Underwater census tells story of fish recovery after disaster

A Japanese research team has published a five-year study that offers a colorful snapshot of the recovery of marine life in areas struck by the massive tsunami following the March 2011 megathrust earthquake.

The researchers, led by Reiji Masuda, set up four underwater stations in waters off Kesennuma, Miyagi Prefecture, and carried out 30-minute visual surveys of fauna every two months from May 2011 to March 2016. Kesennuma was among the cities hardest hit by the massive tsunami. The study appeared in PLOS ONE on Dec. 12.

During the first survey in May 2011, a total of just 41 fish were seen in the highly disturbed and oxygen-deprived waters. The majority of them were juvenile sunrise sculpin, a small fish with a short one-year lifespan that dies after first mating.

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Moon jellyfish in murky waters off Kesennuma, Miyagi Prefecture, two months after the March 2011 earthquake and tsunami

Other early recolonizers included groups of moon jellyfish, which can tolerate low-oxygen conditions and likely had little competition over zooplankton.

The researchers hypothesized that “[t]he unusually high abundance of opportunists after a disturbance might be due to both the absence of competitors and an increase in resources provided by the disturbance.”

The study noted that the tsunami brought in a wealth of nutrient-rich sediment from land, creating favorable conditions for small, short-lived species.

In the second year, striped sandgoby and sevenspine goby were the prevailing species. Although they, too, have short lifespans, these mud-dwellers require more time to recover because they use holes in the seafloor made by shrimp as habitats, the authors explained.

Recovery was slower for larger species with longer lifespans and reproductive cycles. Surfperch and black rockfish, which can live four and eight years, respectively, increased in abundance in the latter half of the survey period.

Meanwhile, sea cucumber required three years to stabilize, while abalone, whose shells take several years to grow, required five.

The authors pointed out that a decline in the number of moon jellies in the third year, likely resulting from the reappearance of plankton-feeding oysters, “may have reflected the recovery of oceanographic conditions after the tsunami.”

This recovery was further evidenced by the ability of the restored fish assemblage to defend against invaders. “Tropical vagrants arrived to the open niches and were presumably expelled by competitors or predators in the fourth year,” the study said.

The researchers concluded that it takes about three years for coastal fish ecosystems to recover from a major natural disaster such as a tsunami, and that “recovery is dependent on species-specific life span and habitat.” Masuda said he plans to continue the survey in January.

 

 

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A sunrise sculpin is seen in waters off Kesennuma, Miyagi Prefecture, in May 2011

 

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Banded goby swim in waters off Kesennuma, Miyagi Prefecture, in November 2011

 

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A sandgoby fish spotted in waters off Kesennuma, Miyagi Prefecture, in Sept. 2013