Actaeon Islands

The notorious Actaeon Reef is in reality a number of reefs situated at the southern entrance to the D’Entrecasteaux Channel, between Bruny Island and the Tasmanian mainland. It is a botanical, zoological, ornithological, fisheries, geoheritage and maritime heritage wonderland.

Geography and Geology

Actaeon Island is a large reef system. Above sea level Actaeon Island itself is a group of three low lying tied islands that are connected via cobble tombolos (cobble banks). A further group of rocks and reefs 150 m south of the southernmost island are accessible at low tide. South of this is Sterile Island. Sterile Island is a small low lying island composed primarily of wave worked dolerite cobbles and small boulders resting on dolerite bedrock. It is surrounded by reefs, many exposed at low tide. These geomorphological features have been recognised as qualities of Outstanding Significance at a state level. The islands are composed of dolerite largely surrounded by, and in places overlain by, wave worked dolerite cobbles and boulders. On the eastern side of Actaeon Island is a layer of friable sandy soil which has developed over thousands of years due to shearwaters and little penguin poo. This area is a popular fur seal haulout. The northern island is the largest and highest at only 14 m above sea level. The total area of Actaeon Island occupied by terrestrial vegetation is 12.9 ha, comprising of the northern (10.25 ha), middle (0.95 ha) and southern (1.7 ha) islands. The total area of Sterile Island can be as big as 4.5 ha at low tide, with the vegetated portion less than 2 ha. The southern and western coasts have a gentle slope into the sea and are partly protected by the large reefs to the south. The remains of a wreck and driftwood litter the shores.

History

The bands of indigenous Tasmanians who were living in the area included the Lyluequonny and Nuenonne who used of bark canoes to get to the islands. Stone arrangements that were likely made by aboriginal visitation were previously recorded on both islands. Aboriginal people may have introduce the swamp antechinus that they called tore,er to the island as a food source. They are so numerous that Europeans thought the islands were infested with rats. In April 1792, through an error of navigation, the crews of the French naval vessels Recherche and Esperence were the first Europeans to see the Actaeon and Sterile Islands and gave the name of Ile Steriles (the Sterile Islands) to the reefs.

The islands were too small, stony and waterless to be put to much use by Europeans. In the 1830’s or 40’s a whaler’s lookout and signal flag pole were placed on the islands. They were periodically visited by hunters and naturalists. In 1884, naturalist Colonel William Legge noted wildlife on the island and recorded white bellied sea eagle and crested terns. Neither of these species have been noted as breeding on the island since. It might have had something to do with burning of the islands. Actaeon Island has substantial mutton bird or short-tailed shearwater (Ardenna tenuirostris) colonies. For many years it was visited annually by locals for mutton birding, party’s taking around 20 dozen birds each. In late November 1886 a yachting party were “menaced by a goat” left on the island. Rabbits were also introduced, but they have since died out. Even in the 1970s the island was crossed by trails, kept open by regular firing. On the first weekend of the mutton birding season in 1947, 200 dozen birds were taken on Actaeon. Sterile and Actaeon Islands were designated a Game Reserve in 1984.

Actaeon Islands Underwater Life

Although nominally in the Davey Bioregion that dominates South Western Tasmania, the Actaeons contain many features in common with both the Davey and Bruny bioregions. The outer coast in this area is subject to the heavy swells that characterise the Davey Bioregion. Inshore, it also includes an extensive network of coastal reef and delicate Macrocystis forests. The shallow reef area forms a barrier that provides some protection to a substantial portion of coast and reef inside this barrier. The reef in the vicinity of the Actaeon Islands is vast. Inshore reef in Tasmanian waters is usually limited to a narrow coastal fringe, making this reef system an unusual feature. Reef habitats on the highly exposed outer Actaeon Islands are dominated by bull kelp Durvillaea to depths of 10 m or more. As the swell force lessens, crazyweed Phyllospora dominates from 5 to 15 m . Strap weed Ecklonia is then common from 10 to 30 m. Red algae is common in the shade of the larger plants below 10 m and sponge communities dominate below 30m. Inside the Actaeon Islands, exposure is moderate. Macrocystis is common in the moderately exposed areas, forming extensive beds on reefs of 5-15 m depth inside the Actaeon Islands.

Unique Handfish

The Ziebell’s Handfish has been found on the edge of giant kelp forests (Marcrocystis pyrifera) at the Actaeon Islands. The species is found at depths of between 3m and 20m. The Ziebell’s Handfish was named after the diver that handed in the first specimen back in the 1970s. Other material says the fish appears to prefer softbottomed habitats, with patches of rock that support sponge and algae communities, which they use as spawning substrate. The area also once boasted Red Handfish. Graeme Blight found some here in the 1980s. One abalone diver recently told me that they would see a couple of Ziebell’s handfish every year on the edge of the kelp, but none in the last 10 years, coinciding with a dramatic loss of kelp density.

Premier Abalone Fishery

The Actaeon Reef system is Australia’s most valuable region of coast in terms of economic yield from abalone fisheries harvests. This remarkably productive region has average production of around 350 tonnes, comparable to the entire WA and NSW catch combined. Abalone sub block 13E (which contains the Actaeons reef system) has maintained a high level of productivity since the commercial abalone industry commenced in the mid sixties. At current prices, this sub block generates about $10 million each year in export revenue for Tasmania. An abalone fishery reserve at George III Rock has existed in this region since 1985 and has probably helped to keep the area productive. Tests indicated that abalone populations are largely (90–100%)  from other local abalone, rather than distant reefs. The relatively large apparent export of larvae from George III Rocks ‘no take’ area may be important at least for local replenishment of abalone stocks and to contribute to the productivity of a fishery. Catch and catch rate from this region have declined over the last few years showing that perceptions that this area is robust to high fishing levels could be wrong. The basis for the remarkable productivity of the location is a mystery. There is no information available to assess risk or protect future production.

The Actaeons are critical to the Tasmanian fishery because any decline in catch from this location sends a cascade of shifting catch effort to other areas that is difficult to manage.

Giant Kelp Disappearance

“There is one marine production, which from its importance is worthy of a particular history. It is the kelp, Macrocystis pyrifera. This plant grows on every rock, from lowwater mark to a great depth, both on the outer coast and within the channels…The number of living creatures of all Orders, whose existence intimately depends on the kelp is wonderful. A great volume might be written, describing the inhabitants of one of these beds of seaweed….I can only compare these great aquatic forests of the southern hemisphere, with the terrestrial ones in the inter-tropical regions.” (Charles Darwin 1845)

The sheltered giant kelp forests of the Actaeons are home to schools of pike, trumpeter and sweep. Sweep are a more recent arrival this far south, a symptom of the warming waters of the East Coast. The area is still blasted predominantly by colder Southern Ocean currents rich in nutrients, that previously allowed some of the forests to hold on, when 90% of the kelp forests on the East Coast of Tasmania disappeared.

The Far South has always been acknowledged for its large kelp forests, “Southport is situated on the south part of Recherche Bay,…but that on the NW side, although there is a depth of 3 fathoms, is so choked with weeds, that it was with difficulty a boat could pass.” The Australian Directory, 1830

According to abalone divers in the last ten years the density of the kelp forests has fallen dramatically. The kelp canopy is a major habitat for marine life. When growth is vigorous, the kelp forest is crowned by a dense surface canopy. The canopy acts as a nursery for juvenile fishes, attracting swirling schools of small bait fish and predatory fish. The kelp canopy also provides a resting place for seabirds and seals. The Giant Kelp plant an essential source of nutrients for many animals. For example, limpets ingest broken pieces of kelp from offshore beds and detritivores ingest microscopic pieces of kelp after it has broken down in the surf. On the seafloor, large numbers of filter-feeders or suspension feeders (such as sponges), feed on kelp detritus. This abundance is also probably due to the concentrated settlement of planktonic larvae beneath the kelp canopy, which has accumulated because of the reduced water flow (compared to more exposed hard substrata). Reproduction and growth in Macrocystis pyrifera is strongly coupled to environmental fluctuations. Cold, nutrient-rich springtime waters are optimal for growth and reproduction. In Tasmania a number of factors-linked with El Nino episodes-have probably dampened reproductive success including rising in minimum water temperatures, declining influence of SubAntarctic waters, increasing influence of subtropical Eastern Australian Current waters, reductions in seasonal and interannual variability in sea temperatures on the east coast.

A report by Dr Karen Edyvane in 2003 was seen at the time as unduly pessimistic, but if anything it got worse. Some kelp forests have fluctuated but 99.9% historic kelp loss on the East Coast is now the ‘new normal’. It ebbed and flowed but now is effectively gone. The area inshore of the Actaeons has thinned dramatically since about 2004. NOAA landsat, A September 2014 photo shows missing kelp beds in the area, however, In October 2015 I noted that the bed north of Southport Island has returned. Then in 2019 the last beds at the Actaeons completely disappeared. 

Land Flora

The islands have been dramatically changed by fire particularly evident on the northern Actaeon island. The deeper soils mean that it should be covered in tall scrub or forest. However, localised disturbance by burrowing penguins and mutton birds, and small scale patch burning by Aboriginal people prevented this. Instead, it has had a relatively diverse mosaic of vegetation. The dense regrowth scrub, bracken and weeds that currently dominate the island have resulted from a succession of high intensity island-wide fires. The centre part of the northern island is covered in dense blackwood scrub with, blackberries and mirror bush (Coprosma repens) tangling in between the trees. The understorey is thick with leaf litter and bracken (Pteridium esculentum). Around the margins of the scrubby centre is very dense, head high bracken. Mutton bird rookeries are covered by tussock grass, bracken, scattered herbs and small shrubs. On the edges of the coastal rocks where there is less soil depth, the vegetation is dominated by coast groundsel. The other islands are relatively bare, but have weeds including large amounts of brome. The vegetation is dominated by fireweed, coastal saltbush, native pigface (Carpobrotus rossii). In total, 49 species of vascular plant, seven of which are introduced, have been recorded from Actaeon Island. The vegetation on Sterile Island also boastd sawleaf daisybush (Olearia stellulata) and coast groundsel (Senecio pinnatifolius). The centre of the island is dominated by tussock grass (Poa poiformis) On the lower energy southern shore, low mats of bower spinach (Tetragonia implexicoma) occur above the high tide mark. This area is a favoured nesting spot for gulls. Actaeon Island was noted by John Gould in 1838 and Colonel William Legge in 1886, as being dominated by “barilla bush”, also known as Grey saltbush [Atriplex cinerea]). This species appears to have been wiped out by fire. The vegetation of the island is presently in a state of flux, with scrub and weed taking over as fire and seabird disturbance lessens. Maybe one day it will stabilise back to a more natural state