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Canadian Science Advisory Secretariat
Research Document - 2009/022
State of physical, biological, and selected fishery resources of Pacific Canadian marine ecosystems
By W.R. Crawford and J.R. Irvine
Abstract
Despite continuing increases in overall global water temperatures, the waters off the Pacific coast of Canada were the coldest in 50 years, and the cooling extended far into the Pacific Ocean and south along the American coast. Near-shore temperatures dropped as well, as did temperature in deep waters of the Strait of Georgia. Only the surface temperatures in the Strait of Georgia remained at or above normal. This cooling is associated with weather patterns typical of La Niña and of the local cold phase of the Pacific Decadal Oscillation (PDO).
Surface phytoplankton and zooplankton concentrations were the highest in a decade of observations across the Gulf of Alaska in August and September 2008. The cause is as-yet uncertain, but injection of iron by winds or currents is suspected (Iron is a limiting nutrient in this region), along with higher levels of nitrate and silicate in spring. Ship-based sampling for phytoplankton in Juan de Fuca Strait revealed high near-surface concentrations in early September. Deep-sea and coastal zooplankton populations continued their recent shift to cold-water species and delayed spring blooms.
In the Gulf of Alaska, the ocean mixed-layer depth was relatively deep in early 2008, and surface oxygen concentrations were relatively high in early 2009. However, oxygen concentrations have generally declined in deep waters along the continental slope over the past several decades. A sudden decline in bottom-water oxygen concentration in 2008 on the continental shelf was likely due to denser water with naturally low oxygen levels moving up onto the shelf in this year, rather due to anomalous winds and currents. This oxygen drop may have been a factor in the movement of some groundfish species to shallower depths in 2008.
Cool marine conditions generally improve marine survival for salmon. However, despite relatively cool ocean conditions in 2007 and 2008, many BC populations remain depressed due to low numbers of brood-year spawners, partially attributed to warm oceans in 2003 to 2005. Sockeye returns remain generally low coast-wide, with one notable exception being Okanagan sockeye that returned in record numbers in 2008. High pre-spawn mortality was observed for many Fraser River watershed sockeye populations in 2008, and river entry of returning adults was generally early. Coho populations in southern BC remain extremely depressed, while northern coho populations have improved. For chinook, the situation is somewhat reversed – northern populations continue to decline while the status of southern chinook is highly variable.
Classification of salmon marine survival expectations based on a “weight of indicators” approach continues to show promise. In general, survivals of coho and sockeye that went to sea in 2008 are predicted to be at average- to above-average levels, meaning improved coho returns in 2009, and sockeye in 2010, relative to brood year strengths. One possible exception is Strait of Georgia coho.
Herring biomass has declined recently for all five major BC stocks. In the Georgia Basin where herring biomass was at record high levels earlier this century, the biomass declined almost to the fishery-closure limit in 2008. Three other Canadian herring stocks were at or below the fishing limit. Eulachon populations remain depressed. Although there was no widescale hake survey in 2008, their numbers on the BC continental shelf, particularly on the traditional fishing grounds around La Pérouse bank, appear to have been very low, continuing a trend that began developing around 2003-04. Smooth pink shrimp and English sole along the west coast of Vancouver Island increased in numbers in 2008.
For many of our fish species including salmon, Pacific Ocean conditions have been improving since the extremely poor year of 2005. Cool water generated bottom-up changes to the food web that have contributed to improving marine survival for many juvenile fish. Linkages between ocean conditions and fish survival are not completely understood and additional exploration of existing data is warranted.
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(129 pages; 6942K)
