Marine

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Changes in the physical environment of marine ecosystems

Sea temperature, salinity, wind patterns, and ocean circulation have significant impacts on marine biodiversity. For example, zooplankton community composition and several fish trends are correlated with largescale climate signals in the Pacific Ocean, including the El Niño Southern Oscillation and the Pacific Decadal Oscillation.5

Sea temperature, Newfoundland and Labrador Shelves
Mean annual temperature
Graph: sea temperature in the Newfoundland and Labrador Shelves. Click for graphic description (new window).
Long Description for Sea temperature, Newfoundland and Labrador Shelves

This graph displays sea temperatures around Newfoundland (Bonavista) and Labrador (Seal Island) from 1950 to 2005. The water temperatures at both sites were very similar, fluctuating around 2 degrees Celsius, with no apparent trend up to 1990. Both locations experienced peaks of approximately 3 degrees Celsius in 1965, and lows of approximately 1 degree Celsius in 1984 and 1990. Since 1990, the temperature at both locations has been increasing steadily, reaching nearly 3 degrees Celsius in 2005.

 

Source: adapted Oceans Canada (DFO), 20077
Sea temperature, Pacific Coast
Mean annual temperature ºC, up to 2006
Three graphs: sea temperature on the Pacific Coast. Click for graphic description (new window).
Long Description for Sea temperature, Pacific Coast

This series of graphs shows the mean annual temperature for three locations on the Pacific Coast of British Columbia. On each graph, annual values are plotted and a horizontal line shows the average sea temperature for the reference period 1961 to 1991. At the first location, Langara Island (North Coast/Hecate Strait), the average sea temperature was just below 9 degrees Celsius. At the second location, Amphitrite Point (West Coast of Vancouver Island), the average sea temperature was 10.5 degrees Celsius. At the third location, Departure Bay (Strait of Georgia), the average sea temperature was 11.4 degrees Celsius. At all three locations, temperatures in most years before 1978 were below the average (with an exception of warmer temperatures grouped around 1940), while most years after 1978 have had warmer than average sea temperatures.

 

Note: the horizontal line represents the average temperature for the reference period, 1961 to 1991.
Source: adapted from Fisheries and Oceans Canada (DFO), 20105

Mean sea surface temperature has increased:5

  • from 1978 to 2006 in the North Coast and Hecate Strait and West Coast Vancouver Island, following a period of colder surface water in the previous 25 years, although 2007 and 2008 were cooler than average;6
  • since the 1970s in the Beaufort Sea;
  • since the late 1970s in the Canadian Arctic Archipelago and in the Hudson Bay, James Bay, and Foxe Basin;
  • since the early 1990s in the Newfoundland and Labrador Shelves;
  • since the 1980s in the Estuary and Gulf of St. Lawrence.

The ocean has become fresher (less saline)5 in several ecozones+:

  • since 1978 in the North Coast and Hecate Strait, following a 30-year period of high salinity;
  • since the 1970s in the Beaufort Sea, as a result of melting sea ice, input from the Pacific Ocean, and surface water from the Arctic Ocean.

Ocean acidification

Photo: mussels © iStock.com/eddyfish

When carbon dioxide dissolves in the ocean, it lowers the pH, making the ocean more acidic.8 Since pre-industrial times, the oceans have become more acidic by a pH of approximately 0.1. This seems like a small amount – but the biological effects of small changes in ocean acidity can be severe. For example, a pH change of 0.45 from pre-industrial times, which is predicted by the end of this century, could have dire consequences for marine organisms that build a calcium carbonate skeleton or shell, such as corals, molluscs (oysters, mussels, scallops), crustaceans (crabs, shrimp), echinoderms (starfish, and many species of plankton.9 Impacts are expected to occur first in the polar regions.10

Ocean acidification is already occurring in four marine ecozones+: West Coast Vancouver Island, Beaufort Sea, Estuary and Gulf of St. Lawrence, and Gulf of Maine and Scotian Shelf. It is predicted to occur in all oceans and to have severe consequences for biodiversity as early as the end of this century.5

Oxygen depletion in marine waters

Dissolved oxygen in the St. Lawrence Estuary
Percentage, 1930 to 2008
Graph: dissolved oxygen in the St. Lawerence Estuary. Click for graphic description (new window).
Long Description for Dissolved oxygen in the St. Lawrence Estuary

This graph displays the percentage saturation of dissolved oxygen in the St. Lawrence Estuary from 1930 to 2008. A horizontal line at 30% marks the critical oxygen concentration threshold. Samples in the earlier part of the period of record were only occasionally below this level, but samples from the early 1980s to the late 2000s were all well below this threshold, with dissolved oxygen levels being stable at around 20% saturation since 2000. There are no data points from 1936 to 1959 and from the mid 1970s to 1981.

 

Source: adapted from Dufour et al., 201014
 

Critically low oxygen concentrations have been observed at some sampling points in the Estuary and Gulf of St. Lawrence and the three ecozones+ in the Pacific. In the St. Lawrence Estuary, low oxygen conditions have been observed since 1984.5 Declines in oxygen concentration are caused by a number of factors, including changes in ocean circulation patterns, freshwater inputs, rising temperatures, and increases in organic matter on the sea floor. The latter may be caused by increases in primary production on the surface and by human activities.11

Observed effects of low oxygen content on biodiversity in Canadian waters include declines and mortality of bottomdwelling animals and altered food webs.5 Some impacts observed globally include fish and crab kills,12 more prevalent jellyfish blooms,13 changes in marine biochemical pathways that favour some species over others,11 creation of dispersal barriers for larval fish and crustaceans that are less tolerant of low oxygen than adults,11 and altered food webs.11

Globe

Global Trends

Low-oxygen zones where ocean species cannot live have increased globally by close to 5.2 million km2 since the 1960s.11
 

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Marine food webs

Photo: great blue heron © iStock.com/BirdImages

Plankton are passively drifting plants and animals that move on ocean currents. Some species can reach very high densities (up to 20 million cells per litre, over very large areas (thousands of square kilometres), and their "blooms" can be captured by satellite. Planktonic plants, bacteria, and algae (phytoplankton) are the foundation of the marine food web. Planktonic animals (zooplankton) provide a key link between the phytoplankton, that they eat, and the fish, seabirds, and other marine species that eat them.2

 

Seasonal change in zooplankton bloom, Strait of Georgia

Date of peak bloom
Graph: change in zooplankton bloom, Strait of Georgia. Click for graphic description (new window).
Long Description for Seasonal change in zooplankton bloom, Strait of Georgia

This graph shows the date of peak bloom of zooplankton in the Strait of Georgia. Points representing annual dates of the blooms are plotted and two trend lines are shown for an earlier and a later set of years. First, from 1968 to 1996, the date of peak bloom advanced from approximately May 13th to April 24th. The values over these years were determined by back-calculation. Second, from 2002 to 2005, the date of peak bloom advanced steadily and at a more rapid rate than during the earlier time period, changing from approximately April 7th to March 10th. The values over these later years were determined by direct observation.

 

Source: adapted from Fisheries and Oceans Canada (DFO), 20105

The timing and duration of the peak zooplankton bloom has changed over the past 40 years in all Pacific and Atlantic marine ecozones+. For example, the peak abundance of Neocalanus, the dominant zooplankton species in the Strait of Georgia, occurs approximately 50 days early in the 2000s compared to the 1960s to 1970s. This has created a mismatch in timing between small fish and their zooplankton prey. Juvenile salmon that enter the Strait early in the season, such as chum, pink, and sockeye, have benefitted, while species that arrive later in the season, such as chinook and coho, have declined.15Neocalanus has also Photo: Saanich Inlet herring, predator of zooplankton © VENUS at UVicdeclined sharply since 2001 and the decline in abundance may be accelerating and affecting species that depend on it for food.15

Spring phytoplankton blooms start earlier, are more intense, and last longer on the Scotian Shelf than they did in the 1960s and 1970s.16

 

Decline in krill in the western North Atlantic and Scotian Shelf

Mean number of krill (log(x+1)) per 3 m3 of filtered sea water, 1961 to 2008
Graph: Decline in krill in the western north Atlantic and Scotian Shelf. Click for graphic description (new window). Photo: krill © VENUS at UVic.
Long Description for Decline in krill in the western North Atlantic and Scotian Shelf

This graph shows an overall declining trend in the mean number of krill (log x+1) per 3 cubic metres of filtered seawater from in the western North Atlantic and Scotian Shelf from 1961 to 2008, with change mainly occurring in recent years. In 1961 the mean number of krill was 6.3. Annual values fluctuated until 1978, with a peak in 1975 of 11.6 krill, and low points of about 4 krill in two years during this period. Overall, there was no clear trend in krill abundance from 1961 to 1978. No data are available from 1979 through 1990. From 1991 to 2008 the mean number of krill declined sharply and steadily from about 6 to a value of below 1 in 2008.

 

Note: no data are available for 1979 to 1990.
Source: adapted from Johns, 201017

 

Several zooplankton species that are considered to have a key role in the marine food web are declining. Euphausiids, or krill, in the western North Atlantic and Scotian Shelf, feed on phytoplankton in their youngest stages and are preyed upon by juvenile groundfish, pelagic fish, and baleen whales. Their abundance has declined between the 1960s to 1970s and the 1990s to 2008.18
 

Population trends for northern shrimp and four of their predators

Population measures specific to each species, 1976 to 2000
Compound graphic: population trends for northern shrimp and their predators. Click for graphic description (new window).
Long Description for Population trends for northern shrimp and four of their predators

These four graphs show the population of northern shrimp from 1976 to 2000, compared against four of its predators. The shrimp population (measured as an index of catch per unit effort, abbreviated as CPUE) decreased over the first ten years, from approximately 0.5 to 0.2 CPUE in the period from 1976 to 1985. After 1985 the population increased, reaching a high of 1.0 CPUE in 2000, the last year for which data are presented. The datasets for the four predators are all based on research surveys. Three of the northern shrimp’s predators (all fish species) declined, while snow crab, an invertebrate, increased with increasing shrimp populations.

Each graph is described in the following set of points:

  1. Redfish abundance fluctuated widely until 1985, with peaks of over 2 tonnes and lows of about 0.6 tonnes. After 1985 redfish abundance dropped to below 0.5 tonnes and, from 1992 to 2000, declined to annual measurements of zero to slightly above zero tonnes. The graph includes the shrimp population trend, showing that shrimp abundance increased rapidly, coinciding with the period of sharp declines of redfish.
  2. Atlantic cod abundance fluctuated between 1 and 2.5 tonnes from 1983 to 1991, then rapidly declined to around zero tonnes in annual surveys from 1994 to 2000. The graph includes the shrimp population trend, showing that shrimp abundance increased rapidly, coinciding with the period of sharp declines of Atlantic cod.
  3. Skate abundance increased from 1981 to 1984, reaching a peak of nearly 15 kilograms per tow. The population then declined over the next 10 years to 2 kilograms per tow in 1994, remaining close to this value from 1994 to 2000. The graph includes the shrimp population trend, showing that shrimp abundance increased rapidly, coinciding with the period of sharp declines of skate.
  4. Snow crab abundance fluctuated over the period of measurement, with overall higher values since 1990. Prior to that time, annual surveys resulted in measurements varying between about 3 and 12 kilograms per trap. After 1990, measurements were between 12 and 15 kilograms per trap. The graph includes the shrimp population trend, showing that snow crabs did not undergo a decline as did the fish predators of shrimp.

 

Note: Measures are: catch per unit effort (CPUE) for shrimp, millions tonnes for cod and redfish, kilograms per tow for skate and snow crab.
Source: adapted from Fisheries and Oceans Canada (DFO), 20105

In the Newfoundland and Labrador Shelves Ecozone+, in the 1990s, a decrease in groundfish abundance was accompanied by a dramatic increase in invertebrates such as shrimp and crab. A combination of several factors has potentially led to these changes in the marine food web, including overfishing of groundfish, change in water temperatures, and decreased predation on the invertebrates. In response, the commercial fishery has shifted from groundfish to species lower on the food web, such as shrimp, snow crabs, and, more recently, sea cucumber, whelk, and hagfish. The shift from a higher to a lower trophic level fishery is a worldwide phenomenon often referred to as “fishing down the food chain”.5

An equivalent shift in ecosystem structure occurred in the Gulf of Maine and Scotian Shelf, and the Estuary and Gulf of St. Lawrence ecozones+ between 1985 and 1990. The shift is reflected in decreases in groundfish and zooplankton and concurrent increases in seals, small pelagic fish, and invertebrates. A moratorium on the commercial groundfish fishery was implemented in the Gulf of Maine and Scotian Shelf in 1993, with only limited recovery of some groundfish species.5

Diet of thick-billed murre at Coats and Digges islands

Graph: thick-billed murre's diet at Coats and Digges islands. Click for graphic description (new window).
Long Description for Diet of thick-billed murre at Coats and Digges islands

This graph shows the percent of the diet of thick-billed murres that was composed of Arctic cod and of capelin over the period 1981 to 2007. The diet composition fluctuated from year to year, with Arctic cod gradually declining in importance in the diet and capelin gradually increasing. From 1981 to 1990, Arctic cod consistently made up the majority of the diet. By 1997, capelin had replaced Arctic cod as the main component in the diet of thick-billed murres. An inset map shows the location of Coats and Digges Islands north of Hudson Bay.

 

Source: adapted from Gaston et al., 200919


Hudson Bay and James Bay, the small Arctic cod is recognized as a keystone species that plays a central role in food web dynamics. Arctic cod is important in the diet of seabirds and marine mammals such as ringed seals and belugas, although it does not appear to be the sole food of any one species.20 Arctic cod can be extremely abundant – densities of 11 kg cod per square metre were recorded in ice-covered Franklin Bay in the Beaufort Sea.21

The major food of thick-billed murre nestlings at Coats and Digges islands shifted from Arctic cod to capelin in the mid-1990s. The shift reflects a change in the relative abundance of Arctic cod and capelin. Photo: thick-billed murre © Garry DonaldsonAs the extent and duration of sea ice declines, the abundance of Arctic cod, which is a sea-ice associated species, is declining, while capelin, which prefers warmer waters, is increasing.19 In contrast to Hudson Bay, capelin is decreasing as a proportion of the diet for murres in the Newfoundland and Labrador Shelves,19 where capelin abundance and size has declined.22

Globe

Global Trends

Over the past 50 years there has been a decline in size, a change in species composition, and earlier onset of phytoplankton blooms worldwide.2
 

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Marine mammals

Marine mammals may play a role in structuring marine ecosystems as top predators (for example, killer whales, belugas), fish-eaters (for example, sea lions, seals), or bottom feeders (for example, sea otters, bowhead whales, gray whales). However, the effects of marine mammals on the functioning of marine ecosystems are poorly understood. Some marine mammals, such as sea otters, are known to be keystone species because their removal results in a significant ecosystem shift. Sea otters feed on sea urchins, which, in the absence of predation by sea otters, overgraze kelp.

Photo: killer whales, west coast Vancouver Island, B.C. © John Ford, Fisheries and Oceans CanadaSeveral marine mammal populations are recovering from past overharvesting including grey seals in the Scotian Shelf and Gulf of St. Lawrence,23 harp seals in the Gulf of Maine and Scotian Shelf,24 western Arctic bowhead whales in the Beaufort Sea,25 the B.C./Alaska sea lions,26 sea otters,5 and the Pacific harbour seal.27 Resident killer whale populations off the coasts of Vancouver Island have also recovered from previous commercial overexploitation but have begun to decline in recent years, possibly related to declines in chinook salmon, an important food source.28

Map and graphs: marine mammal population trends at various locations in Canada. Click for graphic description (new window).
Long Description for Marine mammals

This graphic shows the populations of seven species of marine mammals in locations along Canada’s coasts over a range of time periods. A separate line graph is shown for each of the seven locations, which are indicated on a central locator map. Overall, the graphs show increasing populations of marine mammals.

Each graph is described in the following set of points:

  1. Bowhead whales in the Beaufort Sea, 1978 to 2001. At the beginning of this period, the population was around 4,000 whales. Since then, the population generally increased, with a count of more than 10,500 whales in 2001.
  2. Harp seal populations in the Gulf of St. Lawrence from 1952 to 2009. Populations declined slowly from 2.7 million seals in 1954 to 1.9 million seals in 1970. From 1970 to 2009 the population increased, reaching 6.9 million in 2009.
  3. Sable Island grey seal pups from 1962 to 1997. In 1962 there was a low of fewer than 1000 pups; however, the number of pups increased exponentially over the study period, reaching more than 25,000 pups by 1997.
  4. Resident killer whale populations of northern and southern B.C. The northern population (north of central Vancouver Island) increased from 120 whales in 1974 to 220 whales by 2004. The southern population (southern B.C. coast, including Georgia Strait and Puget Sound) did not show the same increase, fluctuating in the range of 70 to 100 whales, with no clear overall trend, from 1974 to 2008.
  5. Sea otters off Vancouver Island from 1977 to 2008. Numbers of otters increased from 100 in 1977 to more than 2,700 otters in 2008.
  6. Steller sea lion population off the central coast of B.C. from 1913 to 2005. The population declined from 13,000 in 1913 to 7,000 in 1961. After 1961, the population began to increase, reaching 28,000 by 2005.
  7. Harbour seal population off Haida Gwaii, B.C. The historical reconstruction of harbour seal populations shows a cycling population with a high of 80,000 seals in 1890 and lows of approximately 10,000 seals in 1917 and 1970. Population estimates from the mid 1980s to 2009 are based on surveys. Harbour seals increased in abundance after about 1970, with the trend showing signs of leveling off in the last decade, with approximately 100,000 seals in 2009.

 

Source: adapted from Fisheries and Oceans Canada (DFO), 2010.5 Primary references noted in the text.

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Marine fisheries

Declines in several fish stocks have occurred in the Atlantic and Pacific oceans as well as in the Hudson Bay, James Bay, and Foxe Basin, as a result of overexploitation in combination with other stressors, such as increased temperature, decreased salinity, and increased acidity. Declining stocks include groundfish, such as Atlantic and Pacific cod, lingcod and rockfish, pelagic fish such as herring and capelin, and anadromous fish such as coho, chinook salmon, Atlantic salmon, and Arctic char.5 Management measures designed to reverse longterm fisheries declines have been largely unsuccessful. Depending on the fishery, Photo: netted fish © iStock.com/Irishka1rebounds have been hampered by large-scale oceanographic regime shifts, loss of spawning and rearing habitat, and contaminants.5

Not all fisheries are in decline. For example, turbot, sablefish, and Pacific sardine are all increasing in the West Coast Vancouver Island Ecozone+ and pink and chum salmon are increasing in the Strait of Georgia.5

Map and graphs: population trends of marine fisheries at various locations in Canada. Click for graphic description (new window).
Long Description for Marine Fisheries

This graphic shows the populations of five species of fish in locations along Canada’s east and west coasts over a range of time periods. A separate line graph is shown for each of the five locations, which are indicated on a central locator map. Overall, the graphs show strongly declining fish populations.

Each graph is described in the following set of points:

  1. Spawning biomass of “Grand Bank” Atlantic cod from 1960 to about 2008. The population showed large variations, including a high of more than 120,000 tonnes in 1965 and a low of 10,000 tonnes in 1957, increasing again to 80,000 tonnes in the early 1980s. By the early 1990s biomass values below 10,000 tonnes were consistently recorded.
  2. Atlantic salmon returning to a Bay of Fundy river from the early 1960s to 2002. There was a peak of approximately 5,000 salmon returning in 1965. Following the 1960s, numbers were mainly in the range of 1,000 to 2,000 until the early 1990s, when they decreased and leveled off at approximately 100 salmon.
  3. Percent survival of wild coho in the Strait of Georgia from 1986 to 2006. Survival rates were about 12% in the first two years, rose to 18% in 1988, and then declined fairly steadily to 2% in 2006.
  4. Sockeye returning to Barkley Sound from 1970 to 2006. The number of sockeye fluctuated over a range from lows of approximately 300,000 to peaks of up to 1,800,000 sockeye, with below-average peak-year returns from the mid 1990s to 2006.
  5. Pre-fishery biomass of herring on the central coast of B.C., from 1950 to 2009. Biomass of herring fluctuated from year to year, generally within a range of about 30,000 to 60,000 tonnes, with peaks as high as almost 80,000 tonnes (1950) and an outlying low value of less than 20,000 tonnes in the late 1960s. In the last decade, however, the pattern changed, with biomass declining steadily to less than 10,000 tonnes in 2009.

 

Source: adapted from Fisheries and Oceans Canada (DFO), 2010,5 Johannessen and McCarter, 2010,15 and Worcester and Parker, 201016

Fish length at age 5, Scotian Shelf

cm, 1970 to 2002
Graph: fish length at age 5, Scotian Shelf. Click for graphic description (new window).
Long Description for Fish length at age 5, Scotian Shelf

This graph shows the change in length of four species of fish at age 5 years from 1970 to 2002. Annual average lengths are plotted and trend lines are shown for each fish species. All species – cod, haddock, pollock, and silver hake – had decreasing trends over this period. The average length of pollock at age five decreased from approximately 67 centimetres in 1970 to approximately 51 centimetres in 2000. Cod average length decreased from approximately 57 centimetres in 1970 to approximately 46 centimetres in 2000. Haddock average length decreased from approximately 67 centimetres in 1970 to approximately 51 centimetres in 2000. Silver hake average length decreased from approximately 39 centimetres in 1970 to approximately 32 centimetres in 2000.

 

Source: adapted from Fisheries and Oceans Canada (DFO), 20105

 

Size of fish is an important determinant of reproductive success. Since the 1970s, several species have been getting smaller, including Pacific herring in the Strait of Georgia and five species of groundfish in the Scotian Shelf. Smaller size is implicated as a factor hampering recovery of some fisheries.5

 

 

Globe

Global Trends

Over 30% of fish stocks are over-exploited, fully exploited, or depleted.29
 

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