Threats to fish and fish habitat

Aquatic ecosystems face numerous threats, from human activities in or near water, land use in the broader watershed, and even natural events. The fish and fish habitat protection policy statement 2019 lists the following broad threats that threaten the health and sustainability of fish and their habitat:

• modification or degradation of habitat
• aquatic invasive species (AIS)
• overexploitation of fish
• pollution
• climate change

Habitat modification or degradation

Habitat modification refers to changes to the function of an aquatic ecosystem that makes it less suitable to support fish and other aquatic species. For example, the alteration of flow in a watercourse by a dam, water diversion, stream crossings or water extraction can result in negative effects on spawning or rearing, or the death of fish. The loss or degradation of habitat may result from activities like infilling lakes, streams, or wetlands to create dry land or creating barriers to fish passage (i.e., disrupting the connectivity of fish habitat).

Aquatic invasive species

Aquatic invasive species are fish, invertebrate or plant species that have been introduced into a new aquatic environment, outside of their natural range. Canada has numerous AIS, in both freshwater and marine ecosystems. Canada’s federal Aquatic Invasive Species Regulations list over 164 aquatic species as invasive. Some of the more visible freshwater invaders in Canada include Asian carp, sea lamprey, and zebra mussels.

AIS can be introduced accidentally or intentionally in Canadian waters through several pathways such as:

• ballast waters from ocean-going vessels
• boats and equipment associated with watersports and recreational fishing
• aquaculture
• aquarium trade (animal and plants in aquariums) and other commercial sales of live species
• disposal / release of aquarium pets, water garden plants, live food (e.g., fish, crabs, shellfish, snails)

With few or no natural predators, AIS can outcompete native species for food and space. They can degrade habitat by affecting water quality, and by transporting diseases or parasites.

Economic impacts of AIS include:

• control and management costs
• reduced recreational opportunities and economic benefits
• reduced productivity in resource sectors, including fisheries and aquaculture
• the imposition of trade restrictions from other authorities
• impacted property values

Overexploitation of fish

Overexploitation of fish, or overfishing, refers to catching amounts of fish that are not sustainable, i.e. fishing at a rate that hampers the ability of the remaining population to rebuild or recover to healthy levels. The depletion of fish stocks can lead to severe economic consequences and alter entire food webs.

Other ways that fishing can lead to depleted and vulnerable fish stocks is through:

• Bycatch, which is the unintentional or incidental capture of other species, including fish and invertebrates, marine mammals, migratory birds, and sea turtles, and/or the capture of juveniles or other undesired individuals of the target species.
• Illegal, unreported and unregulated fishing, which accounts for about 30 per cent of all fishing activity worldwide, removing up to 26 million tonnes of fish from oceans annually.
• Ghost gear and other harmful fishing practices. Ghost gear refers to any fishing gear that has been abandoned, lost, or otherwise discarded (for example nets, line, rope, traps, pots, and floats). This debris can be fatal to fish, marine mammals, and other marine life, poses a navigation hazard, and breaks down into other forms of pollution such as micro-plastics.

Pollution

Pollution can enter freshwater habitats from a wide range of sources, including but not limited to:

• atmospheric deposition of air pollutants arising from industry, transportation (i.e., vehicle emissions) and electricity generation
• plastic and other post-consumer wastes migrating to waterways
• industrial and wastewater effluent discharges to water systems
• surface water and ground water contamination from sources within the watershed (pesticide application to crops, road salts, chemical spills, etc.)

These inputs can harm fish and aquatic ecosystems in many ways. For example, phosphorus is a nutrient that occurs in nature, as well as in human-made products, such as fertilizer and detergent. On its own, phosphorus helps to nourish life in the lake, but when we add to that natural load, phosphorus over-feeds the lake, causing algae and aquatic plant populations to explode, a process called eutrophication (or nutrient enrichment). This can block the sunlight from reaching water depths below the surface, deplete oxygen levels and change species composition and biodiversity.

Climate change

Climate change refers to long-term shifts in temperatures and weather patterns, caused by human activities, especially the burning of fossil fuels.

Canada’s Changing Climate Report (2019), led by Environment and Climate Change Canada and part of Canada’s national assessment process, provides a comprehensive summary of current knowledge about how and why Canada’s climate has changed, and what changes are projected for the future. Some key findings show that:

• on average, Canada is warming about twice as fast as the global average, and three times faster in the North
• Canada’s oceans are becoming warmer, more acidic, and less oxygenated
• precipitation is projected to increase for most of Canada, on average, although summer rainfall may decrease in some areas
• the seasonal availability of freshwater is changing, with an increased risk of water supply shortages in summer
• a warmer climate will intensify some weather extremes in the future (extreme hot temperatures will become more frequent and more intense, increasing the severity of heatwaves, and contribute to increased drought and wildfire risks)
• coastal flooding is expected to increase in many areas of Canada due to local sea level rise