As promised here is a statement from the high level working group who met to discuss the recent death of a number of stingrays in Orakei basin:
Agreed Statement - Stingray deaths in Orakei Basin during August 2020
Orakei Basin is a managed body of water that is generally kept with water levels around those of a high tide for the purposes of water sports and recreation in the Basin. Part of the Basin’s normal operating protocol is to permit tidal exchange of the Basin for a period of time on a regular fortnightly basis; in-part to manage any potential water quality issues within the Basin.
On 14 May 2020 (under Covid Level 4 lockdown) a malfunction of the sluice gates that control the exchange of water in and out of the Basin was reported. Subsequently, repairs to the sluice gates took two and a half months to complete and opening of the sluice gates did not occur again until Friday 7 August. During this period the Basin would have received significantly reduced tidal exchange, although some tidal exchange would still have taken place occasionally at the high end of the tidal range.
In the days following the operation of the sluice gates on Friday 7 August, dead stingrays began to be reported within the Basin. Subsequently Auckland Council operations contractors found and retrieved 13 dead stingrays from various locations on the shoreline and floating in the water.
Subsequent to these events, marine and other scientists, and other key staff from Auckland Council, Department of Conservation, Ministry for Primary Industries and Kelly Tarlton’s Aquarium met to discuss the likely causes. The following key points regarding the cause of the stingray deaths were agreed:
• The causes of the deaths of the rays cannot be known with absolute certainty.
• It was noted that there was a mixture long tail and short tail stingrays. Long tail stingrays, tend to be more commonly expected in channels
• Any rays in the basin are likely to be occasional visitors rather than resident.
• Based on the degree of decomposition of the rays it seems likely that they died in the period immediately following the opening of the sluice gates on 7 August.
• It seems unlikely that the rays entered the basin following the opening of the gates on 7 August. It is considered more likely that the rays were in the basin for the period from 14 May to 7 August but did not die during this period.
• If the rays were in the Basin prior to 14 May, this may have been due to the absence of boat traffic during this period (due to Covid Level 4 lockdown) as they are generally averse to such disturbance.
• Whilst it is possible that one or more water quality issues (e.g. wastewater overflows or salinity changes within the Basin) may have contributed to the death of the rays, decomposition of the carcasses prevented any testing of those for contaminants or pathogens and there is no specific water quality evidence in this regard.
• It is considered possible that the extended period of impoundment of the rays within the Basin may have led to behavioural changes whereby they failed to respond to tidal cues to move to deeper water on a receding tide. As such, it is considered possible that the rays died on the mudflats on the first outgoing tide. It was noted that there have been similar incidences of deaths of rays trapped in intertidal lagoons on an outgoing tide within the region previously, but not specifically at Orakei Basin.
All scientists endorsed increased water quality monitoring within the basin to help better understand if poor water quality conditions do occur during periods with limited tidal exchange.
Meeting participants:
• Clinton Duffy (Department of Conservation)
• Nick Ward (Ministry for Primary Industries)
• Madeline Seaman (Kelly Tarlton’s Aquarium)
• Megan Carbines (Auckland Council Research and Evaluation Unit)
• Samantha Happy (Auckland Council Biosecurity)
• Darryl Thompson (Auckland Council Environmental Health)
• Nick Vigar (Auckland Council Healthy Waters)
• Cath Duncan (Auckland Council Community Facilities)