Decline of Giant Kelp

Following surveys of M.pyrifera in south-eastern and eastern Tasmania (eg. Cribb 1954, Olsen 1966, Sanderson 1987), it is clear that the size and number of beds of M.pyrifera has dramatically declined over the past 30 years, to perhaps only 5% of the original area (Edgar 1997).

Beds have declined from approximately 120 km² in 1954 (Cribb 1954), to 8 km² in 1986 (Sanderson 1987), to approximately 0.5 km² in 1988/89 (SeaCare, pers.comm.). Possibly the greatest loss occurred on the east coast, with a greater than 90% decline occurring during 1978-81 (Edgar pers.comm.). A number of possible causes or factors have been identified by researchers, including:

• Large scale oceanographic changes, specifically the increased penetration and influence of the warm-water, low-nutrient Eastern Australian Current southwards along the east coast of Tasmania, which has resulted in a 1.5^oC rise in sea temperature since 1940 (Edgar 1999, Crawford et al.2000) (see Figure 1);
• Large scale ecosystem changes along the east coast (as a result of the above oceanographic changes) , particularly the increased abundance, since the 1960s, of the black sea urchin (ie. Centrostephanus rodgersii), which are known to graze on large kelps (Edgar 1999);
• The effects of marine pollution, particularly in the Derwent estuary and D’Entrecasteaux Channel (SeaCare, pers.comm.);
• And/or the introduction of the Japanese Kelp (Undaria pinnatifida) on the east coast of Tasmania, which has colonised many areas formerly occupied by M.pyrifera (Sanderson 1987).

Figure 1. Surface Sea Temperatures, NE Maria Island (20m depth) 1944-1998 (from Crawford et al. 2000) (Click thumbnail for larger version)

Other potential causes of kelp loss include, the commercial harvesting of kelp in the 1970s, coastal runoff, scallop dredging in the 1950s, and ecosystem changes due to fishing.

As the only and largest population of M.pyrifera in Australia, these factors mean that this species is likely threatened with local extinction (Edgar 1997), with potentially large-scale ecological and economic consequences. For this reason, the need for protection of Giant Kelp habitats and the recovery of degraded beds, has been highlighted as a major policy commitment by the Government of Tasmania (in it’s Environment Policy), and also, has been highlighted as a major national and state issue in recent State of the Environment Reports (Commonwealth of Australia 1996, SDAC 1996).

However, despite the considerable ecological and economic significance of Giant Kelp forests in Australia, the basic fundamental knowledge required for their long-term conservation and management – ie. distribution, ecology, health, and potential threats or risks to their survival – is poorly known.

While specific kelp surveys have been conducted on the south-east and east coasts of Tasmania (eg. Cribb 1954, Olsen 1966, Sanderson 1987), almost nothing is known of the distribution of forests on the southern, northern and western coasts or the health or ecology of kelp forests generally. Unfortunately, the large-scale loss of Giant Kelp forests in Tasmania has also been exacerbated by the lack of any government policy or integrated research program to assess the status and management of these marine ecosystems in Tasmania.