Fish vanished from sonar during solar eclipse

Fish detected by the trawler Kaldbakur EA disappeared from its depth sounders during the solar eclipse, mbl.is reported. The vessel was fishing for cod and saithe on the Halamið fishing grounds northwest of Iceland, directly beneath the path of totality.

The crew faced heavy cloud and dense fog, but the captain, Oddur Jóhann Brynjólfsson, was in the bridge monitoring the depth sounders when all signs of marine life vanished from the displays.

Depth sounders detect biomass through echoes, including fish and plankton. Dense shoals produce a clear return, while scattered organisms are harder for the instruments to detect.

“It was all gone suddenly, as if someone had snapped their fingers,” Oddur told Morgunblaðið. “It was as if the fish had disintegrated or something.”

Oddur said he was used to seeing changes in marine life during twilight, but found it particularly unusual that the fish did not return for a long time after the eclipse ended.

It was not until nearly 9pm, less than three hours after the eclipse, that he noticed signs on the depth sounders that marine life was returning.

“It was so strange. There was no real darkness, only for a short while, and yet the fish behaved as if something significant had happened,” he said.

Oddur had prepared for the possibility that the sudden darkness would affect the fishing. He raised the trawl so it would be ready for deployment during the eclipse, expecting either an exceptionally large catch or very little.

“I raised it so I would be ready with the trawl down during the eclipse, to see whether we might catch more, but it did not turn out that way,” he said. “I thought the fish might gather more tightly, and I wanted to be ready, but there was simply nothing.”

Few studies on eclipses

Jónas Páll Jónasson, head of the pelagic division at the Marine and Freshwater Research Institute, said few studies have examined the effects of solar eclipses on marine life, and even fewer have focused on waters around Iceland.

“This has not been studied systematically in relation to solar eclipses,” he told Morgunblaðið. He said relatively recent research had examined how light affects the distribution of various organisms in the upper layers of the ocean.

That research has mainly focused on the effects of lights on vessels, Jónas said. Ship spotlights can affect the distribution of species down to considerable depths, perhaps 50 to 100 metres.

Such effects have even been used in developing new fishing gear that does not touch the seabed, but instead uses lights to herd fish into nets.