Australian Marine Parks

Graham J Edgar, Daniela Ceccarelli, Rick D Stuart-Smith, Antonia T Cooper. Report to Parks Australia, Department of the Environment, 2018.


Executive summary

This latest survey of Elizabeth and Middleton Reefs in the Lord Howe Marine Park (referred to here as the EMR) follows on from a decade of surveys conducted approximately every 2-3 years, primarily by James Cook University or Reef Life Survey divers. The primary changes noted in the time between 2013 and 2018 were a sharp increase in the number of species and abundance of cryptic fishes, a decline in turf cover on the reefs, and signs of increasing prevalence and biomass of large tropical herbivores. Functionally important fishes recorded in high densities, compared with other locations, included the regional endemic doubleheader wrasse Coris bulbifrons, the black cod Epinephelus daemelii, and the Galapagos shark Carcharhinus galapagensis, which remained relatively stable between 2013 and 2018. Black cod biomass was similar in 2018 to that recorded in 2013 at Middleton Reef; however, biomass significantly declined at Elizabeth Reef, perhaps due to incidental mortality associated with catch and release by recreational fishers.

Increases in cryptic and herbivorous fishes were also recorded on the southern Great Barrier Reef (GBR) and Coral Sea during the same period (Stuart-Smith et al. 2018), and were hypothesised to be related to warmer seas across the broader region in recent years, and especially the heatwave that caused the 2016 mass coral bleaching event along the GBR and Coral Sea. The consistency in the observations of herbivorous fishes and populations of small cryptic fishes with patterns observed much further north along the GBR and Coral Sea suggest the same large-scale drivers, and strongly point at elevated temperature as most likely responsible.

The cover of living benthos on both reefs was still dominated by low-lying turf growing on a dead coral base in 2018. This appears to be typical of highly exposed reef fronts, but also of reefs that have suffered past disturbances and coral mortality. A decline in turf cover observed at both reefs could be a direct result of the increased abundance of herbivorous fishes. Coral cover was slightly higher at both reefs than that recorded on RLS surveys in 2013, and by other researchers in 2011 and 2014, suggesting that corals have either been recovering from earlier disturbances or at least remained stable over the last decade. Slow recovery is expected on isolated sub-tropical reefs such as the EMR, where connectivity to source reefs and growth rates of corals are naturally low.

The key threats to these reefs are likely to be increasing sea temperatures, physical damage from severe storms, outbreaks of crown-of-thorns seastars, and illegal fishing. A further concern for the EMR in the future will be climate change induced deviations in the East Australian Current, potentially leading to significant changes in larval supply and physical environmental conditions. Recovery from disturbance is likely to be prolonged by the isolation from potential source reefs. These reefs remain a stronghold for Galapagos sharks and black cod because their isolation potentially protects them from illegal fishing. However, illegal and unreported fishing is still an issue (Edgar et al. 2016), and sharks are sometimes killed because they take fishers’ catches. Given the globally