Mark Langdon

Sea urchins can have a significant influence upon the ecological structure of coral reefs through bioerosion of substrata and also by affecting competition for space. They are important grazers in many marine systems and can cause major ecosystem changes when their populations reach high levels (generally after a decline in the numbers of their fish predators). However, the relative importance of the role of sea urchins in influencing the composition and structure of coral reef habitats has rarely been explored. This study is linked to the CSIRO Wealth from Oceans Ningaloo Collaboration Cluster, Component 1: Habitat Mapping and Biodiversity. It has so far examined coral reef habitats and macroinvertebrate (particularly urchins) distribution and abundance within Ningaloo Marine Park. Field sampling has been undertaken at over 100 sites within the Park, focussing on near shore, lagoonal and back reef areas within Sanctuary zones and adjacent Recreation zones. Data analyses so far indicate that the distribution of urchins is not affected by the management zones of the park (i.e. no significant evidence has been found of indirect effects from fishing of urchin predators). However, habitat type has a major influence on urchin distribution, e.g. urchin populations were higher on nearshore intertidal and sub-tidal reef platforms, lagoonal patch reefs and shallow backreef platforms than other habitats. In coral reefs in other parts of the world, unusually high urchin populations can indicate overfishing. So far, this study has found no indication of fishing pressure indirectly affecting urchin densities, which suggests that the current zoning may be effective. However, further analyses of the data, particularly those from the nearshore sanctuary areas where shore based fishing activities are allowed, are yet to be completed. If fishing pressure is having an effect, it is likely to be seen in those areas where urchin abundance is predicted to be highest.

Surveys were undertaken of key invertebrate and plant groups in lagoon areas throughout the Ningaloo Marine Park, focussing on identifying and quantifying soft corals, sponges, echinoderms (urchins and sea cucumbers) and seagrasses; this information was linked to the underlying habitat structure in the Ningaloo Reef lagoons, used in a parallel habitat mapping project based on hyperspectral aerial photography. The biodiversity surveys conducted throughout the Ningaloo Reef system have shown that there are clear differences in the lagoon systems in different parts of the Marine Park, with clear biogeographic separation of sponges, soft corals and seagrasses. This has important implications for the distribution and abundance of many animals, including some of commercial importance. The northern section of the Marine Park (north of Point Cloates) is more strongly tropical than the southern section, which has many temperate species present. There are also several locations within the Ningaloo Reef system that are quite unique and don’t necessarily match the surrounding lagoon environment. Examples include Coral Bay, Bateman Bay and the Point Cloates region. These observations will be important in the management of the Ningaloo Marine Park; the northern and southern sections of the Marine Park may need to be managed differently. Surveying biological groups over the whole Marine Park is difficult and time consuming; further sampling in more locations and at different times of year are required to build on the findings of our study. There are clear seasonal differences in primary productivity in the lagoons, which likely drive significant community-wide changes throughout the year. These seasonal variations will also affect the validity of the habitat maps, which were based on imagery collected at only one time of year. Seasonal sampling will improve the reliability of the habitat maps and also give a better understanding of how the Ningaloo system operates.

As part of the CSIRO Wealth from Oceans Ningaloo Collaborative Cluster programme currently underway in Western Australia, this study aims to examine lagoonal habitats and biodiversity within Ningaloo Reef. Key habitat types were identified using information from hyperspectral remote sensing and were used to develop a stratified sampling approach. Two focal areas were selected, based on sanctuary zones within Ningaloo Marine Park: Osprey Bay and Coral Bay in the north and south respectively. A nested quadrat sampling regime was used to attempt to link field-collected data with remotely-sensed data, collected at different scales. Preliminary results confirm that northern sections of Ningaloo Reef differ greatly from the south, with a greater diversity of habitats present in the broader lagoons in the south. Greater areas of coral are found close inshore and across the entire reef at the southern location, compared with the northern section, which has a broad expanse of sand and limestone pavement before grading to corals further offshore (back-reef and reef-crest). These differences in habitat may have implications for the overall biodiversity of the two locations and more broadly along the reef.