Habitat loss and over-exploitation are leading to a decline in salmon populations

One-quarter of freshwater animals are now threatened with extinction, and population declines in fresh waters outpace those in marine and terrestrial systems. Reports of salmonid fish decline are stark, with many populations and species listed as threatened or endangered.

Salmonids are a large family of ray-finned fish. In North America, it includes Pacific salmon on the west coast, Atlantic salmon on the east, and trout and char species such as brook trout and the introduced brown trout.

Salmonid fish are extremely important. Ecologically, they provide food and nutrients for other animals and are indicators of ecosystem health. Culturally, they hold places in stories and worldviews, provide sustenance for humans and foster a deep connection to our rivers, lakes and oceans.

Economically, they sustain communities around the world, as people travel to see these captivating creatures. Beyond all this, they have intrinsic value within river ecosystems.

Although the salmonid family is undoubtedly one of the most studied groups of fish, we still don’t know much about the relative abundance of these fishes globally.

We did a systematic review of published literature for reports of salmonid biomass (the total weight of fish in a particular area) in rivers around the world. The result was the largest dataset of salmonid biomass as we know it: more than 1,000 rivers across 27 countries, with fish sampling spanning 84 years (1937-2021).

Habitat degradation

This unique dataset enabled us to test several hypotheses, including temporal trends in salmonid populations. We found that average biomass declined 38% from pre-1980 levels compared to post-2000 levels. Real declines are likely to be even higher, due to a publication bias towards reporting on rivers with higher biomass.

Reasons for the decline will be unique to each population and often due to a combination of factors that include habitat loss and degradation, river regulation, over-exploitation, aquaculture and climate change.

Freshwater ecosystems are among the most threatened and degraded environments in the world. The way we use the land surrounding waterways can have devastating impacts on aquatic life.

Excessive nutrient loads from agriculture can lead to harmful algal blooms, which can choke waterways and lead to oxygen depletion, killing fish.

Contaminants from pesticides, mining waste, oil and gas production, and urban areas can lead to decreased abundance and growth, declines in genetic diversity and effects on reproductive potential.

Forest clearing destroys stream habitat by removing shade and shelter-providing plants along stream banks. Without this vegetation, excessive sediment can be washed into the stream, filling gaps between rocks and stones and further degrading important fish habitats and increasing water temperatures.

Human activity disrupting migrations

Many salmonid species are anadromous, meaning they migrate from freshwater to the sea and return to freshwater to complete their life cycle. Adult salmon will swim into the headwaters of streams to spawn, so access to these habitats is essential.

Dams and other structures sever the pathway for many migratory fish and are perhaps the most significant disturbance in river ecosystems. Sixty-three percent of large rivers (over 1,000 kilometers) are no longer connected across their whole length.

While many salmonids may be able to scale small waterfalls, a dam or structure with smooth surfaces and no water are virtually impossible to pass. Fish passes (human-made pathways alongside barriers that fish can move through) can provide access upstream of dams. However, not all fish passes work as intended, and older dams will likely lack these facilities.

Even if fish can migrate above a dam, the natural flow of water and movement of substrate is disrupted, causing major effects downstream. These natural regimes of water and substrate are crucial for maintaining habitat for aquatic species.

Dam removal is becoming more common as a restoration technique, which leads to improved connectivity of sediment and fish. For example, fish numbers increased after removal of two dams on the Elwha River in Washington state, which reconnected 60 kilometers of previously inaccessible salmonid habitat.

Climate change

A warming climate, with more frequent droughts and flood events, is predicted to have negative impacts on salmon growth and survival, leading to deteriorating habitats and a reduction in abundance.

Warming waters may cause shifts in salmonid abundance and distribution, with some species unable to adapt or move in time. Warming can also lead to increased stress and mortality for these cold-water fishes, reductions in body size and spawning success.

Unfortunately, it was not possible to include temperature in our global dataset, as it is not systematically reported in studies.

Biomass not evenly distributed

In our study, we found that salmonid biomass is not evenly distributed. Most streams have a relatively low biomass (average of 5.2 g/m²). However, a few outstanding streams exhibit much higher biomass than average (over 36.5 g/m²).

It remains difficult to determine which variables contribute the most to this high productivity. High biomass may be related to local factors (temperature, flow, rock sizes in the river, presence of wood), which are not represented in our global dataset.

Investigating what makes these streams so productive is a key question for scientists. Our dataset can help fuel researchers’ curiosity and promote habitat restoration and enhancement for all freshwater life.

The dataset, which currently includes biomass data for 11 salmonid species and contains multiple variables that could affect biomass (stream width, season, sampling methods, area sampled and elevation), is publicly available. Scientists around the world can update the dataset in the coming years with additional data, such as temperature, which will help us understand the impact of climate change.

Restoring habitats

A lot of effort has gone into restoring and enhancing the habitats of salmonid species.

While we are seeing local improvements in some populations—for example, after habitat restoration with large wood or boulders—restoration efforts are often short-lived and target very small areas. These efforts should encompass whole watersheds to be most effective.

Rivers are naturally dynamic, shifting their course as they move across floodplains. Improving river mobility, by allowing a river to restore itself and providing it space to move, will lead to more long-term sustainable restoration. This will be beneficial for not only salmonids but other aquatic animals.

This article is republished from The Conversation under a Creative Commons license. Read the original article.