Coral bleaching which has severely affected significant amouts of coral in the Torres Strait has been attributed to a combination of warmer than normal sea temperatures and low tides levels, according to a report from the TSRA Land and Sea Managament Unit.
The ARC Centre of Excellence for Reef Studies report, compiled by Andrew Baird (JCU) and Vic McGarth (TSRA), finds most-sensitive species, such as the soft corals and staghorn, will die within the next three to four weeks.
Most-affected colonies of the less-sensitive species will survive, although many will experience partial mortality.
Some of the coral from Thursday Island, Hammond Island and Entrance Island. Photos courtesy of Andrew Baird.Overall, particularly in areas such as soft corals and Acropora, there will be a significant loss of coral cover and, over time, fish and other species that rely on these species and the structure they provide will be expected to decline in abundance.
Recovery to similar level of cover in these areas, in the absence or further disturbance, is likely to take between five to 10 years.
The earliest evidence of bleaching was from Hammond Island on March 14. This was followed by reports of “a phenomena of a white and yellow discolouration appearing in large areas of soft coral around the Prince of Wales, Thursday and Horn Islands”.
Field surveys were conducted at two sites on Thursday Island - Hospital Point and Quarantine Beach; underwater surveys were conducted at Entrance Island; and aerial survey of the reefs around Thursday Island.
The report says coral bleaching is a stress response that can be triggered by many factors, including high temperature, sunlight (in particular UV radiation) and low salinity or a combination of these factors.
A preliminary examination of predicted tidal heights and satellite records of sea surface temperatures (SST) in the area suggest that both may have been involved in triggering the event.
A long period of calm hot weather in February which also may have contributed to this heating
In addition to this heat stress, low tides in early March occurred in the mid-afternoon potentially exposing corals to high UV.
The report says that, in addition, there were many anecdotal reports of tides being exceptionally low during the middle of the day on some occasions in February and/or March. Low salinity does not appear to have been a factor, because average rainfall over the past few months has been lower than usual.
The most likely cause of the bleaching was higher than normal sea surface temperatures through February and March combined with low tides.