GREAT BRAK RIVER NEWS - The Great Brak River Conservancy has informed the Great Brak River Estuary Forum and those who participate in it about the death of several adult spotted grunter fish in the river.
Reports of the dead grunter were received from the eastern side of Great Brak Island, adjacent to the Suiderkruis beach parking lot and in front of the police station.
Fishermen sent pictures of the dead fish to the Mossel Bay Advertiser this week.
Mossel Bay municipal spokesperson Nickey le Roux says scientists who participate in the Great Brak Estuary Forum responded that it is rare to have mortalities confined to a single species.
"This usually happens as a result of disease or the dying fish being the only large-bodied fish in the system at the time. This happens when juveniles can cope with stress such as low oxygen, but adults succumb due to their higher oxygen demand."
Low oxygen
The conservancy says low oxygen may be a cause as this is frequently recorded at all three sites mentioned, these sites also receiving runoff from stormwater outlets from the roads, picnic sites, parking lots and the golf course.
"High nutrient levels from these and other catchment sources feed the dense algal growth currently highly visible in the system. During daytime, algal photosynthesis produces sky-high supersaturated oxygen levels but nighttime respiration sees oxygen levels plummet to levels stressful to fish. Eventual algal die-off and decay result in low oxygen levels throughout the day," Le Roux says.
The conservancy says it is unlikely the causes of death are high nitrogen loads and ammonia toxicity as these would have led to more species and all sizes dying. However, the estuary is dynamic and this situation could change.
"Breaching is not going to solve the problem as there is likely to be an insufficient head of water to flush the system, and tidal exchange will be extremely limited."
Samples of the dead fish have been retrieved and will be preserved for analyses. Le Roux says the municipality is continuing to take water samples and monitoring the oxygen levels.
Fishermen took pictures of the dead fish that washed up on the banks of the Great Brak River.
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