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Research Document - 2010/010

Ecosystem Status and Trends Report for the Strait of Georgia Ecozone

By S.C. Johannessen and B. McCarter

Abstract

The Strait of Georgia is a semi-enclosed sea located between Vancouver Island and mainland British Columbia. Water circulation in the Strait is dominated by estuarine exchange (out at the surface, in at depth) and by tidal and wind mixing. The Strait is highly productive, supporting commercial, aboriginal and recreational fisheries. It is also surrounded by a growing urban population, which is putting pressure on the ecosystem of the Strait. Global climate change acts locally through changes in seawater and river temperature, in the oxygen concentration and pH of inflowing deep water and in the timing of river discharge. Other changes have resulted from local human activities, such as shipping, fishing, discharge of contaminants and habitat destruction, including the construction of hard edges, which will interact with sea level rise. The ecosystem has shown resilience in the past, having recovered from numerous stressors and climatic variations. However, the combination of accelerating climate change with urbanization and fishing pressure is new. We do not know how the ecosystem will respond to the cumulative effects of human- and climate-driven changes in the future.

The Strait of Georgia is warming at all depths (1970-2006), while the concentration of oxygen in the deep water is decreasing. The Fraser River's summer temperature has increased (1942-2006), while its summer flow has decreased, resulting in increased pre-spawning mortality of Pacific salmon.Zooplankton abundance is decreasing in the Strait of Georgia, and the maximum biomass is peaking as much as 50 days earlier now than in the 1970s. These changes threaten the survival of some marine birds, and may threaten late-migrating juvenile salmon, but a directly attributable effect on planktivorous fishes has not been demonstrated. The populations of several piscivorous fishes (coho and Chinook salmon, ling cod, Pacific cod and inshore rockfish) have declined (1986-2006), while those of predominantly planktivorous fishes (chum and sockeye salmon, Pacific hake, spiny dogfish, walleye pollock) are relatively stable or within the normal range of historical variability (1981-2006). Resident killer whales are threatened because of contaminants, traffic and declining availability of prey (primarily Chinook salmon). The populations of all the pods were stable or increasing locally until the mid-1990s, when they all began to decline simultaneously, closely correlated with a coast-wide decline in the population of Chinook salmon. Since 2001 the killer whale population has increased again, though more gradually than it declined.

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