Although sometimes heart-wrenching to discover on our nature outings, the animal carcasses that we may come across play an important role in the functioning of healthy ecosystems. The animal remains not only provide food for many carrion-eating species, their nutrients also contribute to the significantly increased growth of surrounding plants. This, in turn, attracts many herbivorous insects and their predators, thus increasing biodiversity.
There’s no better illustration of this than the grizzly bears-salmon cycle in Alaska. When salmon runs are good, bears can find themselves a fresh dinner without much effort. During these high times, grizzly bears eat only the most nutritious parts of the fish, going after fat-rich brains, eggs and skin. The rest of the carcass is tossed aside for an enterprising bird, an assertive river otter or left to decompose.
When salmon carcasses are left to decay, they become a crucial source of forest nutrients in riparian areas (the land immediately around a lake or a stream). Amazingly, because bears often carry fish away from riverbanks and a short distance into the woods, decomposing salmon provide up to 24 percent of soil nitrogen in these riparian zones. Researchers describe this process as a “marine-to-land nutrient transfer,” and it can have major impacts on a forest.
Just how big those effects can be have recently been quantified by researchers.
Salmon carcasses left on the shore aided the growth of surrounding plant species and even caused some flowers to grow bigger and more plentiful. Unfortunately, some regions are rapidly losing salmon biomass.
Salmon flowers
In a study that is the first to demonstrate a connection between salmon and coastal plant growth and reproduction, scientists from Simon Fraser University in Burnaby, British Columbia, conclude that nutrients from salmon carcasses can substantively alter the growth and reproduction of plant species in the surrounding habitat—and even cause some flowers to grow bigger and more plentiful.
During a three-year field study, the results of which were published in the journal Royal Society Open Science in January 2023, researchers experimentally added pink salmon carcasses into the estuary of a small river in Heiltsuk territory on British Columbia’s central coast. The area features a large meadow of grasses and wildflowers.
Following the experiments, some wildflowers grew larger leaves where a salmon carcass had been deposited; and in some years, some plants grew larger flowers or produced more seeds.
It all begins with the bears. Bears are apex predators who influence their ecosystems through predator-prey relationships. Likewise, salmon, in turn, “feed” their ecosystems not only by feeding bears, but also with their carcasses, which provide nutrients to streams and riparian vegetation. ©Jitze Couperus, flickr
The scientists undertook similar experiments using rockweed seaweed, which provides a different set of nutrients. They also experimented with a combination of rockweed and salmon carcasses, and a control; then examined their impacts on four common wildflower species: Douglas aster, common red paintbrush, silverweed and yarrow. The addition of salmon carcasses led to larger leaves—particularly in common red paintbrush and yarrow—and a greater seed set in yarrow in the third year.
This work extends what was previously known about a nitrogen isotope that’s found in some animals and plants in the ecosystem and has been generally attributed to the nutrients from salmon. It also sheds light on the bigger picture of how to best manage the rivers and streams traveled by salmon in these times of great climate change. Currently, lands and waters in many areas are managed under separate provincial and federal jurisdictions.
Sockeye spruce
As mentioned in the above paragraph, a similar study published in 2018 had shown the advantages to landscapes from the nutrients in salmon.
In one study, white spruce trees on the side of a river that was the depository for salmon carcasses grew faster than their counterparts on the other side. ©Kpsudeep, Wikimedia Commons
For the past 20 years, dozens of University of Washington researchers have walked Hansen Creek—a small stream in southwest Alaska—every day during salmon spawning season, counting the live fish and recording information about those that had died. For salmon, death is inevitable here, either after spawning or in the paws of grizzly bears, who catch fish in the streams and often eat just part of the carcass.
After counting a dead fish, researchers would throw it on the shore to remove it and avoid double counting it the next day.
When this effort began in the mid-1990s, the lead scientist on the project decided that everyone should throw sockeye carcasses to the left side, facing downstream. They might as well be consistent, he thought, and who knows: maybe someday they could see whether the tossed carcasses had an effect on that side of the river.
Striped cleaner wrasses (cleaner fish) tend to dwell in coral or rocky areas on coral reefs. They benefit humans by increasing the survival of various larger, economically important fish by eating and removing harmful parasites and diseased tissue from the bodies and scales of the bigger fish. ©Elias Levy, flickr
Twenty years later, researchers found that two decades of carcasses—nearly 600,000 pounds of fish—tossed to the left side of Hansen Creek did have a noticeable effect: white spruce trees on that side of the stream grew faster than their counterparts on the other side. What’s more, nitrogen derived from salmon was found in high concentrations in the needles of the spruce trees on the side where the tossed carcasses landed.
Essentially, then, the dead sockeye were fertilizing the trees.
Fish faces
I couldn’t leave this topic of our deep connection and relationship to fish without mentioning a fun fact that has just come to light: like us, fish seem to be self-aware.
In findings published in the journal Proceedings of the National Academy of Sciences on February 6, 2023, a research team from Osaka Metropolitan University in Japan has recently demonstrated that fish think “it’s me!” when they see themselves in a photograph. The determining factor wasn’t seeing their bodies but looking at their faces.
We humans are so accustomed to staring at our own reflections that we mostly take for granted that this form of self-recognition is an advanced act of cognitive awareness. But new tests challenge the notion that humans are the only species capable of higher cognitive abilities, such as self-consciousness.
In this study, experiments were conducted with cleaner fish (Labroides dimidiatus), which are known to be able to recognize themselves in mirrors and regularly attack other, unfamiliar cleaner fish who intrude upon their territories. Each cleaner fish was presented with four photographs: a photo of themselves, a photo of an unfamiliar cleaner, a photo of their own face on an unfamiliar cleaner’s body, and a photo of an unfamiliar cleaner’s face on their own body. Interestingly, the cleaner fish did not attack photos with their own faces but did attack those with the faces of unfamiliar cleaner fish. These results indicate that cleaner fish determined who was in the photograph based on faces but not bodies, like humans.
To negate the possibility that the fish considered photographs of themselves as very close companions, a photograph mark-test was conducted. Fish were presented with photographs where a parasite-like mark was placed on their throats. Six of the eight individuals that saw the photograph of themselves with a parasite mark were observed to rub their throats to clean it off. Showing those same fish pictures of themselves without parasite marks or of a familiar cleaner fish with parasite marks did not cause them to rub their own throats.
The researchers say that this study is the first to demonstrate that fish have an internal sense of self; and since the target animal is a fish, it suggests that nearly all social vertebrates also have this higher sense of self.
Scientifically and management-wise, say researchers, we think of the land and water as separate and unconnected entities. But, in truth, they are interdependent. ©Bureau of Land Management, flickr
Interconnection importance
Unfortunately, we are continuing to see population declines in salmon in the B.C. region. Research conducted last year showed that chum salmon here had decreased almost 50 percent within the last 15 years, and more than 70 percent within the last 50 years.
We often don’t realize these two truths: 1) even after death, animals are important in ecosystems, and 2) the ocean has a connection to life on land. Even large ecosystems don’t exist in isolation, and what happens in one can influence the other.
Understanding that confluence and interconnection is incredibly important for knowing how to protect them both.
Here’s to finding your true places and natural habitats,
Candy
