Quantitative surveys of reef communities across the Lord Howe Island Marine Park (LHIMP) were undertaken during February/March each year from 2006 to 2010, with the exception of 2007. A total of 49 different sites were investigated overall, including 33 core sites resurveyed most years, and up to 46 sites surveyed in a single year. A total of 285 fishes (250 identified to species level), 31 crustaceans, 51 molluscs and 38 echinoderms were recorded, including eight new fish records and the first sighting of the eastern rock lobster Sagmariasus verreauxi at the island. Threatened species observed during surveys included the green turtle (Chelonia mydas), black cod (Epinephelus daemelii), blotched fantail ray (Taeniura meyeni) and humphead wrasse (Cheilinus undulatus). Fish species protected under New South Wales fisheries regulations sighted comprised the elegant wrasse (Anampses elegans), bluefish (Girella cyanea), and Ballina angelfish (Chaetodontoplus ballinae). No introduced fish, invertebrate or algal species was seen during surveys.
Underwater visual censuses indicated that the composition of the fish community changed through time from 2006 to 2010 in SZs, where fishes are fully protected, relative to HPZs, which remain open to fishing. The density of large (>40 cm length) fish increased by nearly an order of magnitude in SZs while remaining stable in HPZs. Densities of bluefish (Girella cyanea) declined dramatically within HPZs while remaining stable in SZs, a possible consequence of overfishing in HPZs. Densities of spotted sawtail (Prionurus maculatus) unexpectedly increased within SZs while remaining stable in HPZs. No changes in macro-invertebrate communities within SZs were detected relative to changes within HPZs. Such changes are, however, predicted through the longer term as predation by increasingly-high densities of large fishes and lobsters in SZs affects populations of invertebrate prey.
A threefold increase in density of the most abundant urchin species—the destructive-grazing black urchin Centrostephanus rodgersii—was recorded between 2006 and 2010, while densities of all four other common urchin species also rose significantly over this same time period. An increasing population of black urchins with associated expansion of ‗barrens habitat‘, in particular, greatly increases extinction risk for the rich local seaweed flora, which includes 47 species not recorded outside the LHIMP.
A total of 124 categories of sessile taxa were identified from photoquadrats. Prominent species recorded comprised the reef-building corals Acropora palifera (6.3%), Porites heronensis (3.9%), Pocillopora damicornis (1.1%), Acropora solitaryensis (0.8%), Cyphastrea cerialia (0.7%), Stylophora pistillata (0.5%) and Acropora yongei (0.5%), the soft corals Cladiella sp. (2.0%) and Xenia sp. (1.7%), and the algae Asparogopsis taxiformis (4.2%), Dictyota sp. (3.7%), Caulerpa racemosa (1.4%), Dilophus sp. (1.0%), Sarcodia ciliata (0.8%), Codium spongiosum (0.8%), Chlorodesmis major (0.4%), and Lobophora variegata (0.4%). These taxa were not evenly distributed across the LHIMP, but responded to a large degree to variation in wave exposure, and generally fell within three major groupings — (i) a coral grouping, (ii) a macro-algal grouping, and (iii) a sponge, hydroid, crustose coralline algae and hydrocoral grouping. Communities of sessile organisms at the Algal Holes, South East Rock and North Bay were distinctly different from each other and from other community types observed.
Total coral cover was stable between survey years, with 20% mean cover of the seabed between 2006 and 2010. By contrast, mean cover of large foliose macro-algae across all sites declined from 36% to 25% over the same period. This decline was not evenly spread across sites, but a precipitous decline decline occurred between 2006 and 2008 at sites with high recruitment of the urchin Tripneustes gratilla, with later major declines of macro-algal cover between 2008 and 2010 at other sites. The most likely cause of the macro-algae decline at T. gratilla outbreak sites is that this urchin overgrazed the seabed, causing expansion of bare areas of substrata at the expense of the foliose macro-algae. Declines in macro-algae at other sites more likely reflected the continuing population increase of other urchin species, particularly Centrostephanus rodgersii.
An additional apparent threat to LHI biodiversity values is coral bleaching, a threat exacerbated by the transitionary oceanographic position of LHI on the Tasman Front. Most coral colonies within the northern and eastern sectors of the Lagoon were greatly affected by an extreme heating event that extended through the first three months of 2010.
Negligible bleaching had been recorded in prior marine park surveys, although a widespread coral bleaching event was recorded at LHIMP in summer 1998 (P. Harrison unpubl. data), albeit with limited detectable impact on coral cover at that time (Harrison et al. 2011). Bleaching in 2010 was most severe (>90% of coral) amongst sites investigated at North Bay and Sylphs Hole, with Comets Hole, Horseshoe and Signal Point also badly affected (>50%). These sites were the most sheltered, and also were subjected to temperatures above 26 oC for a period that exceeded two months – an apparent threshold. Bleaching disproportionately affected Pocillopora damicornis, Acropora yongei, Seriatopora hystrix and Porites heronensis compared to faviid corals and Acropora palifera, including when the different coral taxa co-occurred at the same sites.
A full evaluation of bleaching impacts on coral communities through the medium term should be undertaken through follow-up surveys at the core ecological monitoring sites in February 2012. In addition to allowing an assessment of coral mortality, such surveys should include fishes and mobile macro-invertebrates to allow assessment of any persistent ecosystem-level impacts. Collection of coincident density data on fishes, macro-invertebrates, corals and macro-algae during the peak of the 2010 bleaching period provides an unprecedented opportunity to assess such broad-scale impacts.
The LHIMP ecological monitoring program has increasingly relied on skilled volunteers associated with the Reef Life Survey program to achieve monitoring goals. Assistance of volunteer divers trained in underwater visual census techniques should continue to be encouraged as it has allowed a major expansion of surveys in both time and space.
Management recommendations arising from the study are summarised as follows:
1. The magnitude of ongoing ecological change between surveys indicates that the ecosystem may be transitioning, and that frequent ecological monitoring is desirable. Follow-up surveys are particularly needed to quantify persistent impacts of the 2010 bleaching event on sessile invertebrate, macroalgal, mobile invertebrate and fish communities. These should be undertaken as soon as possible, ideally February 2012.
2. The reef monitoring program should be extended through the long term, with surveys undertaken at core monitoring sites on at least a three-yearly basis.
3. In intervening years, data collection from core monitoring sites should be undertaken as resources allow, including through facilitation of mechanisms that allow further assistance from volunteer divers trained in underwater visual census techniques.
4. Additional surveys of impacted and reference sites should be undertaken following exceptional events (e.g. oil spills, extreme bleaching).
5. Factors contributing to the formation of urchin barrens, and impacts of urchin barren formation on inshore flora and fauna, should be assessed.
6. An expanded network of long-term monitoring sites should be identified and developed in the intertidal zone.
7. Stakeholder negotiations should be undertaken with the aim of creating a sanctuary zone that includes the unique Algal Holes community type, as this community likely includes many globally endemic species and may be threatened by increasing grazing of sea urchins.
8. With the exception of suggested modification of boundaries associated with the Algal Holes, the boundaries of sanctuary zones should remain stable through the long term. The South East Rock sanctuary zone has particular importance within the existing sanctuary zone network due to exceptionally high fish biomass.
9. The 2006 baseline evaluation of marine pests should be updated through the establishment of a five-yearly monitoring program.
10. Consultation with the fishing sector should be undertaken with respect to management of declining bluefish numbers observed in HPZs.
11. Research studies on topics relevant to LHIMP management should be actively encouraged.
Graham J. Edgar, Joe Valentine, Antonia Cooper, Rick Stuart-Smith, Sallyann Gudge, Ian Kerr
Report for New South Wales Marine Parks Authority by Aquenal Pty Ltd, Kingston, Tasmania