The following information was provided to the City by Susan Libes and Danielle Viso of the Burroughs & Chapin Center for Marine & Wetland Studies at Coastal Carolina University. Except for a few instances, we have not tried to edit the information:
Based on information collected by weather and water quality sensors located at the Apache Pier and the Cherry Grove Pier, the large number of dead menhaden fish that washed ashore in North Myrtle Beach on September 11 was associated with a period of low dissolved oxygen and salinity.
The data collected suggests the following causative oceanographic phenomena were ongoing leading up to the fish kill:
-- A typical summertime episode of low dissolved oxygen (< 4 mg/L) occurred from September 10 to September 11, with very brief periods of hypoxia, a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. This coincided with very brief periods of upwelling favorable southwesterly winds. Wind speeds were consistently low (< 10 mph) on both days.
-- The unusual occurrence of low salinity water at Cherry Grove also played a part. This started on September 9 and persisted, despite the lack of local rainfall. From September 5, onward, winds have been mostly out of the north. This has likely set up a southbound longshore current bringing fresher waters from Hog Inlet, Lockwood Folly, and/or the Cape Fear River to the Grand Strand. Also, significant rain fell inland from September 8 to September 11, totaling close to 2 inches. The low salinity water reached Apache Pier on September 12. The USGS sensor in the Waterway suggests significant freshwater was discharging from August 22 to September 6.
-- An algal bloom was detected in the bottom waters at both piers from September 8 to September 11.
Fish kills usually occur due to sustained periods of very low oxygen (hypoxia and anoxia) that fish cannot escape from. This may have been present offshore with the pier sensors detecting only the outer perimeter of the low dissolved oxygen zone. The algal bloom might have contributed to the low dissolved oxygen but could also contribute to a fish kill if comprised of species “toxic” to the menhaden. The freshwater carried by the southbound current could have supplied nutrients fueling the algae bloom.