We usually think of nutrients moving down rivers to the ocean, not the other way around. But in many temperate rivers of the northern hemisphere migration of salmon brings nutrients from the ocean to rivers and surrounding vegetation.
Salmon migrate to freshwater to spawn, then they die. Their carcasses thus become fertilizer for local vegetation. These nutrients have remarkable effects on stream-side ecosystems, like increasing [plant diversity]](https://science.sciencemag.org/content/331/6024/1609?casa_token=SbM0_-Wr3WIAAAAA:k9ajEC6j5fa5EdyYDDI2Hn7tpkR5RgV3j4tiSi7bYhidXBi13kd6P87WGkC_HFZzh8Bwn_T3nPNJ__M), raising bird diversity and increasing tree growth.
We wanted to know if all this fertilization of local vegetation showed up in satellite images. If we could measure the salmon fertilization effect with satellites, then we could build a complete picture of how far and across how many streams this fertilization effect is important.
Measuring the footprint of salmon nutrients is important for the management of streamside ecosystems. This connection may be weakened if fisheries catch too many salmon. Salmon populations in many places are also threatened by development and mining in their catchments.
It may seem odd for an Australian researcher to work on northern salmon, but this work was facilitated by a trans-Pacific connection. Griffith has a kind of sister-school relationship with Simon Fraser University in Vancouver.
I won a travel grant from Griffith to visit Simon Fraser University. On my travels I heard about the remarkable work that John Reynolds and his team are doing on documenting how salmon bring nutrients to streamside ecosystems. I started wondering if such effects would show up in satellite images.
We decided to look at the ‘greenness’ of satellite image. Greenness is a common indicator of vegetation productivity that reflects chlorophyll density in leaves. Chlorophyll is the pigment that helps plants turn sunlight into energy.
We studied two different places. In central British Columbia John and his team have detailed surveys of 50 salmon spawning streams. We compared greenness across these sites and found trees were greener near streams with higher salmon spawning densities.
We also compared greenness across different years on the lower Fraser river in southern British Columbia. Salmon spawning numbers in the lower Fraser are dominated by pink salmon, which tend to only return in odd numbered years to spawn. So we expected an ‘on-off’ pattern in greenness.
In the lower Fraser we found such an ‘on-off’ pattern.
So we’d like to use this technique to measure the footprint of salmon across all spawning streams. However, the science isn’t quite there yet.
At both sites the salmon signal was well hidden under other drivers of greenness. For example, at the central coast site in particular, alders, a type of tree that can make its own nutrients, weakened the effect of salmon on greenness. At the lower Fraser site, patterns of greenness were dominated by the weather first, with salmon fertilization playing a secondary role.
These other drivers of greenness mean we aren’t confident enough in the method to extrapolate it more widely. Follow-up work should use methods like chemical tracers to validate increases in greenness against the supply of ocean nutrients.
Our study does offer a tantalizing hint as to the possibility of using satellite images to better measure the footprint of salmon nutrients in trees.
The paper Salmon abundance and patterns of forest greenness as measured by satellite imagery was published in Science of the Total Environment.