Surveys of the Subtidal Reef Biota of the Cradle Coast 1992-2018

This report documents changes on rocky reefs in the Cradle Coast NRM region over the past 25 years, by examining the results of approximately decadal-scale surveys undertaken during the mid 1990s, 2006/2007 and 2017/2018. Overall, reef communities appear to have experience some changes during the past 25 years in response to climate change and fishing pressure. These changes follow, to a lesser extent, the ones reported for reef systems in eastern Tasmanian waters, where rapid warming as a result of increasing EAC influence has seen major changes in biological assemblages. The 0.4 ℃ rise in sea surface temperature in the NW of Tasmania during the 25 years of study, may have driven the increases in some warm-affinity species in the region, particularly amongst the herbivorous fishes. Furthermore, there was a decline of canopy algae in the north region of the Cradle Coast as well as a drop to zero percentage cover of Macrocystis pyrifera during the time of study. Additionally, invertebrate’s species richness dropped to almost half of the 1990s values. This drop in species richness was also mirrored by a drop in abundance of key species such as the common urchin Heliocidaris erythrogramma, the seastar Meridiastra calcar and blacklip abalone Haliotis rubra. Declines in harvested species have also occurred, including declines in size and abundance of the major fish target species, abalone and lobster. Population decline in these important predatory and herbivore species may affect some ecosystem processes, in addition to the fishery related management issues raised. Introduced marine pests did not represent a major threat, although the introduced seastar Astrostole scabra appears to be gradually increasing in abundance along the west coast. Declines in industrial effluent to the coastal waters surrounding Burnie region have coincided with an increase in algal cover of some groups (i.e. foliose brown and filamentous red alga); apparently previously impacted by lack of light to the reef due to pollution increasing turbidity. However, surprisingly few signs of major recovery were apparent overall. To facilitate ongoing examination of reef health in the Burnie region, seven additional monitoring sites were added, with a further three sites added in the far north-west to improve regional coverage for future reporting.