" Climate variability and change are already impacting and will increasingly impact Canadian fish and fisheries."⁽²⁾

The impacts of climate change on fish and fisheries will result from both biological and abiotic changes, as well as shifts in the man-made environment. Changes in water temperature, water levels, extreme events and diseases, and climate-driven shifts in predator and prey abundances will all impact Canadian fisheries. Changes in lake and ocean circulation patterns and vertical mixing will also be important. However, the limited understanding of the mechanisms controlling the behavioural response of fish to climate change,⁽¹⁴⁾ limitations in data, and the inability of models to account for the delayed impacts of environmental variability⁽¹⁵⁾ reduce our ability to project net impacts at present.

Pacific Coast

In British Columbia, provincial revenues from commercial fishing, sport fishing, aquaculture and fish processing exceed $1.7 billion.⁽¹⁶⁾ Over the past 10 years, significant changes have been noted in the British Columbia marine ecosystem⁽¹⁷⁾ that may be related to shifts in climate, although other factors, such as fishing practices, salmon farming, freshwater habitat destruction, and freshwater dams and irrigation facilities, have also been implicated.^{(18, 19)}

In recent years, much of the climate change research on the Pacific coast has focused on salmon species, owing to their importance to this region's commercial, recreational and subsistence fisheries, and to the alarming declines in the salmon catch observed since the late 1980s.^{(2, 19)} Low population sizes and survival rates of steelhead and coho salmon have caused significant fisheries reductions and closures in recent years.⁽²⁰⁾ In addition, salmon require at least two different aquatic habitats (marine and freshwater) over their life cycle, making them susceptible to a wide array of potential climate impacts, and studies have concluded that climatic forcing has been a key factor regulating northeastern Pacific salmon stocks over the last 2 200 years.⁽²¹⁾

The relationship between water temperature and salmon is complex, with numerous studies documenting diverse results. Higher temperatures have been associated with slower growth,^{(22, 23)} enhanced survival,⁽²⁴⁾ faster swimming rates,⁽²⁵⁾ reduced productivity⁽²⁵⁾ and shifts in salmon distribution.⁽²⁵⁾ As water temperatures increase, energy requirements tend to rise, which often reduces growth, productivity and, ultimately, population size.⁽²³⁾ Higher water temperatures have also been shown to decrease salmon spawning success,⁽²⁶⁾ and to enhance survival rates by improving the physiological state of the salmon.⁽²⁴⁾

Temperature changes will also affect fish indirectly, through changes in food and nutrient supplies and predator-prey dynamics. Temperature anomalies and changes in current patterns have been associated with large changes in the type and seasonal availability of plankton.⁽²⁷⁾ Furthermore, higher surface water temperatures have been shown to both prevent nutrients from reaching the water surface⁽²⁸⁾ and increase the rates of salmon predation by other fishes.⁽²⁹⁾

Future climate changes are projected to result in more variable river flows, with more frequent flash floods and lower minimum flows (see Water Resources chapter). The timing of peak flows is also expected to shift due to climate change.⁽²⁶⁾ These changes would influence salmon mortality, passage and habitat. Lower flows may benefit juvenile salmon by reducing mortality and providing increased habitat refuges.⁽³⁰⁾ When combined with higher temperatures in the late summer and fall, however, lower flows could increase pre-spawning mortality.⁽²⁾ An increase in flash flooding could damage gravel beds used by salmon for spawning.⁽³¹⁾ Flooding also has the potential to cause fish kills from oxygen depletion, owing to the increased flushing of organic matter into estuaries.⁽²⁾

Other climate factors that may significantly affect west coast salmon populations include synoptic-scale climate changes and the frequency of extreme climate events. For example, widespread decreases in coho marine survival have been shown to correspond to abrupt changes in the Aleutian Low Pressure Index.^{(32, 33)} Other studies have suggested that recent declines in Pacific steelhead populations are related to the increased frequency of winter storms and summer droughts observed during the 1980s and 1990s.⁽³⁴⁾ These extreme events may have impacted salmon survival and production through habitat disruption and loss.

It is important to note that, although most of the recent literature on the Pacific coast focuses on salmon, climate change would have implications for other types of fish. Groundfish and shellfish are both important economically to the region, with landed values in 1998 of $115.8 million and $94.9 million respectively.⁽⁴⁾ Changing marine conditions will have implications for sustainable harvests, fishing practices and subsistence fisheries.

Atlantic Coast

The fishing industry remains extremely important to the economy of the Atlantic coast, although its dominance is weakening.⁽³⁵⁾ Shellfish catches currently represent the greatest landed value,⁽⁴⁾ with aquaculture quickly growing in importance. There are an estimated 43 000 fishermen in the Atlantic region, most of whom are highly dependent on the fishing industry.⁽³⁵⁾ As is the case for the Pacific coast, the main climate change issues for the Atlantic fishery in Canada relate to impacts arising from changes in ocean temperatures, current, and wind and weather patterns, as well as increases in extreme events.⁽³⁶⁾ Key species of concern include cod, snow crab and salmon. The impacts of climate change on different varieties of plankton are also a concern.⁽²⁾

Long-term trends suggest that climate influences which species of fish are available for harvesting.⁽³⁷⁾ While the recent shift in harvesting from groundfish to shellfish appears to have been driven primarily by fishing practices, climate is also believed to have played a role. For example, reduced growth rates and productivity, resulting from lower than average water temperatures during the late 1980s and early 1990s, are believed to have contributed to the decline in groundfish stocks.^{(38, 39)}

It is important to emphasize that the relationships between water temperature and factors such as growth rate and productivity are complex, with different species having different optimal thermal conditions. Researchers have demonstrated that snow crab, for example, are particularly sensitive to changing environmental conditions, and that changes in water temperatures affect their reproduction and distribution (see Box 1). Another example is the observation that egg survival, hatch rate and initial hatch size of winter flounder tend to be higher in cooler waters, leading researchers to suggest that in some regions, recent increases in water temperatures have contributed to observed declines in the abundance of the fish.⁽⁴⁰⁾

BOX 1: Water temperature and Atlantic Snow Crab⁽⁴¹⁾

Snow crab, an important component of Atlantic marine fisheries, are sensitive to climate warming. This is especially true on the eastern Scotian Shelf and the Grand Bank of Newfoundland. Researchers found a strong relationship between water temperature and snow crab reproduction and distribution, although the relationship was found to depend on the crab's stage of development. Some key findings include: Females incubate their eggs for 1 year in waters warmer than 1°C, as opposed to 2 years in waters colder than 1°C. This suggests that females in warmer waters may produce twice as many eggs as females in colder waters over their reproductive lifetime. The survivorship and long-term growth of juveniles is optimized at intermediate water temperatures (0 to +1.5°C). The spatial distribution of adolescent and adult crab is influenced by water temperature. Cooler waters are occupied by smaller, younger crab, whereas warmer waters are inhabited by larger, older crab. No crab, however, were found in waters exceeding 8°C. Atlantic snow crab (Photo courtesy of D. Gilbert)

Higher water temperatures, an increase in sea level and changes in salinity could all affect marine pathogens,⁽⁴²⁾ resulting in changes in the distribution and significance of certain marine diseases. This is supported by historical observations, such as the northward extension in the range of eastern oyster disease along the American coast during the mid-1980s as the result of a winter warming trend.⁽⁴²⁾ Conversely, some diseases of salmon have been shown to decrease or even disappear at higher temperatures.⁽⁴²⁾

Another concern for Atlantic fisheries is a potential increase in toxic algal blooms.⁽⁴³⁾ Researchers believe that climate warming may stimulate the growth and extend the range of the organisms responsible for toxic algal blooms, such as red tides (see Box 2). These blooms threaten shellfish populations through both lethal effects and chronic impacts. Aquaculture operations are particularly sensitive to toxic algal blooms because they operate in a fixed location. Clams are generally more affected than other shellfish, such as lobster, shrimp and scallops. Exposure to the toxins may negatively affect fish habitat, behaviour, susceptibility to disease, feeding ability and reproduction.⁽⁴⁴⁾ Infected shellfish are also a danger to human health, potentially resulting in paralytic shellfish poisoning.

BOX 2: An increase in toxic algal blooms?⁽⁴³⁾

Harmful algal blooms (HABs) are recurrent in the estuary and Gulf of St. Lawrence in eastern Canada. There is concern that these blooms will increase in frequency and intensity due to climate change. To determine the role of climate on algal blooms, Weise et al. (2001) analysed 10 years of hydrological, biological and meteorological data. They found that rainfall, local river runoff, and wind regime greatly affected the pattern of bloom development, with the development of blooms favoured by high run-off from local tributary rivers, combined with prolonged periods of low winds. More intense algal outbreaks were associated with extreme climate events, such as heavy rainfall. If conditions such as these become more common in the future, we can expect to see an increase in the onset and proliferation of toxic algal blooms in eastern Canada. Electron microscope image of Alexandrium tamarense, an algae responsible for toxic algal blooms (Image courtesy of L. Bérard)

The impacts of climate change on Atlantic salmon are similar to those described for Pacific salmon. During their time in freshwater, Atlantic salmon are sensitive to changes in both river water temperatures and flow regimes (see Box 3). Changes in temperature have been shown to significantly affect sustainable harvests and fishing practices. For example, researchers studying the influence of water temperatures on recreational salmon fisheries in Newfoundland's rivers found that, between 1975 and 1999, about 28% of rivers were temporarily closed each year due to warm water temperatures or low water levels.⁽⁴⁵⁾ In some years, more than 70% of rivers were affected. These closures led to a loss of 35 to 65% of potential fishing days in some regions, the worst period being between 1995 and 1999. The researchers concluded that climate change may increase the frequency of closures, and potentially decrease the economic importance of recreational fishing in Newfoundland.⁽⁴⁵⁾

BOX 3: How will climate change affect juvenile Atlantic salmon?⁽⁴⁶⁾

Atlantic salmon are cold-water species, and warmer waters resulting from future climate change could negatively impact fish growth, increase susceptibility to disease and infection, increase mortality rates, and decrease the availability of suitable habitat. New Brunswick's Miramichi River is located near the southern limit of Atlantic salmon distribution, and hence its populations are very sensitive to changes in both water temperature and streamflow. Modelling suggests that climate change could increase river water temperatures by 2 to 5°C, and produce more extreme low flow conditions. Using 30 years of data, Swansberg and El Jabi (2001) examined the relationships between climate, hydrological parameters, and the fork length of juvenile salmon in the Miramichi River. Fork length is an indicator of growth, which also affects competition, predation, smoltification, and marine survival of salmon. In association with the warming observed over the time period studied, fork length of juvenile salmon parr was found to have declined significantly. Researchers have therefore suggested that future climate change will adversely affect the growth of juvenile salmon in the Miramichi River. Atlantic salmon (Image courtesy of Atlantic Salmon Federation and G. van Ryckevorsel)

While it is broadly acknowledged that changes in the intensity and frequency of extreme events have the potential to impact marine fisheries, relatively few studies have addressed this issue. A recent study, examining the impact of summer drought and flood events in the Sainte-Marguerite River system of eastern Quebec, concluded that these events influence the average size of salmon at the end of the summer through selective mortality of salmon fry.⁽⁴⁷⁾ During drought, mortality rates were higher in smaller salmon fry, whereas during floods, greater mortality rates were recorded among larger fry. However, other studies suggest that salmon are relatively resilient to flood events.⁽⁴⁸⁾ In a study of New Brunswick streams, average feeding rates and long-term growth were determined to not be significantly reduced by flooding, despite temporary reductions in juvenile salmon growth in response to specific flood events.⁽⁴⁸⁾

Aquaculture is generally considered to be relatively adaptable to climate change, and is even recognized as a potential adaptation to help fisheries cope with the impacts of climate change. On a global basis, aquaculture production has been steadily increasing since 1990 and is expected to surpass capture harvests by 2030.⁽⁸⁾ Nonetheless, the aquaculture industry is concerned about how an increase in extreme events and shifts in wind patterns could affect the flushing of wastes and nutrients between farm sites and the ocean.⁽³⁷⁾ Furthermore, higher water temperatures may increase the risk of disease and compromise water quality by affecting bacteria levels, dissolved oxygen concentrations and algal blooms.⁽⁸⁾ Climate change may also affect the type of species farmed, with water temperatures becoming too warm for the culture of certain species, yet better suited for others.

The impacts of climate change on coastal wetlands could also significantly affect Atlantic fisheries, as salt marshes are an important source of organic matter for coastal fisheries and provide vital fish habitat. Researchers have found that increasing rates of sea level rise as a result of climate change could threaten many of these marshes (reference 49; see Coastal Zone chapter), with resultant consequences for fish productivity.

Arctic Coast

Future climate change is expected to impact many aspects of life in northern Canada, including fishing practices.⁽²⁾ Though not of the same economic magnitude as the fisheries of the Atlantic and Pacific coasts, Arctic fisheries are important for subsistence, sport and commercial activities, as well as for conservation values.⁽⁵⁰⁾ There is growing recognition that recent changes in climate are already impacting fish and marine mammals, and that these changes are, in turn, impacting subsistence activities and traditional ways of life. For example, there have been reports from the Northwest Territories of salmon capture outside of known species ranges, such as sockeye and pink salmon in Sachs Harbour, and coho salmon in Great Bear Lake,⁽¹¹⁾ that may be early evidence that distributions are shifting.⁽¹³⁾ In Sachs Harbour, recent warming and increased variability in spring weather have shortened the fishing season by limiting access to fishing camps, and local residents have noted changes in fish and seal availability.⁽⁵¹⁾

Some of the most significant impacts of climate change on Arctic marine ecosystems are expected to result from changes in sea-ice cover (see Coastal Zone chapter). Using satellite and/or surface-based observations, several studies have documented significant reductions in the extent of sea ice over the past three to four decades (e.g., reference 52), with up to a 9% decline in the extent of perennial sea ice per decade between 1978 and 1998.⁽⁵³⁾ Although significant decreases in the thickness of Arctic Ocean sea ice, on the order of 40% over past three decades, have also been reported,⁽⁵⁴⁾ some researchers believe that the observed decrease likely relates to sea ice dynamics and distribution, rather than a basin-wide thinning.⁽⁵⁵⁾ However, most climate models project that both the extent and thickness of sea ice will continue to decline throughout the present century,⁽⁵²⁾ eventually leading to an Arctic with only a very limited summer sea-ice cover.^{(53, 56, 57)}

Sea ice is a major control on the interactions between marine and terrestrial ecosystems, and the undersurface of sea ice is a growth site for the algae and invertebrates that sustain the marine food web.⁽⁵⁸⁾ Some studies suggest that a decrease in sea ice could threaten Arctic cod stocks because their distribution and diet are highly dependent on ice conditions.⁽⁵⁹⁾ However, a decrease in sea ice could, in the short term, increase the number and extent of highly productive polynyas (areas of recurrent open water enclosed by sea ice),⁽¹³⁾ enabling some species to benefit from an increase in food supply. Fishing practices would also be impacted by changes in the extent, thickness and predictability of sea-ice cover. Changes in sea-ice conditions would affect the length of the fishing season, the safety of using sea-ice as a hunting platform, and potentially alter the fish species available for harvesting.

Marine mammals, including polar bears, seals and whales, which contribute significantly to the subsistence diets and incomes of many northerners, are known to be sensitive to climate change. For example, polar bears are directly and indirectly affected by changes in temperature and sea-ice conditions, with populations located near the southern limit of their species distribution being especially sensitive.⁽⁶⁰⁾ For example, observed declines in bear condition and births in the western Hudson Bay region have been associated with recent warming trends, which have caused earlier ice break-up, thereby restricting access to the seals that are a critical source of nutrition for the bears.^{(60, 61)} Seals, in turn, may be affected by reduced predation,⁽⁵⁸⁾ as well as by habitat degradation or loss.⁽⁵⁹⁾

Other marine mammals would also be impacted by changes in sea-ice conditions.⁽⁵⁹⁾ Reductions in the extent of sea-ice could result in decreased amounts of sub-ice and ice-edge phytoplankton, a key source of food for the copepods and fish, such as Arctic cod, that provide nutrition for narwhal and beluga whales.⁽⁶²⁾ Conversely, a decrease in ice cover could enhance primary production in open water, and thereby increase food supply. In the winter, the risk of ice entrapment of whales may increase, whereas decreased ice cover on summer nursery grounds may increase rates of predation.⁽⁶³⁾ Finally, decreased ice cover would likely result in increased use of marine channels for shipping, which could have negative impacts on marine ecosystems as a result of increased noise and pollution.⁽⁶²⁾

Freshwater Fisheries

Canada has the world's largest freshwater system, with over 2 million lakes and rivers that cover more than 755 000 square kilometres.⁽²⁾

For freshwater fisheries, changes in water temperature, species distributions and habitat quality are the main direct impacts expected to result from climate change. As is the case with marine fisheries, it is important to recognize that the effects of non-climatic ecosystem stresses will continue to impact fisheries, making it important to understand how climate change will interact with these stressors. For freshwater fisheries, these stressors include land-use change, water withdrawals⁽⁶⁴⁾) and the introduction of non-native species.⁽⁶⁵⁾ Inland fisheries will also face additional challenges stemming from increased competition for water between sectors, as supply-demand mismatches become more common due to climate change (see Water Resources chapter).

Higher temperatures will affect different freshwater fish species in different ways. The magnitude of potential temperature changes in freshwater sites is significantly greater than that for marine environments. Fish are commonly divided into three guilds (cold, cool and warm water), based on the optimal thermal habitats around which their thermal niche is centred. A fourth guild, for Arctic fish that prefer even lower temperatures, has also been suggested.⁽¹³⁾ Both laboratory and field research support the conclusion that warm-water fish, such as sturgeon and bass, generally benefit from increased water temperatures, whereas cold-water fish like trout and salmon tend to suffer (e.g., reference 13). For instance, a 2°C increase in water temperature was found to reduce the growth rate,(66) survival(67) and reproductive success⁽⁶⁸⁾ of rainbow trout. In contrast, higher temperatures were found to increase population growth of lake sturgeon.⁽⁶⁹⁾

Climate change will also impact freshwater fisheries through its effects on water levels (reference 70, see Water Resources chapter). Lower water levels in the Great Lakes, resulting from increased evaporation and shifts in surface-water and groundwater flow patterns, would threaten shoreline wetlands that provide vital fish habitat and fish nursery grounds.⁽⁷¹⁾ In the St. Lawrence River, lower water levels would expose new substrate, and may facilitate the invasion of exotic and/or aggressive aquatic plant species.⁽⁷²⁾ Lower water levels in lakes on the Prairies have been shown to result in increased salinity, and have significant effects on aquatic organisms.⁽⁷³⁾

Shifts in seasonal ice cover^{(74, 75, 76, 77)} and extreme climate events would also be an important result of climate change. Ice cover affects lake productivity by controlling light availability and dissolved oxygen concentrations. Dissolved oxygen levels decline progressively through the ice-cover period, and can drop to levels that are lethal for fish. A decrease in duration of ice cover could therefore reduce overwinter fish mortality from winterkill.⁽⁷⁸⁾ Temperature extremes, high winds, extreme precipitation and storm events have all been shown to impact the growth, reproduction and metabolism of fish species.⁽⁷⁹⁾ Increases in the intensity or frequency of such events as a result of climate change could substantially increase fish mortality in some lakes.⁽⁷⁹⁾

Climate change is expected to alter the regions of suitable habitat for fish,⁽⁷³⁾ both within lakes and within or between drainage basins. Within many lakes, there exists a range of thermal habitats due to seasonal stratification (e.g., a warm surface layer and cooler deep waters). The timing and size of the different thermal zones are strongly influenced by climatic conditions (see Box 4), as well as by the characteristics of the lake. For example, studies have found that clear lakes are more sensitive to climate warming than lakes where light penetration is more limited.⁽⁸⁰⁾ Climate change could potentially result in earlier onset of stratification,⁽⁸¹⁾ an extended summer stratification period⁽⁷⁷⁾ and changes in the volume of each of the various layers.⁽⁷³⁾ These changes could, in turn, alter the dominant species found in a lake and potentially cause the extirpation of certain fish species.⁽⁸²⁾

BOX 4: How will lake stratification affect changing water temperatures?⁽⁸²⁾

Climate change is expected to affect both the size and temperature of the different thermal zones in lakes. Spatial and temporal shifts in thermal niche space are expected to affect the feeding patterns, productivity and reproduction of such fish as yellow perch and lake trout. The surface layer will warm in response to higher air temperatures, but there is less certainty concerning how the deeper layers would be affected. To address this issue, Hesslein et al. (2001) applied a modelling approach and concluded that the deeper layers would warm primarily through increased penetration of solar radiation due to an increase in lake clarity. Lake clarity could be altered by changes in runoff from surrounding lands resulting from changes in precipitation. Changes in lake clarity are expected to be most significant in shallow lakes. Diagram of a stratified lake.

Climate change would also result in shifts in the distribution of fish species. It has been suggested that the warming associated with a doubling of atmospheric CO2 could cause the zoogeographical boundary for freshwater fish species to move northward by 500 to 600 kilometres,⁽⁷⁰⁾ assuming that fish are able to adapt successfully. A number of factors could impede this shift, including a lack of viable migration routes and warmer waters that isolate fish in confined headwaters.⁽⁶⁵⁾ Such changes in species distribution would affect the sustainable harvests of fish in lakes and rivers.

Additional stress would be added to aquatic ecosystems by the invasion of new and exotic species. For example, it is expected that warm-water fish will migrate to regions currently occupied by cool- and cold-water fish. In the Great Lakes, exotic species are expected to continue to be introduced through ballast waters discharged from freighters.⁽⁸³⁾ As most of these species originate from warmer waters of the Ponto-Caspian region, their competitive advantage over the native cold-water species of the Great Lakes should increase, as lake waters warm in response to climate change.⁽⁷³⁾ As well as increasing fish extirpations,⁽⁷⁰⁾ the introduction of new species can also have significant effects on aquatic food webs and ecosystem functioning.⁽⁸⁴⁾

Climate change could also impact fisheries through exacerbating existing water quality problems (see Water Resources chapter). For example, although fish contamination from metals has always been a concern in the Arctic, new evidence suggests that warming may worsen the situation by enhancing the uptake of heavy metals by fish. Elevated accumulations of cadmium and lead in Arctic char have been attributed to higher fish metabolic rates, induced by higher water temperatures, and longer ice-free seasons (reference 85; see 'Human Health and Well-Being' chapter). Poor water quality can impact fisheries by displacing fish populations, causing large fish kills or rendering fish unsafe for consumption.

A large number of studies show that climatic factors, including temperature and drought, are important controls on water acidity and a wide range of biological and geochemical processes.^{(75, 86, 87, 88, 89)} For example, higher water temperatures have been shown to increase microbiological activity, which enhances the release of metals from the substrate to the water.⁽⁸⁸⁾ As fish tend to be well adapted to a certain range of environmental conditions, shifts in any of these factors could cause stress and higher mortality rates in certain fish species.