Madagascar may be a secret stronghold for 'living fossil' fish

Madagascar may be a secret stronghold for coelacanths, the "living fossil" fish that were considered extinct until a fisherman caught one in 1938.

That incredible first specimen hailed from the coast of South Africa, but coelacanths of the same species — Latimeria chalumnae — have since turned up off Tanzania, the Comoros (a group of islands off the eastern coast of Africa) and Madagascar. Now, a new review of the Madagascar fishery bycatch, or accidental catch, reveals that at least 34 confirmed specimens have been caught and that many more likely have been pulled up that never reached the attention of biologists or conservationists. Though the overall population numbers remain a mystery, the authors of the new study suspect that Madagascar may be an important habitat for coelacanths and that it may even be their ancestral home. 

With 420 million years of history behind them, coelacanths are older than Madagascar, which has had a coastline for 88 million years and has been in its current location for about 40 million years. But they're best known from the Comoros, which are only about 15 million years old. Researchers think the fish may have been living in Madagascar longer, colonizing the Comoros later in history.

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Madagascar "has got a vast shoreline, and we know that there are canyons along its coast," study co-author Mike Bruton, an ichthyologist based in Cape Town, South Africa, told Live Science. "And we know that coelacanths like to live in canyons from depths of about 150 to 500 meters [500 to 1,600 feet]."

Madagascar is also much older than the Comoros, where most recorded coelacanth catches come from. Because coelacanth fossil history stretches back 420 million years, Bruton and his colleagues believe that, compared with the Comoros, Madagascar might have been home to coelacanths for longer.

Fossil fish

That long history is what makes coelacanths fascinating, said Bruton, the author of "The Annotated Old Fourlegs: The Updated Story of the Coelacanth" (University Press of Florida, 2018). These fish evolved 180 million years before the dinosaurs first emerged, surviving even as continents shifted and an asteroid wiped out much of life on Earth, including marine "sea monsters" like mosasaurs. Known first from fossils, coelacanths were believed extinct until a trawler caught one in a gill net in December 1938 near South Africa. According to the Australian Museum, the crew was intrigued enough by the large, bizarre-looking fish that they alerted a museum in East London, South Africa, whose scientists brought the specimen to the attention of South African ichthyologist J.L.B. Smith. Smith confirmed that the creature was a coelacanth and gave it a scientific name. (Another species, Latimeria menadoensis, was discovered in 1998 near Indonesia.)

Information on coelacanths in Madagascar waters has always been scattered and disorganized, Bruton said; there has never been a coelacanth specialist based on the island. Given the promising habitat around the coast, the researchers started to gather reports of coelacanth catches. They found an increasing number of reports with time, possibly because of the rising popularity of large-mesh gill nets used to catch sharks for the shark-fin market. These gill nets, called jarifa nets, are left in deep water and sometimes baited with small fish. The nets probably go undetected by coelacanths until it's too late, as the fish hunt by night and mostly by using electroreception, the detection of tiny electric fields made by prey in the water. The nets don't generate electric fields. Making matters worse for coelacanths, the nets can be deployed in the rocky canyons they prefer, unlike trawl nets, which must be used on relatively smooth seafloor.

Of 34 catches with enough detail recorded to be confirmed coelacanths, the fish ranged in weight from 66 to 198 lbs. (30 to 90 kilograms). The lengths ranged from almost 4 feet to more than 6 feet (121 to 190 centimeters).

Protecting the coelacanth

The catches occurred across 620 miles (1,000 kilometers) of the western coast of Madagascar, from the southernmost point of the island all the way to the northwestern coast. The largest cluster was caught in Onilahy Canyon, off the southwest coast of the island. This level of bycatch could be dangerous to the coelacanths' survival. The species is critically endangered and has many of the features that put fish at risk of extinction, Bruton said: It is slow-growing, it reproduces rarely, and it is a high-level predator easily threatened by habitat loss and environmental degradation. 

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It's possible that coelacanths also live off the east coast of Madagascar, the researchers reported March 29 in the South African Journal of Science. Fishing activity is lighter to the east, so surveys using remotely operated vehicles would be helpful to search that side of the island for the ancient fish, Bruton said. The African Coelacanth Ecosystem Programme, a project of the South African Institute for Aquatic Biodiversity, has a submersible that is capable of doing these surveys, Bruton said, but a planned expedition that included Madagascar was postponed because of the COVID-19 pandemic. He and his colleagues hope the expedition will be rescheduled.

"Only then will we know the full extent of the population, how healthy it is, and be able to recommend measures to ensure their survival," Bruton said.

For now, the researchers recommend that Madagascar set up a coelacanth sanctuary in Onilahy Canyon and pass legislation adding L. chalumnae to the country's protected species list. Fishing with jarifa nets should be banned in coelacanth-rich areas, they wrote in the South African Journal of Science, and fishing crews should be given incentives to tag, photograph and throw back any live coelacanths they catch. The fish make for poor eating, as their tissues contain a variety of rancid-tasting oils and a waste product called urea (the main ingredient in urine), but they are sometimes consumed in Madagascar.

If fishing crews can be brought onboard the conservation bandwagon, they might also help advance coelacanth research, Bruton said. Fishers could be taught to deep-freeze any dead coelacanths they do catch to preserve tissue for genetic analysis. Gene sequencing could help reveal whether there is any breeding of coelacanths between Madagascar and other West Indian Ocean populations, Bruton said.

"That would be very valuable information that we don't know at this stage," he said. 

Originally published on Live Science.