Why Do Forests Grow When Salmon Swim Up the River?
― Some of the Nutrients in Riverside Trees Swam In from the Sea
In autumn, salmon swim up rivers in great schools. Once they have laid their eggs, they die in the river where they spawned. This autumn ritual is, in fact, an important "fertiliser delivery" for the forest along the banks. Trace the nutrients in a tree's leaves, and some of them turn out to have come from the sea.
From October to November, you can see salmon runs in rivers in Hokkaido and the Tohoku region. Fish push upstream through the shallows, backs breaking the surface and bodies twisting. It is a familiar autumn scene in the news.
Once salmon reach the end of their journey, they lay their eggs and soon run out of strength. Pale, spent fish lie along the riverbank.
It looks like a bit of a waste. But what would you think if you heard that these bodies then go on to feed the trees of the forest?
Two "Carriers" Explain Why the Forest Grows
A young salmon heading downriver weighs only about a gram. After several years at sea, it returns weighing a few kilograms. Almost all of its body is built from nutrients of the sea.
Bears and eagles take salmon into the forest to eat. Leftovers and droppings soak into the soil, and tree roots take them up.
Normally, nutrients flow from high ground to low. Rain dissolves nutrients from the soil, and rivers carry them to the sea. Salmon are carriers that move against that flow, in the opposite direction.
A Salmon Is a Lump of Sea Nutrients Swimming Upstream
Of the materials that make up a body, the one plants most often run short of is nitrogen. Nitrogen is an essential part of protein. Forest soil is often poor in it, so it acts as a "bottleneck" on how well trees grow.
Then along come salmon, which are lumps of protein. The nitrogen carried by one salmon is estimated at roughly 100 grams (the calculation is in the fold-out section at the end). In terms of household fertiliser, that is about a quarter of a bag per fish.
Look at Figure 1. Nutrients in river water and fallen leaves flow downhill toward the sea at lower left. Only salmon lift nutrients upstream to the upper right, like the thick arrow. Use the slider to change the number of salmon swimming up.
How Do We Know the Nitrogen Came from the Sea?
Nitrogen comes in two types that differ very slightly in weight. The bodies of sea creatures contain a little more of the heavier type. Land plants usually have more of the lighter type.
Researchers measured the share of heavy nitrogen in the leaves of riverside trees. Trees beside rivers that salmon climb showed values closer to those of sea creatures. In surveys in North America, about 20% of the nitrogen in the leaves of riverside trees and shrubs was estimated to come from salmon. Trees along such rivers were reported to thicken faster than trees along rivers without salmon.
The star carrier is the bear. When fish are plentiful, bears are said to eat only the fatty heads and eggs and leave the rest in the forest. Flies gather on the leftovers, then birds, and in the end it all returns to the soil.
Riverside trees shade the water and keep it from getting too warm. Their fallen leaves feed river insects. Those insects feed young salmon born in the river. Salmon grow the forest, and the forest grows the next salmon. The autumn run is one part of a loop that links sea and forest.
Summary
A salmon's body is almost entirely a lump of nutrients grown in the sea. Bears and birds carry spent salmon from the river into the forest. The nutrients soak into the soil and feed trees in a forest short of nitrogen. Nutrients usually only flow downhill, but every autumn salmon bring some back upstream.
Salmon carry the sea's nutrients on their backs up the river.
The autumn run is also a fertiliser delivery to the forest.
How salmon find their way back to the river where they were born is explained in "How Do Salmon Find Their Way Back to the River They Were Born In?", and how fallen leaves in the forest return to the soil is explained in "Why Don't Fallen Leaves in the Forest Pile Up into a Mountain?".
- Find out whether there is a place near a salmon river where you can watch them (a hatchery or a visitor facility).
- On the riverbank, look for where spent salmon lie. Note whether they are only in the water, or also on the bank and in the grass.
- Look again a few days later. Compare how the amount of bodies has changed and whether birds or insects have come.
In areas with bears, never go to the riverside or into the forest alone. Follow the guidance of the facility and local rules. On most rivers, taking salmon home is prohibited by law.
For Those Who Want to Know More ― Terms, Formulas and Links to TextbooksFrom middle-school science to university courses, each part is labelled by level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school Basic Biology / Biology and Basic Chemistry
- High school+Advanced high-school material, or textbook sidebars
- UniversitySpecialist university subjects (ecology, biogeochemistry) not taught in high school
- ResearchNot yet settled even at university level; researchers are still investigating
Middle schoolTerms: This Phenomenon Has a Name
- Run (upstream migration): fish swimming up a river against the current.
- Material cycling: nitrogen, carbon and other elements moving, changing form, among living things, soil, water and air.
- Decomposers: organisms that break down dead bodies and droppings into forms plants can use, such as fungi, bacteria, earthworms and fly larvae.
Middle schoolHigh schoolChecking with a Formula: How Much Nitrogen Does One Salmon Carry?
We estimate the nitrogen in a salmon's body from its weight. Then we multiply by the number of fish to get the amount carried upstream by the whole river.
| In symbols | N ＝ n × M × p × q |
| In words | Nitrogen carried ＝ number of salmon × weight of one fish × share of protein × share of nitrogen in protein |
| Where it comes from | Conservation of matter (mass balance). Nitrogen is neither created nor destroyed, so what the fish took into its body at sea is carried upstream as it is. |
| N | Nitrogen carried upstream (in kg) |
| n | Number of salmon swimming up (fish) |
| M | Weight of one salmon (kg) |
| p | Share of body weight that is protein |
| q | Share of nitrogen in protein |
| Weight of one chum salmon (rough guide) | Said to be 3.5 kg |
| Share of protein in fish flesh | About 0.20 (20%) |
| Share of nitrogen in protein | About 0.16 (basis of the conversion factor in food composition tables) |
| Weight of a young salmon heading to sea | Said to be about 1 g |
| Share of nitrogen in urea fertiliser | 0.46 |
| Nitrogen share of body weight (p × q) | 0.20 × 0.16 ＝ 0.032 |
| Nitrogen in one fish (kg) | 3.5 × 0.032 ＝ 0.112 |
| For a river with 10,000 fish (kg) | 0.112 × 10000 ＝ 1120 |
| As urea fertiliser (kg) | 1120 ÷ 0.46 ≒ 2435 |
| Weight gained at sea (how many times) | 3500 ÷ 1 ＝ 3500 |
By this calculation, about 110 grams of nitrogen per fish, and about 1.1 tonnes for 10,000 fish, travel upstream. Body weight grows 3,500 times at sea, so almost all the nitrogen carried was gained at sea. What actually reaches forest soil is the part that bears, birds and floods carry onto the bank.
High schoolHigh school+Nutrients Do Not Only Flow Downhill
High schoolIn Basic Biology you learn that matter cycles through ecosystems while energy flows in one direction. Nitrogen on land is an element that easily dissolves in rain and leaves for the sea by way of rivers. Little flows back from sea to land, and migratory fish are one of the few routes.
High school+Nitrogen has stable isotopes with mass numbers 14 and 15. A salmon has passed through many steps of the marine food chain, so its body has a higher share of nitrogen-15 than land plants. Using this difference as a marker, we can estimate how much sea-derived nitrogen has been taken into forest leaves and insects.
UniversityMarine-Derived Nutrients and Tracing with Stable Isotope Ratios
In ecology, the nutrients salmon carry are called marine-derived nutrients (marine-derived nitrogen). A "two-source mixing model" compares the nitrogen stable isotope ratio (the value written δ15N) with end-member values for sea and land, and estimates the share from each source. In a study on the west coast of North America, a little over 20% of the nitrogen in the needles of riverside conifers was estimated to be marine-derived. This is a classic example of "biological transport" in the nitrogen cycle, where the movement of living things carries nutrients.
📖 Derivations and further reading: Nitrogen cycle (Wikipedia, in Japanese) ／ Isotopes and stable isotopes (Wikipedia, in Japanese)
ResearchWhat Is Still Not Clear
- How far into the forest does it reach? Beyond a few tens of metres from the river, the sea-nitrogen marker is said to fade quickly. How far it reaches varies greatly with the river and the number of bears.
- What happens to the forest on rivers where salmon have declined? Forests change over decades, so it is not easy to pick out the effect of a smaller run.
- Is it the same in Japanese rivers? In Japan, conditions such as dams, hatchery releases and bear numbers differ from North America. Studies have been done in places like Hokkaido, but the size of the contribution to forests still varies from region to region.
So the content of this article is also "an explanation of what is known so far." In particular, how much forests grow is thought to vary considerably from place to place.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Balance in Nature" (food chains, decomposers) | Leftovers returning to the soil |
| High school | Basic Biology: "Ecosystems and Material Cycles" | The route nitrogen takes from sea back to forest |
| High school+ | Basic Chemistry: "Isotopes" | Telling origin by heavy nitrogen |
| University | Ecology, biogeochemistry | Marine-derived nutrients and mixing models |
| Research | Watershed ecology | How far it reaches, effects when runs decline |
| ― | Link to daily life | Protecting river fish also helps protect forests |
- Helfield, J. M. & Naiman, R. J. (2001) Effects of salmon-derived nitrogen on riparian forest growth and implications for stream productivity. Ecology 82: 2403–2409.
- Gende, S. M., Edwards, R. T., Willson, M. F. & Wipfli, M. S. (2002) Pacific Salmon in Aquatic and Terrestrial Ecosystems. BioScience 52: 917–928.
- Ministry of Education, Culture, Sports, Science and Technology (文部科学省), Standard Tables of Food Composition in Japan (日本食品標準成分表) (conversion factor between protein and nitrogen)
- Salmon - Wikipedia (サケ, in Japanese)
※This article is a science explainer for general readers. The figures given are rough guides and estimates to help you understand the mechanism. Salmon weight and nitrogen share differ by species and individual, and the size of the contribution to forests varies greatly by region. When observing in places where wild animals live, follow the guidance of local authorities and facilities.