The urban curse on Coho salmon

Lets perform a small thought-experiment: what are the first three things you think about when you hear the term urbanization? I would guess people would give answers akin to stone houses, big cities, paved roads and a decrease in greenery. And yes, those are big aspects of urbanization. But all of those aspects have an important thing in common: they are about urbanization on land. Which is interesting, given that aquatic environments such as lakes and rivers are also greatly affected by the environmental changes that accompany urbanization. For example, the flow of rivers needs to be altered when building cities while the decrease in canopy cover leads to an increase in water temperature (Spanjer et al., 2018).

An example of a species that is severely affected by urbanization, and that I will discuss in this post, is the Coho salmon (Oncorhynchus kisutch). Spanjer et al. (2018) looked at differences between Coho salmon in urban and non-urban streams. The normal lifecycle of a Coho salmon starts in small- to medium-sized Puget Sound lowland streams. There, they are usually born during autumn. The juvenile salmon generally spend about a year in the lowland streams, after which they migrate to the open ocean. In the research by Spanjer et al. (2018), six streams were chosen over an urbanization gradient. This gradient ranged from a highly urbanized stream to a stream in a forest. Juvenile Coho salmon were released in each stream and analyzed a year later. It was hypothesized that urban rivers would be worse habitat for the juvenile salmon, with those rivers containing smaller individuals compared to the less urbanized streams. As it turns out, this hypothesis was not entirely correct.

When measuring the size of the grown salmon, the most urbanized streams yielded larger salmon than the forest streams. Thanks to the higher water temperature in urban streams, the salmon emerged earlier and kept growing for a longer period of time. However, the growth efficiency, meaning how well the fish were able to grow over time, was not as favorable. During the summer the growth efficiency was about 17% lower in highly urbanized streams compared to other streams. This was the result of lower food quality and a higher amount of stress in urban streams. In essence, the salmon had longer growing periods but a more difficult time growing up in urban streams. And although not lethal, the effects are quite negative for the Coho salmon. As a possible solution, restoration of the urbanized streams was suggested.

So urbanization affects Coho salmon size and growth efficiency. That is hardly a reason to say Coho salmon are severely affected by urbanization, as I stated earlier. But in 2011, research from Spromberg & Scholz pointed out a worrying increase in the mortality rate of Coho salmon in the Pacific Northwest of the USA. When Coho salmon want to reproduce, they do this via a process called spawning. When a fish spawns, this means it freely releases either its eggs or its sperm into a body of water. The fertilization occurs when the drifting sperm and eggs find each other and fuse. When looking at restored habitats for Coho salmon spawners, it was found that an increasing number of spawners were dying before being able to spawn (Spromberg & Scholz, 2011). This phenomenon was named Coho prespawn mortality (PSM). PSM turned out to be the result of urban stormwater finding its way to the restored habitats during autumn. The stormwater was carrying chemical pollutants. Although not known at the time, later research by Tian et al. (2021) would identify the chemical pollutant as a rubber tire–derived chemical.
When Spromberg & Scholz (2011) ran a model to predict the number of years it would take for the Coho salmon to go extinct, the results were shocking. The model predicted that at best it would take 115 years, and at worst just 8 years before this species would be extinct. Other salmon species did not seem to be affected, nor Coho salmon in other life stages than spawner (Feist et al., 2011). But in Coho salmon, wild individuals as well as hatchery salmons are affected (Feist et al., 2011).

Since the chemical pollutant has been identified, there is hope that this increase in Coho salmon mortality can be reversed. Luckily, there is still time to intervene and save this species from extinction. But the clock is ticking, as the lowest threshold of the predicted timeline from Spromberg & Scholz (2011) has already passed.

 

References

1. Feist, B. E., Buhle, E. R., Arnold, P., Davis, J. W., & Scholz, N. L. (2011). Landscape Ecotoxicology of Coho Salmon Spawner Mortality in Urban Streams. PLOS ONE, 6(8), e23424. https://doi.org/10.1371/JOURNAL.PONE.0023424
2. Spanjer, A. R., Moran, P. W., Larsen, K. A., Wetzel, L. A., Hansen, A. G., & Beauchamp, D. A. (2018). Juvenile coho salmon growth and health in streams across an urbanization gradient. Science of The Total Environment, 625, 1003–1012. https://doi.org/10.1016/J.SCITOTENV.2017.12.327
3. Spromberg, J. A., & Scholz, N. L. (2011). Estimating the future decline of wild coho salmon populations resulting from early spawner die‐offs in urbanizing watersheds of the Pacific Northwest, USA. Integrated Environmental Assessment and Management, 7(4), 648–656. https://doi.org/10.1002/IEAM.219
4. Tian, Z., Zhao, H., Peter, K. T., Gonzalez, M., Wetzel, J., Wu, C., Hu, X., Prat, J., Mudrock, E., Hettinger, R., Cortina, A. E., Biswas, R. G., Kock, F. V. C., Soong, R., Jenne, A., Du, B., Hou, F., He, H., Lundeen, R., … Kolodziej, E. P. (2021). A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon. Science, 371(6525), 185–189. https://doi.org/10.1126/SCIENCE.ABD6951/SUPPL_FILE/PAPV2.PDF

Photo by the Bureau of Land Management of Oregon and Washington