Introduction to the Connectivity of Hills, Humans and Oceans (CoHHO*):
Figure out how they interact with each other*We call it “Mori - Sato - Umi Renkan - Gaku” in Japanese.

(3) Important factors other than “nutrients”
Emeritus Prof. Yoh Yamashita

In the second article of the Introduction to the CoHHO, I focused on "nutrients" supplied
from land to the sea, based on the catchphrase, "Rich nutrients from forests nurture productive
coastal seas." This time, I would like to talk about important factors other than nutrition.

1.

Estuarine and coastal food webs and utilization of organic matter

Organic matter produced on land and transported to the sea through rivers includes
trees, grass, animal carcasses, discarded human food, sewage, and many others. Organic
matter is broken down or decomposed during transport through rivers. It is carried to thesea in a variety of forms, ranging from ultrafine particles dissolved in water (elements that
pass-through pore sizes of 0.45 to 1 µm), called dissolved organic matter, to solid organic
matter that remains in its original form, such as plant fragments. When organic particles
and solid organic matter accumulate on the seafloor, they are decomposed by
microorganisms and cause hypoxic conditions. Previously, there were no methods toinvestigate the role of such diverse organic matter in food webs of rivers and seas. However,recent advances in stable isotope analysis of carbon, nitrogen, and other elements haveenabled us to analyze the origin of organic matter and the dietary relationships of theanimals that use it. This has led to dramatic advances in research on the actual utilizationof organic matter by aquatic animals. The authors have been conducting a detailed
investigation of the supply of organic matter from rivers and its utilization by animals in
Yura River and Tango Bay in northern Kyoto Prefecture. I would like to introduce some of
the results in the following sections.

In Yura River, physical and biological processes can be broadly classified into two
major periods. The high discharge period (December to March) is in winter when rain andsnowfall causes a large volume of discharge, and the river fills with fresh water to its mouth.
The low discharge period is from spring to autumn (April to November), when the river
forms a stratified estuary where seawater intrudes as a salt wedge along the river bottom(Figure 1). From early summer to autumn, seawater intrudes up to about 18 km from the1© Field Science Education and Research Center, Kyoto University

river mouth in the downstream and estuarine areas. The distance of seawater intrusion isdetermined mostly by river discharge and sea level which is higher during the summer(Kasai et al. 2010). During the low discharge period, brackish and marine phytoplankton
actively undertake primary production in the lower reaches using abundant nutrients in theriver water. A distinct chlorophyll maximum forms in the halocline layer, which appears
between the upper and lower layers. During the high discharge period, phytoplankton
density becomes extremely low in the river. Contrastingly in Tango Bay, phytoplankton
blooms usually occur from February to April owing to nutrients supplied not only from theriver but also from offshore through estuarine circulation (see Kasai et al. (2010), Watanabe
et al. (2014) and “Introduction to the CoHHO (2)” for details).

Figure 1. Distribution structure of salinity and chlorophyll in the Yura River estuary. Upper
panels (low discharge period): August 22-23, 2007; lower panels (high discharge period):
January 28-29, 2008. Modified from Watanabe et al. (2014).

We examined the dietary relationships between 135 species of benthic animals(macrobenthos) from the lower reaches to the offshore waters of Yura River using stable
isotope analysis of carbon and nitrogen (Antonio et al. 2010a, 2012). We considered five
types of organic matter sources as the basis of the food web: terrestrial plants, river
particulate organic matter, macroalgae, benthic microalgae, and marine particulate organic
matter. River particulate organic matter includes freshwater phytoplankton and broken
fragments of terrestrial plants, and marine particulate organic matter consist of mainly ofphytoplankton. In the downstream and estuarine areas during the spring and autumn
seasons, when seawater intrudes upstream, the analysis showed that various types oforganic matter was used as food for benthic animals, from terrestrial plant-derived organic
matter to marine phytoplankton (Figure. 2). High utilization of terrestrial plant-derived
organic matter was found in the lower reaches mainly in winter when discharge is high. In
2© Field Science Education and Research Center, Kyoto University

contrast, in the coastal and offshore areas, there was little utilization of terrestrial plants
and river particulate organic matter, and the contribution of marine phytoplankton
increased with water depth.

Terrestrial plant-derived organic matter contains refractory substances such as
cellulose and lignin. It is known that these substances can only be used by animals withcellulase that breaks down cellulose. In Yura River, cellulase activity was observed in theestuarine bivalve Corbicula japonica and estuarine snails Cipangopaludina japonica,
Semisulcospira libertina, and Clithon retropictus (Antonio et al. 2010b). These shellfish
fed heavily on terrestrial plant-derived organic matter in rivers, whereas in the sea they
preferred benthic microalgae- or phytoplankton-derived organic matter to terrestrial plantderived organic matter. Although human preferences are not directly comparable, I canunderstand shellfish’s desire to eat seaweed if seaweed and leaves are placed on the samedinner table.

Figure 2. Contribution of five carbon sources to macrobenthos production in the lower
reaches of Yura River and Tango Bay estimated from carbon and nitrogen stable isotope
ratios, modified from Antonio et al. (2012)(Note: POM: particulate organic matter)3© Field Science Education and Research Center, Kyoto University

The amphipod Anisogammarus pugettensis, which is abundantly distributed inestuarine areas in Hokkaido, chomps on and digests leaf litter deposited along the coast(Sakurai & Yanai 2008). In comparison, only a limited number of animals feed onterrestrial plant-derived organic matter in the sea, and terrestrial plants which aretransported and deposited on the seafloor are mainly decomposed by microorganisms. This
has important implications for river management. Natural rivers usually meander and arecomposed of riffles and pools. It is thought that organic matter of terrestrial origin
accumulates in river pools and is decomposed and removed by animals that have cellulases
to utilize it. However, rivers are often straightened and modified to have a riffles-only
structure with three-sided embankments to prevent flooding. In such rivers, terrestrial plantderived organic matter is inevitably carried to the sea in a short time and deposited on thebottom of estuaries and inner bays. Since only a limited number of benthos feed on theseorganic materials, it can be inferred that many of them are decomposed by microorganisms,
which in turn consume oxygen and form hypoxic waters. In addition to providing habitats
for river animals, meanders and riffle-pool sequences in rivers play an important role inconserving coastal environments.

2.

Influence of inorganic particles such as sediment

Various human activities on land brings in a large influx of sediment into rivers.
Sediment is supplied from various places, including degraded forests, arable lands(agriculture not used for rice cultivation), rice paddies, construction works, and river
improvement works. On the other hand, the influx of sediment into seas greatly decreased
after World War II, as many dams were constructed in rivers and sediment accumulates indams. In Japan, the average annual rate of sediment deposition in dams is estimated to be
1.1%, which means that dams will be buried in about 90 years. Sandy beaches have beenreceding on most of our nation's coasts due to a lack of sediment supply (Unoki 2015). In
Tenryu River, for example, it is estimated that about 90% of the annual supply of beachforming sediment has been lost due to dams and gravel extraction from riverbeds (Uda
2010). The recession of sandy beaches and the reduction of tidal flats due to the decline in
sediment supply is also thought to be directly related to the decrease in the number ofaquatic animals that live or breed in these areas. To prevent the recession of sandy beaches,
many beaches in Japan have piled up breakwater concrete blocks to prevent erosion
offshore. The typical landscape of Japan, beaches with white sand and green pines, aredisappearing from this country.

On the other hand, there is concern in recent years about the mud (silt and clay)
deposition in rivers and coastal areas. As mentioned above, sediment supply to the sea isdeclining due to dams. When it rains, only fine particles of less than 50 μm (referred to as“microscopic particles” hereafter) in the sediment of the dam flow out and accumulate on
4© Field Science Education and Research Center, Kyoto University

the river bottom and seafloor. Microscopic particles adversely affect aquatic ecosystems
such as rivers, tidal flats, beaches, rocky reefs, and coral reefs (Yamashita 2014). There are
two ways they can do this. The first is the effect of microscopic particles suspended inwater. Microscopic particles can enter the gills of fish and shellfish, causing inflammation,and in some instances, high concentrations have been reported to kill or harm fish.
Microscopic particles also increase mortality rates of algal zoospores, sea urchin andabalone larvae, and turbidity interferes with photosynthesis of seaweed and sea grass. The
second is the effect of microscopic particles deposited on the bottom. It is well known thatmicroscopic particles destroy coral reef ecosystems (Rogers 1990). In seaweed beds androcky reefs, even a very thin layer of microscopic particles deposited on the substrate caninhibit zoospore attachment, gametophyte growth, survival, and maturation of seaweed(Matsumoto et al. 2020). In addition, planktonic larvae of reef-dwelling snails, such as
turban shells and abalone, selectively settle on fine seaweed called coralline algae(calcareous algae) and use it as a nursery ground during the early juvenile stage (Figure. 3).
Coralline algae have been reported to emit chemicals that attract these snail larvae andpromote settlement and metamorphosis. However, the metamorphosis of abalone
planktonic larvae into juvenile snails is severely inhibited when just a thin layer ofmicroscopic particles is deposited on top of the coralline algae (Onitsuka et al. 2008). An
increase in mud content in the bottom also creates an unsuitable environment for edible
bivalves such as Manila clam (Ruditapes philippinarum) and fan mussel (Atrina pectinata),
as well as for juvenile benthic fishes that grow on sandy/muddy bottoms in shallow waters,
including Japanese flounder (Paralichthys olivaceus), stone flounder (Platichthys
bicoloratus), and marble flounder (Pseudopleuronectes yokohamae) (e.g. Ellis et al. 2002,
Yamashita 2014). In addition, the accumulation of microscopic particles containing organic
matter on the seafloor cause hypoxic conditions and facilitate the formation of highly
biotoxic substances such as hydrogen sulfide. These numerous examples show thatmicroscopic particles are a serious destroyer of coastal ecosystems.

Figure 3. Juvenile snails of Ezo abalone (Haliotis discus hannai) settling on non-articulated
coralline algae (Photo by Dr. Hideki Takami)
5© Field Science Education and Research Center, Kyoto University

I have mentioned that degraded forests, arable lands, rice paddies, construction works,
and river improvement works can be the source of sediment containing microscopic
particles. The flow of sediment into the sea through rivers is a natural process that hascontinued throughout the long history of Earth. Basically, river sediment flowing into thesea in its original form and composition does not cause any problems. However, the sudden
appearance of dams in the 20th century physically broke the connectivity between land,
rivers, and the sea. Sediment flowing into rivers are deposited in dams, and only the finest
particles of the sediment are washed downstream and carried to the coast during highdischarge. In other words, sand-sized particles necessary to form sandy beaches aredeposited in dams, while only microscopic particles (silt and clay components) are
discharged. This is thought to have a significant impact on the bottom environment of rivers
and coastal areas.

Although sediment deposition and microscopic particle discharge in dams affectaquatic ecosystems profoundly, I would like to consider other sources of sediment supply.
The first is degraded forest plantations. During Japan's postwar recovery, natural forests
were cleared and replaced by plantations on a large scale. Plantations now account for 40%of the country's forests. However, with the subsequent import of cheap foreign timber, thedomestic forestry industry lost its competitiveness and fell into an extreme depression.
Forestry workers do not have the capacity to care for plantations, and vast areas ofplantations are severely degraded. In these forests that have not been properly pruned andthinned, light does not penetrate within and the understory vegetation does not form. This
leaves the soil bare, and when it rains, the topsoil runs off into rivers (Onda 2008).

Arable land (farmland) and rice paddies are also considered to be important sources of
microscopic particles (Nishimura 2011, Nunes et al. 2011). The efflux of sediment isdetermined by the extent of vegetation coverage on soil surfaces. In particular, arable landis bare soil, with the exception of pastures, and much of it is eroded by rain and washedinto rivers. Soil erosion in arable land not only causes loss of soil and fertilizer components,
but also poses a serious problem as a non-point source of river water pollution. To prevent
soil and fertilizer components from flowing into rivers, measures are being taken such as
sedimentation ponds and vegetation strips at boundaries of farmlands. In our study, we
found a negative relationship between the diversity of endangered fish species in estuaries
and the area of arable land in their watersheds. One possible cause of the negative impactof arable land is the inflow of microscopic particles into rivers (Lavergne et al. 2022).

Paddy fields are another important source of soil (Nishimura 2011). In the postwar
period, intake and drainage routes were separated in the irrigation system of paddy fields
by the government's agricultural development program. In old paddy fields, suspended
particles from plowing were deposited in the paddies downstream, but after infrastructure
6© Field Science Education and Research Center, Kyoto Universityimprovement, turbid water is discharged from individual paddies directly into rivers(Figure. 4). Sediment also flows into rivers from abandoned farmland, which has been
increasing in recent years due to acreage reduction policy of the Ministry of Agriculture
and the retirement of farmers. It is known that re-vegetation of such abandoned farmland(conversion to grassland or woodland) can reduce the degree of topsoil erosion by two to
three orders of magnitude. Another possible factor is the influx of sediment from cities andconstruction sites. In Okinawa, civil engineering and development works are considered
the main cause of red soil runoff that destroys coral reefs.

Figure 4. Old paddy field (left) and paddy field after infrastructure improvement (right).
Modified from Nishimura (2011)3.

Toxic substances flowing into the sea

Pesticides such as herbicides, fungicides, and insecticides represent toxic substancesthat flow into the sea from land. Since these chemicals are designed to kill weeds andinsects, they are bound to be harmful to humans and ecosystems. However, pesticides arefar from my expertise, and I will not discuss in detail here.

The use of pesticides spread rapidly in Japan after World War II. Harmful chemicals
that even killed fish, such as mercury, parathion, DDT, and PCP, were commonly used infields until the 1960s. At that time, not much thought was given to the negative effects onhuman health, let alone ecosystems. Rachel Carson's "Silent Spring" sounded a strong
alarm about this problem. It is said that the book drew worldwide attention to the dangers
of pesticides, which in turn led to research and use of safer pesticides. Even though thedangers have subsided, these chemicals may still have significant adverse effects on
7© Field Science Education and Research Center, Kyoto University

humans and ecosystems because they are designed to kill living creatures. Dioxin, which
is known to be the strongest poison ever created by mankind, was used for pesticides inJapan until the mid-1980s. Moreover, TeCDD, a representative of more than 200
substances included in dioxin, has a relatively long half-life of 8.7 years. Therefore, it may
still be flowing into coastal areas from fields through rivers and depositing on the bottomof enclosed bays (Okumura et al. 2004).

The mainstream insecticides used today are called neonicotinoids. Their principle use
in Japan is to kill stink bugs and other insects that suck rice juice from husks and cause
spots on rice. However, these pesticides are so persistent that they remain in pollen and in
the hydrosphere for long periods. As a result, they end up killing many insects including
aquatic ones, and are said to be the cause of a drastic decrease in honeybees and dragonfliesin recent years. In 2019, a paper was published indicating that neonicotinoid pesticides
destroyed the aquatic ecosystem in Lake Shinji, causing a sharp decline in Japanese eel(Anguilla japonica) and Japanese smelt (Hypomesus nipponensis) (Yamamuro et al. 2019).
While it is said that they have little effect on humans, some argue that it causes
neurodevelopmental disorders, and no conclusion has been drawn yet. EU has banned threeof the five major neonicotinoid pesticides and France has banned their use altogether. In
contrast, Japan continues to take measures that are completely opposite to such global
trends by expanding the application of neonicotinoid pesticides and relaxing food residue
standards. Speckled rice is a serious problem for farmers as the spots reduce its grade, but
the taste is thought to be unaffected. There are people, especially from overseas, who argue
that we are endangering our ecosystem and human health just to eliminate these spots. I
believe this is a typical problem of "individual optimization", and the concept of "overall
optimization" is essential for SDGs, mentioned in the first article of this series.

There are many other substances that disturb ecosystems through human activities,
such as mercury, PCBs that were used in industrial products, cadmium that originated frommines accumulated in paddy fields, and chemicals that received attention as environmental
hormones. However, evaluation of their impact on ecosystems has not progressed a lot.

4.

Production, growth, and movement of animals in rivers and estuaries

Migration of aquatic animals between rivers and the sea is called diadromous migration,and it is categorized into three types: catadromous migration, where aquatic animals growin rivers and descend to the sea to spawn (e.g. eels); anadromous migration, where aquatic
animals grow in the sea and return to rivers to spawn (e.g. salmons); and amphidromous
migration, where aquatic animals move between rivers and the sea without being directly
related to spawning (e.g. Ayu sweetfish, Plecoglossus altivelis). We have been studying
8© Field Science Education and Research Center, Kyoto University

how amphidromous temperate seabass (Lateolabrax japonicus) and catadromous Japaneseeel (Anguilla japonica) utilize rivers and the sea. Here, I would like to share the results witha focus on the role of the estuarine area and explain it in terms of the CoHHO.

(1)

Production of juvenile seabass

Yura River, which appears frequently in this article, is a first-class river flowing
through the northern part of Kyoto Prefecture with a main channel of 147 km, a basin areaof 1882 km2, and a basin population of approximately 170,000 people. Yura Riveroriginates in Ashiu Research Forest, Field Science Education and Research Center, Kyoto
University. It flows into the innermost part of Tango Bay, about 30 minutes by car from
Maizuru Fisheries Research Station, Kyoto University. Because of its location, Yura Riverhas been the most important field for the Field Science Education and Research Center's
CoHHO research. The shallow waters of Tango Bay are the nursery grounds for importantaquatic animals in fishery, such as temperate seabass, Japanese flounder, squids, and sparidand carangid fishes. Many of the juvenile fish in this area feed mainly on a group of small
crustaceans called mysids.

Temperate seabass spawns offshore in Tango Bay from December to February, and thehatched larvae, a few millimeters long, are transported to the coast using shoreward currents
created by strong northwesterly winds in winter. In February and March, they settle downon the seafloor about 10 m deep offshore of the mouth of Yura River (Suzuki et al. 2020).
In mid-April, the decline of melting snow decreases the discharge of Yura River, andjuvenile fish distributed off its mouth enter the river with intruding seawater. We found thattemperate seabass juveniles that move into Yura River migrate as far as 50 km upstream
from the mouth of the river. Once in the river, they remain there until around autumn.

On the other hand, since a large number of temperate seabass juveniles continue toinhabit the shallow waters of Tango Bay after April, it is thought that the juveniles that
migrate up Yura River are a fraction of the juveniles that arrive at the river mouth inFebruary and March. To investigate the benefits of upstream migration, we compared thefood intake and growth rate of juveniles collected in Yura River and Tango Bay. The results
showed that juveniles in the river fed more and grew better than those in the sea (Figure.
5). The primary food source of temperate seabass juveniles is mysids, which are abundantly
distributed in the coastal area from February to June (mainly Orientomysis japonica) andin the lower reaches of the river from May to August (mainly Neomysis awatschensis). In
other words, temperate seabass lives in shallow waters of the coastal area until March,where Orientomysis japonica are abundant, and some move to the lower reaches of YuraRiver from April/May to feed on the dominant estuarine mysid Neomysis awatschensis.
This indicates that temperate seabass effectively utilizes the productivity of both the lower
9© Field Science Education and Research Center, Kyoto University

reaches of the river and the coastal area (Kasai et al. 2018). Given that the coastal nursery
is much larger in area, the large proportion of juveniles is thought to remain in the sea.

Figure 5. Comparison of food intake and growth rate of coastal residents and river migrantsof temperate seabass juveniles in the Yura River and Tango Bay. Feeding index = weightof stomach contents/body weight, modified from Fuji et al. (2014).

To investigate the contribution of rivers and the sea as nursery grounds for juvenile
seabass, we analyzed trace elements (ratio of strontium Sr to calcium Ca) in otoliths, theonly hard tissue in fish that is not metabolized (its components are not replaced like regularbones). Strontium is abundant in sea water, but only a small amount is found in river water.
Therefore, by examining the Sr:Ca ratio of otoliths formed during the juvenile stage, we
can determine the juvenile’s experienced environment (river or sea). Using this method, we
examined otoliths of temperate seabass adults caught by set nets in Tango Bay and foundthat nearly half of the juveniles had a history of using rivers as nursery grounds (Fuji et al.
2016). In addition, it is possible to estimate the daily growth history of each fish fromhatching to capture based on the analysis of daily rings inscribed in otoliths, since theintervals between the rings are proportional to daily growth (Figure. 6). This technique also
revealed that, among the juveniles that had stayed in the coastal area in February and March,
the slow-growing smaller individuals migrated to the river, while the larger ones remainedin the coastal area. We also found that the river migrant juveniles fed on an abundance of
mysids and grew well, and by the time they returned to the sea in autumn, their length hadcaught up with the coastal residents (Fuji et al. 2014). The primary diet of juvenile seabassin Yura River is the mysid Neomysis awatschensis. They usually live near the river bottom
10© Field Science Education and Research Center, Kyoto University

and feed on attached benthic microalgae. These benthic microalgae are mainly producedby nutrients of terrestrial origin in freshwater areas (Omweri et al. 2021).

Large adults of temperate seabass also migrate up Yura River. Temperate seabass up
to 70 cm in length can be caught in mountain streams as far as 40 km upstream from theriver mouth. We are currently conducting research on large adults of temperate seabass that
migrate up Yura River. In the process, we have discovered that Ayu sweetfish is an
important target prey of large adults in the river. Interestingly, Ayu sweetfish feed on thevery same attached microalgae produced by nutrients from forests. If the research
progresses, we may be able to see how temperate seabass in Tango Bay is nurtured by
Tamba Forest (i.e. forests in the central part of Kyoto Prefecture).

Figure 6. Daily rings in an otolith of temperate seabass juvenile.

(2)

Revival of Japanese eel

It is well known that the stock of Japanese eel (hereafter referred to as "eel") has been
drastically depleted. Although the full life-cycle aquaculture (raising fish from fertilized
eggs to matured adults) of eel has been successful at the laboratory level, the number of eelthat can be produced is extremely limited and we are far from developing a method
sufficient for commercial purposes. Therefore, traders catch large quantities of glass eel for
aquaculture. Eel larvae (leptocephali) are carried from their spawning grounds on the
distant West Mariana Ridge by the Kuroshio Current, before metamorphosing into glass
eels in estuarine waters of Japan from winter to spring. Glass eel fishery is far from being
managed and overfishing of glass eel is the primary factor in the decline of the eel stocks.
Only glass eels that are lucky enough not to be caught change their body color from white
to black (i.e. juvenile elver eels). Eels grow in a wide range of habitats from estuaries to11© Field Science Education and Research Center, Kyoto University

rivers. Eels in this stage are called yellow eels because of the yellowish coloration on theirbellies. When yellow eels reach age about five for males and age 10 for females, theychange into silver eels, and go out to the open sea for spawning migration. The second
cause of the decline in the eel stock in Japan is habitat loss due to environmental
degradation of rivers and estuaries, as the rivers and estuaries serve as important habitats
for wild eels. Here, I would like to discuss this second issue from the viewpoint of the
CoHHO studies.

Our research group is conducting ecological study on eels in Oita, Wakayama, andFukushima Prefectures. We made some important discoveries in this process. Eels are quite
tolerant of river water quality, especially organic pollution. In other words, they can live
without any problem in both clear and slightly polluted urban rivers, as long as theconditions are within certain acceptable levels. In a study by Kasai et al. (2021), they usedenvironmental DNA in rivers throughout Japan from Okinawa to Hokkaido. They foundthat rivers with higher concentrations of eel environmental DNA also tended to have higher
concentrations of total nitrogen. Total nitrogen concentration is considered an indicator of
eutrophication, but recently the quality of river water in Japan has improved substantiallycompared to the period of rapid economic growth in the 1960s and 70s. Therefore, a riverwith high total nitrogen concentration can be considered productive and rich today. In thesurveys in the three prefectures mentioned above, wild eels were found in many rivers,
including clear streams inhabited by chars (Salvelinus spp.) and Yamame trout(Oncorhynchus masou), irrigation canals in paddy fields, and even urban rivers withsewage inflows. Some urban rivers were found to be inhabited by wild eels at densities far
exceeding the distribution densities published in academic journals (Kume et al. 2021).

This study revealed several characteristics of rivers where eels can live naturally. First,
it is important that there should be no crossing structures (dams, weirs, water gates) that
impede their migration, as eels come from the sea and migrate upstream. Yet eels are known
to climb weirs tenaciously. We observed that juvenile eels could climb a weir 1.65 m high.
Recently, it has been reported that juvenile eels can climb a waterfall of 46 m in height(Matsushige et al. 2021). However, the structure of weir walls is an important factor, andeels cannot climb walls without some irregularities. In addition, long water plants and othervegetation hanging from upstream of weirs play an important role in eels' ascent. However,even though juvenile eels can climb walls of weirs, there are limits. The number of eels
inhabiting rivers decreases with each additional weir upstream, and weirs undoubtedly
hinder their distribution (Kume et al. 2019, 2020). Attaching irregularities to the wall ofweirs would facilitate climbing by juvenile eels, and therefore, such irregularities would
certainly aid their migration. However, it is not useful for the migration of older yellow
eels or other fish (Ayu sweetfish, Yamame trout and chars) and it is still important to installpassageways that allow these fish to migrate upstream.
12© Field Science Education and Research Center, Kyoto University

We also tracked eels by attaching transmitters to their bodies and examined theirfeeding habits using stable isotope analysis (Kutzer et al. 2020, Noda et al. 2021). The study
found that eels lead a well-regulated life as they used the estuary to roost during the day,entered the river at sunset, fed there during the night, and returned to their roost before
sunrise. It is interesting that some of the eels that roosted at the estuary also went seaward
to feed at night. Individual eel chooses its feeding ground depending on the spatiotemporalchanges in food availability. If rivers and the sea, or rivers themselves, were divided byweirs, it would be difficult for eels to secure sufficient food through such flexible foraging.

In addition to the absence of weirs, the availability of aquatic prey animals and hiding
places is also important for eels to live comfortably in rivers. Eels are known as carnivores
and they can feed on a variety of animals. Eels feed mainly on aquatic insects until theyreach 20 cm in length, and thereafter on shrimp, crabs, and fish. It is also reported that eelsfeed on earthworms in forest rivers (Itakura et al. 2015). Small eels live under pebbles,
while large eels can hide under large stones (stones not buried in the riverbed), amongaquatic plants, or in the mud. In other words, eels can live anywhere in a normal river aslong as the river has riffles and pools, the riverbed is composed of various substrates frommud to large stones, and if there are plants in the water or along the banks(Kume et al. 2019,2020). However, many rivers in Japan are managed with a focus on “water utilization” and“flood control”, and little consideration is given to the “environment and biodiversity”
which is the other core pillar of the River Law in Japan. Many of the natural rivers withmeanders, riffles and pools have been straightened and revetted with concrete to makedrainage channels that simply let water flow. Such rivers also have few living creatures.
Even in rivers where the natural state has been preserved for eels and Ayu sweetfish to
inhabit, heavy machinery is used every year to do civil work on the riverbanks and riverbed.
In these rivers, a large amount of soil is washed out every time construction is carried out,
covering the microalgae on stones that Ayu sweetfish would otherwise eat. This is one of
the reasons for the sharp decline in Ayu sweetfish in recent years. Our river ecosystem iscurrently being destroyed by construction works for a meaningless measure in the name of
reducing unemployment.

Environmental issues such as global warming began to attract attention around the1980s as a worldwide crisis. However, it is only recently that people have begun torecognize it as a reality in their social lives. The global environment is clearly deteriorating.
People are experiencing an increase in extreme weather events such as heat waves, heavy
rains, hyper typhoons, and consequent disasters. Population continues to grow on our finite
planet and is expected to reach nearly 10 billion by 2050. If we do not recover Earth’s
system so that the environment is preserved and resources are recycled, the human race willcertainly face a crisis within our lifetime. The CoHHO studies consider the symbiosis
between humans and nature scientifically through the restoration of the cycle of nature. In
the first article of "Introduction to CoHHO”, I talked about individual optimization and
13© Field Science Education and Research Center, Kyoto University

overall optimization. Instead of chasing short-term profits (individual optimization), we
need to apply wisdom from a broad and long perspective grounded on science (overall
optimization) for the sustainable future of Earth and mankind. The achievement of theSDGs is an issue of personal interest that directly affects our lives.

Reference
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communities from Yura River-Estuary to offshore of Tango Sea. Estuarine, Coastal and
Shelf Science, 86, 107-117.
Antonio E. S. et al. 2010b. Consumption of terrestrial organic matter by estuarine molluscs
determined by analysis of their stable isotopes and cellulase activity. Estuarine, Coastal and
Shelf Science, 86, 401-407.
Antonio, E. S. et al. 2012. Spatial-temporal feeding dynamics of benthic communities in an
estuary-marine gradient. Estuarine, Coastal and Shelf Science, 112, 86-97.
Ellis, J. 2002. Determining effects of suspended sediment on condition of a suspension feedingbivalve (Atrina zelandica): results of a survey, a laboratory experiment and a field transplant
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