“Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid”.
When Albert Einstein said that, he was referring to the fact that while fish are specialists at swimming, their ability to climb trees is non-existent.
Or so he thought.
It turns out that fish actually DO climb trees, so to speak. But not how you’re imagining it.
Within the streams of the North American rainforests, salmon are returning to spawn.
Salmon are anadromous fish. That means that they are born in freshwater, then migrate to the ocean as juveniles where they continue to grow and live out most of their adult lives before migrating back into freshwater to reproduce. They do this great migration just once in their lifetime.
Once the salmon spawn, they die. This reproductive strategy of having one spawn event in a lifetime and producing many offspring at once is called semelparity.
But what does this have to do with trees? Keep reading to find out!
Why the fish rely on the trees
Tree roots act as a huge safety net that holds the soil and sediments together above the river banks. By doing so, they ensure that fewer fine sediments enter the river. This is important because the characteristics of the sediment and gravel are vital to the quality of the salmon spawning habitat and hence the success of salmon spawning.
To be suitable for salmon spawning, substrates need to be coarse enough that they resist being eroded and permit through‐flow of oxygen‐rich water (Riebe et al, 2014). Yet they simultaneously need to be fine enough so that the female can dig a hole in the gravel with her tail. Where she will then lay a few thousand eggs. Streamexplorers.org. (n.d.).
Therefore, activities that reduce the number of trees around a river basin such as, logging, road building, and agriculture can increase the amount of fine sediment entering streams (Riebe et al, 2014).
This leads to clogging of the gaps between gravel, reducing their permeability (Riebe et al, 2014). This will suffocate the growing eggs.
But that’s not the only reason salmon rely on trees. In fact, every part of the tree supports the salmon lifestyle, from its roots to its leaves. Vegetation provides shade which keeps growing eggs cool. Leaves and branches provide habitats and shelter for both terrestrial and aquatic insects which, when fallen into the river, the salmon can eat. The leaves themselves will fall into the water stream and deliver nutrients to the water as they decompose. Fallen trees create pools that shelter young fish and provide a place to rest away from strong currents. Fallen trees can also direct streamflow, store sediment and create riffles and waterfalls. And finally, the organic matter trapped by wooden debris also provides food and housing for aquatic insects, which the salmon can feed on. (Post, A. 2008).
Why the trees rely on the fish
Trees, like all living things, need nutrients to grow and survive. Yet trees can’t move around the forest in search of food.
However, bears can. And what do bears eat? Salmon!
So by migrating upstream into the depths of the forest, the salmon are providing a food source for not only the bears but the trees too. That’s because when a bear catches a fish, he or she will transport it away from the river and into the rainforest to eat, and once they consume the oily roe, belly, brain and skin, the rest of the carcass is left untouched and available to other animals. As the fish carcass decomposes further, the nutrients such as nitrogen and phosphorus become available to the plants! (Post, A. 2008).
Other predators can catch and move the nutrient-rich salmon into the forest too, such as wolves, eagles, ravens, crows and gulls, river otter and mink (Post, A. 2008).
In Post’s article (2008), she explains that various researches have found high levels of salmon-derived nitrogen within the forest foliage. She states that “According to Robert Naiman of the University of Washington, streamside vegetation gets just under 25 % of its nitrogen from salmon. Other researchers report up to 70 % of the nitrogen found in riparian zone foliage comes from salmon. One study concludes that trees on the banks of salmon-stocked rivers grow more than three times faster than their counterparts along salmon-free rivers and, growing side by side with salmon, Sitka spruce take 86 years, rather the usual 300 years, to reach 50 cm thick.”
This ecological connection is just one of many countless examples of how seemingly small human interactions can negatively impact the whole of the surrounding ecosystem. Including the plants, animals and even abiotic factors like gravel size. Nature is strong and resilient, but it’s also very delicate. And it shows just how important it is to understand the ecosystem before we tamper with it.
References:
Post, A. (2008). Why Fish Need Trees and Trees Need Fish. [online] Adfg.alaska.gov. Available at: http://www.adfg.alaska.gov/index.cfm?adfg=wildlifenews.view_article&articles_id=407 [Accessed 7 Aug. 2019].
Riebe, C.S., Sklar, L.S., Overstreet, B.T. and Wooster, J.K., 2014. Optimal reproduction in salmon spawning substrates linked to grain size and fish length. Water Resources Research, 50(2), pp.898-918.
Streamexplorers.org. (n.d.). Salmon Life Cycle | Stream Explorers. [online] Available at: http://www.streamexplorers.org/fish-facts/salmon-life-cycle [Accessed 7 Aug. 2019].
Young, T. P. (2010) Semelparity and Iteroparity. Nature Education Knowledge 3(10):2.