This is a guest post by Mike Aryeh, Marine mammal scientist and GIS analyst with ResourceTrust Ghana.
If you are reading this, you might have heard of dolphins stranding close to Axim on the 4th of April 2021. You might also have read about the washing ashore of thousands of dead fishes at Osu, a suburb of Accra a week before the dolphin stranding. If you have much more interest in the ocean, you might have seen the recent Netflix documentary ‘Seaspiracy’, highlighting irregularities in the commercial fishing industry globally. The big question you may be asking yourself by now is, are there any relation between these events. Follow my thread as I breakdown the series of events and what the causes and implications might be.
Around 5 am on the 4th of April 2021, a group of fishermen returning from sea encountered over sixty (60) very dark coloured dolphins stranded close to shore, near Axim (4.8665° N, 2.2409° W). The sighting of dolphins is common during the Easter period because there is always a high shoal of small fishes close to shore. Once in a couple of years, some dolphins get stranded but never in recorded history has over sixty dolphins of the same species been stranded along the coast of Ghana. To the local fishermen, this was a gift from nature, and they were determined to capture as many as they could. They tied the tail fin of these stranded dolphins with fishing nets and pulled them towards shore, while others parked the dolphins into trucks. Authorities from the fisheries commission arrived at the scene about 4 hours later. By then, over 30 dolphins had been captured and exported off the shore. Investigations with police revealed the location of the captured dolphins and were confiscated, by which time all thirty were dead. Those that were still alive on the shore numbering 21, were guided back to sea. In all, 37 dead dolphins were recovered from local fishermen and buried by authorities.
Locals capturing live stranded dolphin by the tail and loading them into trucks.
What we know
The dolphin species encountered was the Melon-headed whale (Peponocephalia electra) aka Electra dolphin, little killer whale, or many-toothed blackfish. This is a small- to medium-sized toothed whale of the oceanic dolphin family (Delphinidae). They are widely distributed throughout deep tropical/subtropical waters worldwide. They are found near shore mostly around oceanic islands, such as Hawaii, French Polynesia, and the Philippines. Their diet includes pelagic and mesopelagic squid, small fishes, and crustaceans. They are a highly social species and usually travel and forage in large groups of 100 – 500 individuals. Melon-headed whales forage at night and rest during the early hours of the morning. Females reach sexual maturity by age seven, giving birth to one offspring every three to four years after a 12-month gestation period. They are studied to calve throughout the year and stranding is known to normally occurred in huge numbers due to their gregarious nature. Previously, large number of stranding were recorded in Hawaii, Japan, the Philippines, northern Australia, Madagascar, Brazil, and Cape Verde Islands. This was the first recorded high numbered stranding along the coast of mainland West Africa.
What we do not know
The major question is what the main cause of this stranding could be. There is no definite answer to such marine occurrences because little is done to monitor marine life globally. Had the dolphins washed ashore dead, then the killing of cetaceans at sea using sophisticated fishing devices, as shown in the ‘Seaspiracy’ documentary, could be a cause. Alive stranded dolphins leave us with two strong hypotheses.
1) Melon-headed whales are vulnerable to noise, such as those associated with military sonar activities, seismic surveys, and high-power multi-beam echosounder operations. They are very sensitive to mid-frequency active sonar (1 to 10 kHz) used in military operations and other types of sonar activities like oil and gas exploration. If such activity has happened recently it could be the cause of stranding.
2) The other more probable hypothesis is related to the social behaviour of melon-headed dolphins. They might have been following the shoal of fish abundant close to shore at this period, as witnessed at Osu weeks ago, and missed the tides. They might be caught in the low tides and displaced to land unexpectedly, restricting their movement.
None of the two hypotheses have yet been proven. Information on military activities and seismic survey is always hidden from the public. No data have yet been obtained on such activities at sea yet. To assess the second hypotheses, satellite data on recent tides is being analysed. Even though this occurrence is not common, this has been expected in the marine science society due to the changing temperature of the ocean caused by climate change. Until the cause is known, the focus is on sensitizing locals on the need to avoid feeding on stranded marine mammals that are higher up the food chain and could be bioaccumulate for mercury, PCBs and microplastics. Melon-headed dolphins are currently listed as Least Concerned on the IUCN Redlist but there is no information on current levels of bycatch and commercial hunting, therefore the potential effects of such stranding their populations are undetermined. There is the need to keep studying these species and sensitizing locals along the global coast to avoid such a disastrous incident in future.