There is a growing number of studies aimed at estimating the amount of fishery resources and clarifying the ecology of fish by examining environmental DNA -- eDNA -- derived from fish droppings, bodily fluids, fragmented cells and ohter things.
DNA from the environment provides clues to underwater life that was previously not easily understood.
Maizuru Bay in Kyoto Prefecture is famous for its rias view. Fixed-net fishing for yellowtail and horse mackerel is popular in these coastal waters. Prof. Michio Kondo of Tohoku University and other researchers including from Kobe University tried to estimate the number of jack mackerel using eDNA in the bay in an attempt to help conserve fishery resources.
Environmental DNA is useful for determining the presence of specific creatures, but it has been difficult to estimate their populations. It had been thought that the amount of DNA was not always proportional to the number of living organisms, because eDNA is broken down over time or carried by water currents.
Kondo and his team collected eDNA from about 100 locations on Maizuru Bay, and simultaneously examined the number of fish using a fish finder to verify how effective the eDNA estimation method is.
They found that the amount of eDNA increased or decreased, in response to the number of jack mackerel within a radius of several tens of meters to about 150 meters from the point where the water was taken.
In addition, they measured the relationship between the number of jack mackerel and the amount of eDNA in a water tank. The research team has developed a model that can estimate the distribution and number of fish by incorporating water flow data in the bay. The total number of jack mackerel in the bay estimated by this model was about 33 million, close to the 39 million estimated by the fish finder. Fish finders cannot identify fish species, but eDNA can estimate even the number of each species.
"If we can grasp the appropriate amount of fish catches by constantly observing the amount of fishery resources with eDNA, it will lead to sustainable fishing." Kondo said.
--Fixed point observation
In fiscal 2019, Kondo also started a project to observe eDNA at about 60 locations on the sea from Hokkaido to Okinawa more than four times a year. The results will be made available to researchers this fall.
The amount of eDNA allows us to count not only a single species, but all living things.
Kondo is trying to clarify the relationship or chain between living things from the vast amount of data on eDNA mathematically. This seems to be an attempt that was difficult with conventional methods such as capture and observation.
"Ecosystems that have been elusive can now be represented using mathematics. We might find some surprising ecological mysteries." Kondo said.
--The current situation becoming clearer
Environmental DNA has further revealed some aspects of salmon ecology. Assistant Prof. Yuki Minegishi of the University of Tokyo has been collecting salmon eDNA in Iwate Prefecture's Otsuchi Bay in the first half of every year since 2017. It is the time when juvenile salmon leave the river and head out into the sea.
Analysis of the amount of eDNA at 13 sites on the bay showed that the juveniles tended to remain in the bay from February to April and then leave the bay by around June. This long stay in bay waters is unique to this area. The timeline from hatching to going down the river and then out to sea is shorter than that in Hokkaido. The bay is considered to be a "cradle" supporting the growth of salmon fry.
In recent years, the number of salmon returning to the Sanriku coast of Iwate Prefecture has been about one-tenth of the peak seen around 1996, partly due to the impact of the tsunami caused by the Great East Japan Earthquake in 2011.
"Protecting an environment important for the growth of young fish could lead to the protection of fishery resources," Minegishi said.
Meanwhile, Satoshi Kameyama, a senior researcher at the National Institute for Environmental Studies, and his team examined eDNA from eels at 100 sites in about 50 rivers mainly in western Japan from 2017 to 2019. If eDNA is not detected upstream of a river structure such as a flood gate, it is found that the run-up of eels is hindered. "These results lead to measures such as the removal of structures and the installation of fishways," Kameyama said.
Unexpectedly, the researchers could not find eel eDNA at any of the six sampling points on the Kinokawa River in Wakayama Prefecture, which had been conducting river improvement projects with due consideration for the environment, such as constructing fishways and conserving tidelands.
Chikao Shoji, deputy director of the Wakayama river and national highway office of the Ministry of Land, Infrastructure, Transport and Tourism, which manages the Kinokawa River, said, "It may be because the number of eels is small."
Shoji said that in fiscal 2018, 14 fish were caught near the estuary of the Kinokawa River.
"I hope that the results of the eDNA survey will serve as a reference for the conservation of the river environment." Kameyama added.
Read more from The Japan News at https://japannews.yomiuri.co.jp/
