Unusual Mass Krill Stranding on Benguerra Island, Bazaruto Archipelago

On the 20th of September, a mass stranding of unidentified krill or shrimp species occurred in the northern bay on Benguerra Island, adjacent to the BCSS field station, and other parts of the island. Thousands of carcasses washed up ashore, covering the beaches as the low tide exposed them. The event was quickly followed by the arrival of ants, crabs and birds, feeding on the fresh crustaceans. The krill covered at least five kilometres of coastline, at a width of at least ten meters, meaning the approximate coverage area must be at a minimum of 50,000 m2. Regarding density, the team reported an average of 3500 carcasses per m2.

There could be a combination of factors that caused this event, including currents, tides, hydronamics, temperature variability, global climatic changes and oxygen supply levels.

Despite mass strandings being common, these events are still poorly understood. It is difficult to determine the reasons that may have caused the stranding, though the BCSS team hypothesizing that  it may be related to the strong tidal flows that move the krill shoal from deeper waters to the Bazaruto-Benguerra channel. The tide then pushes the shoal in, with the mass stranding as a result. However, there could be a combination of factors that caused this event, including currents, tides, hydrodynamics, temperature variability, global climatic changes, as well as oxygen supply levels in the seawater possibly resulting in anoxia, as it’s been demonstrated that many krill species do not tolerate hypoxic conditions. 

On some occasions, mass strandings of marine life are governed by low oxygen concentrations, which may be indirectly depleted by a phytoplankton bloom, causing oxygen starvation or prompting the animals to swim up to shallower water.

Krill is an important food source for whales, pinguins and seals at high latitudes, though shoals of krill in the Bazaruto Archipelago region have been typically preyed on by sardines and other baitfish, which are then followed by pelagic fish such as tuna. Mass mortality events of marine organisms are not common but have been widely reported in literature – from jellyfish and salps, to squid, fish and multiple cetaceans’ species. When occurring in coastal areas, the strandings might be wind-driven or current-induced. On some occasions, mass strandings of marine life are governed by low oxygen concentrations, which may be indirectly depleted by a phytoplankton bloom, causing oxygen starvation or prompting the animals to swim up to shallower water. Ultimately, this leads to death of the marine life, and the carcasses are pushed onto the coast by currents and the tide. 

Mass krill strandings are known and well described for Antarctic beaches, while are less common for lower latitudes, which makes this event interesting. This swarming species plays a key role in the trophic web of the most productive marine ecosystems of the world, and its abundance is essential for the larger marine wildlife that rely on them. Thus, its unusual mass stranding events are always concerning, however the fact that krill reproduces so quickly leaves scientists positively hopeful.

For questions about this article, please contact:
Mario Lebrato, Bazaruto Center for Scientific Studies
Mario.Lebrato@bcssmz.org

Bazaruto Center for Scientific Studies
Host of the first permanent Ocean Observatory focused on multi-ecosystem time series research in Africa, the Bazaruto Center for Scientific Studies (BCSS) was established in 2017 as in independent, non-profit organisation with a mission to protect and support the fragile ecosystems of the Bazaruto Archipelago, Mozambique. The research station is located on Benguerra Island, off the coast of Mozambique.

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