It is generally accepted that climate change, with its increasing temperatures, will have negative effects on BC salmon. DFO put out a paper in 2009 that studied more than 350 related papers, covering Korea, Japan, Russia, Alaska, BC, WA, OR and CA: http://www.npafc.org/publications/Special%20Publications/LRMP_Synthesis.pdf. Beamish and Riddell, well-known names to sport anglers, took part. Do read it as it is much more than I could summarize along with my own observations in an article of this length. Use caffeine, too.
Factors include: offshore weather patterns, decadal shifts in the Aleutians, winds, ground water discharge, iron concentration, El Ninos/La Ninas, ocean currents, temperature, ice cover, ocean migration patterns, run-timing, interspecies competition, coastal upwelling, ocean acidity, zoo- and phyto-plankton effects and then in freshwater, precipitation and form of precipitation, snow pack and snow melt, increase in ice-free periods in lakes, temperature, size and timing of freshet, composition of stream- or river-type chinook, coastal versus lengthy migration into interior rivers and so on.
Surprisingly, commercial catches have risen dramatically in Russia and climate warming is seen as a good thing into the middle of the 21st century – because ice effects in fresh and saltwater depress salmon numbers – while competition between chum and pink fry results in size and productivity differences greater than other factors right now. Similar ‘local’ variations occur, and sometimes opposite results can occur in different regions from the same stressor.
Sockeye are the first species to come back, and can be as early as April in the Hobiton River on Van Isle, May to September in the Somass, May to June in Skeena and Nass, and from early June to September in the complex, multi-component Fraser run. Sockeye are the most sensitive to temperature, and diversion from west coast to Johnstone Strait can result from a one degree temperature change. Fraser River entry is also partly triggered by water temperature. In fresh water, sockeye tolerate 20 degrees C, and then begin dying. I once stood on a balcony over-looking the Somass confluence and the bottom of the river was littered with what looked like silver bars. It was sockeye dying and little wonder, it was 42 C in the shade where we sat in Margaritaville, sweating like pigs, doing nothing.
The farther a run must go upstream, sockeye to the interior, for instance, the greater the pre-spawn mortality with respect to temperature; but greater marine fat levels brought back into the Fraser result in lower mortality, implying that several factors can affect the portion of escapement that successfully spawns. You will recall Dr. Kristi Miller’s work on the Viral Signature – meaning disease – of Fraser sockeye can result in up to 90% pre-spawn mortality.
And if there is greater ice or rain precipitation, eggs can be wiped out. And sockeye typically spend a year or so in a lake before migrating. Higher temperatures are thought to result in higher fry mortality in saltwater because they may be too small to survive the ocean. Longer periods of sunlight on saltwater are consistent with greater algal blooms, in Georgia Strait, associated with higher mortality of smolts. And when the ocean is warmer, sockeye don’t grow as well, and thus do poorer on entering rivers to migrate the distances.
The Fraser River accounts for 30- to 40-% of all BC salmon production. Because numerous stocks of sockeye, pink and chum are near the southern limit of their range, the early impacts of climate change should be detectable in these stocks first. Warm water during spawning results in earlier hatching of fry and higher fry to smolt mortality.
Chinook with their large bodies have difficulty entering coastal rivers depleted by long, hot, summers, with a lack of precipitation. The same can be said for their and coho fry surviving a long hot summer in rivers. Poor development in rivers leads to coho, sockeye and chinook doing poorer in saltwater. It is not yet clear whether the mechanism that causes ocean-weather regimes to shift will be exacerbated or muted by increasing levels of greenhouse gasses.
Coho come on the tail end of chinook runs, but tend to hang on beyond all other species, waiting for the high rain falls of late autumn and even winter before entering side-streams where they preferentially spawn. Less rain, means fewer coho, which are the second most temperature sensitive of the five species, because the side-streams become isolated pools in summer, with coho fry frizzling in summer temperatures, waiting for fall flows to escape. Most of our rivers have had their trees logged in the past century, resulting in very open, gravel moonscapes that further elevate temperature. Go look at the San Juan for such a devastated river. On the other hand, walk the easy trail and pretty Big Qualicum with its forest cover intact and cooler summer flow. Vastly different productivity.
Where it rains is important, too. I have stood on a gravel bar extending from a side-stream, measuring 25 by 20 by 12 feet deep. Four days later, because the main-stem had received much rain, but not the tributary’s watershed, the entire tongue had been blown out and the water was 12 feet deep. The higher and more concentrated the rain, the greater the problem.
Chum are notoriously poor at spawning in good locations. They tend to spawn on the highest rains of fall and when the river drops, up to 90% of eggs are wasted. So, less, rather than more rain, would conserve them, forcing them to spawn within the river’s usual banks.
It has also been shown that BC populations north of 50- to 55-degrees latitude oscillate in ocean numbers differently from those of more southerly rivers. Warmer weather allows cold water predators like hake and mackerel to move north and eat more salmon. Predation is a big problem, even in Alaska where 75% of Prince William Sound pink fry are lost to predation during their first 45- to 60-days in the ocean.
Temperature differences in the north east Pacific play a role in sorting out different salmon species to different areas. Along with higher temperatures above the water, currents flowing north to south split the near shore and off shore regions and also distribute salmon for foraging. Higher temperatures will influence marine distribution.
Salmon also stray from their own rivers by as much as 10%. Pink and sockeye are now reported in the Beaufort Sea. Chinook are the least temperature-related strayers. I have witnessed pinks and sockeye in rivers where there are no historical runs. Oddly, I have noticed sockeye – because they are so easily identified as red fish before coho turn – spawning in the same patch of gravel in succeeding years, even though the river has no identified sockeye run.
There has been a decline in hatchery coho in Georgia Strait since the mid-‘80s, while wild coho have remained stable but at low levels. Growth, survival and abundance occur earlier in the year for wild coho than hatchery coho. Growth between July and September is inversely related to marine survival, indicating that faster and earlier growth may improve lipid storage, increasing the chances of survival over the winter. This study suggests that fish farms have caused a 50% decline in salmon numbers: http://fishfarmnews.blogspot.ca/2013/01/fish-farms-kill-more-than-50-of-wild.html.