Key finding overview
KEY FINDING 6. Observed changes in marine biodiversity over the past 50 years have been driven by a combination of physical factors and human activities, such as oceanographic and climate variability, and overexploitation. While certain marine mammals have recovered from past overharvesting, many commercial fisheries have not.
This key finding is divided into five sections:
- Key finding overview (this page)
- Changes in the physical environment of marine ecosystems
- Marine food webs
- Marine mammals
- Marine fisheries
The global marine ecosystem dynamic covers over 70% of the Earth's surface. It is a complex system, in constant motion, moving not only nutrients, dissolved oxygen, carbon, and water masses, but also bacteria, algae, plants, and animals, among regions. The millions of species estimated to live in the ocean dwell in a wide range of habitats, including the open ocean, sea floor, sea ice ridges, hydrothermal vents, cold seeps, coral and sponge communities, seamounts, ocean trenches, and continental shelves.1
Marine biodiversity is the foundation of the countless ecosystem services provided by the oceans. Marine plankton plays a major role in the global carbon cycle, and harvest of marine species provides an estimated $21 trillion per year in socioeconomic benefits to the world.2 Marine biodiversity is essential for the functioning of marine ecosystems, their ability to persist under stress, their ability to recover from disturbances, and their ability to provide benefits to people.3 With jurisdiction over 6.5 million km2 of marine waters in three oceans,4 Canada reaps immense benefits from the ocean.
Tidepool, Tofino, B.C.
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
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.
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
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
Marine food webs
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
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.15 Neocalanus has also declined 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
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
Diet of thick-billed murre at Coats and Digges islands
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. As 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
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.
Several 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
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, rebounds 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
Fish length at age 5, Scotian Shelf
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
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