The deaths of six hammerhead sharks at a Hong Kong theme park earlier this month were caused by malfunctioning equipment.
The six female sharks died one by one over a period of seven hours on November 3 in the Grand Aquarium at Ocean Park.
At the time it was thought there was an unidentified disease, and the remaining eight hammerhead sharks in the park's care were being carefully monitored.
But now it has emerged that the cause was due to a malfunction in the aquarium's ozoniser equipment.
In a press release Ocean Park said: "Upon further inspection, it is confirmed that the sensor regulating ozone output to the aquarium’s protein skimmer was affected by weak electrical interference and precipitated the incident by allowing sporadic higher-than-normal dosages of ozone by-products to enter the aquarium.
"The electrical interference from nearby electrical equipment produced a weak electrical current within the water pipe. Given water is a highly conductive medium, the small electrical currents entered the water and impacted the first sensor’s readings and signals. Coincidentally, the system’s built-in safety mechanism, a second sensor to prevent excessive ozone output, also experienced an independent malfunction, which measured lower-than-actual ozone by-product levels."
Ocean Park says the levels had still fallen within an internationally accepted range and would not pose a threat to most fish and shark species.
However it went on to say that hammerhead sharks — and particularly the larger-sized females, were "exquisitely sensitive to ozone and its by-products".
"With the sensors providing readings that were within normal ranges, the sporadic higher-than-normal dosages of ozone by-products and the highly localised point of entry into the aquarium, it made detection of slightly higher ozone by-products almost impossible by the normal in-aquarium sensor and the routine measurement practices employed throughout the exhibit.
"When exposed to excessive dosages of ozone by-products, animals’ gills and metabolic rates may become compromised, and lesions may be caused to the animal’s brain. In the case of the hammerhead sharks, it caused a detrimental impact to their gills and brain. Whilst quick action was taken to turn off the ozone output immediately and flush the aquarium water, the collaborative expert efforts by the Park’s staff were able to minimise the effect of the ozone by-products for the remaining hammerhead sharks, but it could not save the six female hammerhead sharks."
The park says that action has now been taken to identify and eliminate the source of the electrical interference, along with replacement of the sensors and installation of additional independent sensors to cross-check sensor readings.
"Moreover, the operation protocol of the ozone system has been adjusted such that on the occasion that the ozone level reaches certain required safe levels, the ozone machine will be turned off automatically, and can only be switched back on manually after the equipment is checked and confirmed to be normal. This ensures that safe levels are always maintained," the park said in its statement.
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