transport of people and goods to and from Cape Town, is also very important to islanders.