Salish Sea Marine Survival Project

28 Water temperature can also affect dissolved oxygen but in all years surface waters were well oxygenated (> 6 ppm dissolved oxygen). However, oxygen levels were variable between years and at depth. Dissolved oxygen concen- trations less than about 6 ppm concentrations decreased growth/increased stress in juvenile salmon. Based on this level, large portions of the deeper water column in the Strait were not good habitat for juvenile salmon for much of the year for all 3 years. The most notable year was 2015 with deep-water oxygen levels approaching a hypoxic limit at 100 m depth around Lund in September and October, and with the critical 6 ppm contour as shallow as 10m in Powell River and Lund at the end of the summer in 2015. Levels of dissolved oxygen in the water column below which most animals will die occurs at concentra- tions of about 2ppm. While all measurements were above this level for all 3 years, it is known that deeper waters in the Strait have dropped below this hypoxic limit at times. Given that juvenile Chinook appear to have very defined rearing areas within the Strait, relating regional environ- mental conditions to growth and survival of the various stocks caught will be illuminating. These data are also being utilized by the Strategic Salmon Health Initiative to understand whether there is a relationship between levels of stress and expression of disease. IMPACTS OF WARMING CONDITIONS IN THE STRAIT Warmer water temperatures and increased stratification, both in the North Pacific Ocean and in the Strait, are generally predicted to result in more frequent phytoplankton blooms, more frequent and severe harmful algae blooms and a longer season of blooms. In addition, ocean acidification, another effect of climate change, may increase the toxicity of some harmful algal blooms. Phytoplankton dynamics in the Strait during 2015 were quite unusual, with mechanically harmful algal species present in abundance. Another unusual observation was a lack of toxic Heterosigma blooms over 2015-2017, a species that frequently shows up in the Strait and can kill fish. In 2016, there was an exceptional phytoplankton bloom which turned the water in many areas of the Strait a beautiful tropical turquoise colour (Figure 4). This cocco- lithophore bloom was not toxic. Reduced productivity as a result of reduced upwelling is another prediction associated with warming waters. Evidence of this was seen in the Strait of Georgia with nutrients (nitrate and phosphate) trending downwards over 2015 to 2017, with particularly reduced levels of nitrate in 2017 (nitrate is essential for phytoplankton growth). Other impacts in the Strait included above-average biomass for most zooplankton groups for 2015-2017, which would be good for forage fish and salmon. However, abundances of gelatinous zooplankton, not good fish food, also increased.. The very warm condi- tions over the winters of 2014-2016 at mid- and bottom depths in the Strait of Georgia had potential negative implications for some important zooplankton. The large, lipid-rich copepod, Neocalanus plumchrus, overwinters in the deep Strait of Georgia and has experienced periodic population collapse when deep water winter warming occurs. Early indications in the SSMSP are that Neocalanus biomass is now well below average in the Strait of Georgia. This species is considered an important food resource for young of the year fish in the Strait. Figure 4. Unusual coccolithophore bloom in the Strait of Georgia. August 19 2016. Figure from NASA. Figure 3. Maximum surface temperatures in the Strait of Georgia July 2015. Data from PSF Citizen Science Program.

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