Last night, Cermaq Canada's Venture Point site, in the Okisollo Channel, experienced a number of mortalities, following a temporary drop in dissolved oxygen (DO) to 3.6 mg/l.
Seasonal drops in dissolved oxygen (DO) are common for hot summer months, as the solubility of oxygen decreases with increasing temperatures. And, in the Okisollo Channel, low DO levels are even more common.
In the case of Venture Point, monitoring alarms notified staff of the event so they were able to turn on additional compressors to increase water movement.
“Preliminary assessment shows low mortality. As per fish health protocols, samples are being taken to be sent for testing and the proper authorities are being notified,” Cermaq stated in a media release.
When levels drop below 4 mg/l, fish become very stressed and mortalities can occur – and this can be exacerbated by other stressors such as human/boat activity, or even feeding.
Therefore, it is commonplace for farmers to reduce all other stressors during low-DO events in order to prevent fish from dying – including all feeding and related activities.
Sea Shepherd puts fish at risk
During low DO events, the presence of vessels near farm systems leads to increase stress on the fish, putting their health at risk.
Fish Farming Expert has been informed that Sea Shepherd vessels have been observed very close to farm systems in the Okisollo Channel for several hours each day for the last two days - during times where there was low DO.
“This activity [draws] the fish to the surface as they think they may be fed,” said Marine Harvest Canada's Ian Roberts.
On July 27th, Roberts had requested by email that the Sea Shepherd boat and supporting vessels stay a respectable and safe distance from the farm systems.
“Our farms in the same water body as Venture Point have witnessed the same events of low dissolved oxygen over the past few weeks,” says Roberts.
“We usually see these low DO levels occur at this time of year and we have been running air compressors when needed to improve the situation and have not seen any increase in mortality,” he continues.
He went on to explain how this information was conveyed to – but disregarded by – the crew of the Sea Shepherd's vessel.
“I had sent this information about low oxygen levels and its effect on fish behaviour to Alex Morton directly by email, but unfortunately she has chosen to ignore these facts and fabricate her own story,” Roberts reveals.
Fish adapt to low DO
During periods of warm weather and associated lower levels of oxygen, fish are often seen as “finning” on the surface of the water.
Although this behavior may seem unhealthy and indicative of disease to the lay-person, this behavior is totally normal.
“The top metre of the ocean is often richer in dissolved oxygen, so fish know to surface to run their gills through the top layer of water,” explains Roberts.
“At these times, we stop feeding and, if required, we are able to turn on air compressors to provide additional air. The staff also stay off the system during low DO times, so not to disturb the fish or have the fish think it is feeding time. Our fish are under the watchful eye of our fish health and welfare team during these times,” he adds.
As soon as the DO levels exceed 5 mg/l, farmers are again able to feed their fish. Feeding reaction involves fish finning at the surface waiting for feed, and also jumping when they approach the surface – all normal behavior.