What are the main impacts of human activity on salmon health, habitat, migration, and reproduction in British Columbia?
Riparian Vegetation Disturbances
Disturbances to riparian vegetation (areas immediately adjacent to waterways) destabilize riverbanks and cause erosion. The resulting increase of sediment into salmon rivers scours or suffocates incubating salmon. Sedimentation and vegetation loss decreases habitat diversity and alters the biological community present in-river and along riverbanks.
Examples of disturbances to riparian zones include: urban development, roadwork, and forestry activities. The size of a healthy riparian vegetation buffer that is key to successful salmon life cycles needed around a river is still debated, especially when it comes to the smaller, headwater streams that feed into larger rivers (Jyväsjärvi et al., 2020).
Poor Water Quality
Water quality issues from pollutants can arise for salmon in areas affected by industry and urbanization. These can have immediate lethal effects to salmon or can add a toxic-load to the salmon affecting growth, physical condition, vulnerability to disease, and long-term general health.
Point-source pollutants can be traced directly back to their origin. These are often large and highly concentrated pollution spills that can cause significant and immediate salmon mortality as well as long-term legacy effects. The Mount Polley mine disaster that spilled heavy metals into Polley Lake and Quesnel Lake is an example of this (Petticrew et al., 2015).
Run-Off & Water Extraction Issues
Urbanized areas are associated with multiple low concentration, non-point source (cannot be traced back to their origins) pollutants and contaminants that often end up in waterways. These can affect salmon and salmon habitats in multiple ways. For instance, non-point source pollutants can cause eutrophication via nutrient spikes and produce algal blooms. Cultus Lake Sockeye Salmon fry survival has been poor due to lake eutrophication, severely complicating recovery efforts for this endangered population (Azbarzadeh et al., 2021).
Road run-off can add a significant sediment load to salmon rivers, along with other pollutants, further affecting salmon.
Water extraction issues arise when industry, municipalities, and agriculture remove water from salmon rivers. This can decrease the amount of water available to buffer against drought-like conditions especially in the summer, and further concentrate pollutants in-river.
Ocean Wise has an online interactive map to explore pollution types and levels at various tested sites throughout coastal BC: https://pollutiontracker.org/
Migration Barriers
Many waterways are heavily modified, containing obstructions ranging from culverts to hydro-electric dams. These in-river barriers can limit salmon passage and reduce available habitat for salmon to spawn and rear.
Development or industry near riverbanks can also create waterflow barriers by narrowing sections of a river resulting in flow constriction. These areas when under high water-discharge conditions, can produce waterflows too high to allow for salmon migration. Hell’s Gate and the Big Bar Landslide on the Fraser River are examples of this.
Floodplain Loss
Research suggests that up to 85% of floodplain habitat has been lost due to industry, urban development, and waterflow modification (Finn et al., 2021).
Floodplains provide food and shelter for salmon, acting as important nurseries for salmon fry to grow in. The larger salmon fry produced by having access to these habitats translates into increased salmon survival rates in the nearshore marine environment (Duffy and Beauchamp, 2011).
Estuary Modifications
National Oceanic & Atmospheric Administration (NOAA) research suggests that up to 85% of estuary habitat has been lost due to industry, urban development, and shoreline modification (Brophy et al., 2019).
Estuaries include diverse habitats of sea grass, tidal marsh, and sand and mud-flat areas. These habitats are highly productive, and provide vital food and shelter for salmon at a vulnerable stage in their life: when they are smolting to adjust to marine waters.
River Productivity Declines
Spawned-out salmon carcasses import marine-based nutrients into freshwater, increasing the bio-diversity and productivity in these habitats. Not allowing enough salmon to be recycled in this way can lower river productivity and correlates with a decreased number of salmon in the future (Watkinson, 2000).
Research suggests up to 137 species feed on salmon in-river, and up to 24-57% of nitrogen used by riparian vegetation is directly from salmon (Quinn et al., 2018). Salmon are a keystone species and are critical for a healthy river.
Invasive Species Introductions
Invasive species compete with salmon for food and habitat. Many of these invasive species were historically introduced as sport fishes to create increased fishing opportunities. The majority of these species are highly predatory and can be significant predators of juvenile salmon (e.g., Brown Trout (Alvarez and Ward, 2019)).
Increased Harbour Seal Predation
Salmon are keystone species, and provide important food for many invertebrates, fishes, birds, and mammals. Resident Killer Whales feed primarily on salmon, including critically endangered Southern Resident Killer Whales. It is important that there are enough salmon available to support ecosystem functions.
Research has identified the high abundance of Harbour Seals as a potential increased predation threat to salmon (Nelson et al., 2018). However, The Salish Sea Marine Survival Project suggest Harbour Seals increased salmon predation only when forge fish, like Pacific Herring, numbers were low.
Hatchery-Wild Salmon Interactions
Salmon adapt quickly to hatchery conditions, and enhanced salmon do not survive as well as non-hatchery salmon in the wild (Araki et al., 2008). However, the enhanced salmon that do survive and return to spawn in-river can inbreed with wild salmon altering the genetic make-up of the wild fish. Moreover, enhanced fish are prone to straying, and can outbreed into other rivers, mixing genes and further decreasing the genetic diversity of wild salmon (Withler et al., 2018). Hatchery practice reforms are underway to help address these issues.
High hatchery production can introduce significant numbers of enhanced salmon into systems, intensifying competition with wild salmon in areas where there are resource bottlenecks. For example, in the North Pacific, this competition varies in intensity with climate variation cycles connected to marine productivity. This has been correlated with recent reduced size-at-maturity and size-at-age trends in many salmon species (Debertin et al., 2016).
Fish Farm Issues
Open-pen fish farms are being phased-out of BC waters. Concerns remain over escaped Atlantic Salmon competing with Pacific Salmon, and potentially establishing populations in BC.
Additional concerns over open-pen fish farms are related to sea lice exposure (Larsen and Vormedal, 2021) and viral spillover (Romero et al., 2022) from fish farm salmon to native salmon.
Loss of Genetic Diversity
Examination of Columbia River Chinook Salmon DNA extracted from bones found in ancient First Nation middens suggest that as much as 66% of genetic diversity has been lost over the last 7,000 years (Johnson et al., 2018).
Genetic diversity is important because it permits species to adapt to changing environments and is connected species persistence.
Much of the historical loss of salmon genetic diversity has been attributed to the legacy of industrialized overfishing.
Incidental Fisheries Mortalities
Catching salmon comes with an inherent risk to the survival of the fish even if it is to be released. Survival is influenced by factors like: fishing gear-type, amount of time that the fish is out of water, parasite and disease exposure, and how warm the temperature is when fishing (Teffer et al., 2017). For most catch-release fishing, DFO has adopted an estimate that salmon mortality will be 20%. However, the long-term survival effects of salmon that have been caught and released remain an area of on-going research.
Salmon are encountered in other fisheries, such as those targeting groundfish. It is important to document this bycatch to account for salmon loss for management purposes, and to formulate ways to mitigate the amount of salmon bycatch in these fisheries.
Mark-selective fisheries have opened in several areas of BC Inlets. These fisheries intend to target mass-marked hatchery origin Chinook Salmon while trying to minimize fisheries impacts to wild Chinook Salmon. The efficacy of mark-selective fisheries requires the number mass-marked salmon encountered in the fishery to significantly exceed the incidental mortality to released wild unmarked salmon to be feasible. This requires considerable fisheries monitoring, high hatchery mark rates, and a very good understanding of the number of released salmon by this fishery and their survival. Much of this information is currently lacking, and is further complicated by the fishing regulations within these areas permitting retention of numbers of unmarked Chinook Salmon. At this time, there is little conservation benefit to wild, unmarked Chinook Salmon in these areas.
Mixed-Stock Fisheries Interceptions
Many marine fisheries are interception-based fisheries on mixed stocks. This means that multiple populations (and even species) can be caught in a single fishery. Without careful fisheries planning and management based on sound biological data, such fisheries can disproportionately affect the smaller populations of salmon within these co-migrating groups. This can lead to salmon shortages in terminal areas, lowered escapement, and depressed local populations despite apparent sustainable fishing practices.
Furthermore, if good data is lacking on the populations that are present during a fishery, and appropriate regulatory measures are not adopted, there is potential to fish populations that are of conservation concern. This can lead to severe population bottlenecks and increased extinction risk to endangered salmon populations.
Summer Droughts
As of 2021, five of the last six years have been the hottest on record in BC, and even included an intense heat-dome. Climate research suggests that since the 1950’s river temperatures have risen by 1⁰C, and the number of days with river temperatures over 20⁰C has doubled (Islam et al., 2019). River temperatures above 18⁰C are highly stressful for migrating salmon, and research suggests that challenging migrations can be especially harmful for female salmon (Hinch et al., 2021). This sex-biased mortality can significantly impact salmon reproductive success and overall population numbers.
In BC, 2017-2018 marked record-setting numbers of forest fires. Forest fires can add contaminants to salmon streams and denude riparian vegetation leading to erosion issues.
Trends indicate that snowpack melt and freshet are peaking earlier in the spring. This timing may lead to early high waterflow issues for certain early spring salmon runs, and issues with drought-like conditions for later spring-summer salmon runs. In extreme drought, waterways can become disconnected and fragmented, or even completely dewatered.
Glacial melt has doubled in the last 20 years. Salmon in glacial-fed systems will have less water to buffer against drought-like conditions.
Flashy Winter Waterflows
Climate research suggests that in BC, average annual precipitation has increased by 22% over the last century, but is highly variable depending on region. Climate models project that winter precipitation will increase across all regions of BC, and with milder winters, much of it will be in the form of rain rather than snow. This lack of snow means less snowpack build-up, translating into decreased water storage to supply rivers in the summer, and increased levels of immediate run-off in the winter.
High rainfall events throughout the winter, like the atmospheric rivers of 2021-2022, produce large volumes of run-off with sudden, high waterflows in salmon streams. This can greatly affect the survival of fall-run salmon that are incubating in the river at this time, and can lead to significant flooding.
Oceanic Changes
In the Northeast Pacific, mean annual sea surface temperatures have increased by 0.4⁰C per decade. Under certain conditions, a warmwater blob develops here affecting the productivity, growth, and survival of many species of plankton, invertebrates, fishes, seabirds, and marine mammals.
During these times, smaller, less nutritious, warmwater plankton are present in this marine habitat along with increased gelatinous plankton-types (Cavole et al., 2016). Maturing salmon must compete more for less nutritious food, resulting in poorer growth and survival. Salmon that do survive can be in poor condition for the long, strenuous migration back to their natal streams.
Increased sea surface temperatures are associated with less mixing of ocean layers, and can alter the timing of seasonal mixing, an important event to trigger plankton blooms and larval settlement. If these timings are pronounced, there can be a de-coupling between the timing of salmon outmigration from freshwater and available plankton to feed on in the nearshore marine environment.
Ocean acidification can affect plankton in many ways, potentially decreasing available food for salmon. Moreover, ocean acidification has been shown to reduce the effectiveness of salmon olfaction (Williams et al., 2018), a sense which is key for effective salmon migration and homing.
References
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