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I'll defer any comments to Bob on this topic, but just as an intrigued casual observer of this I seem to recall that a noticeable drop in upwelling appears to be associated with el nino patterns. It is interesting to note the surface water temps when plotted across the equatorial Pacific (Australia to Ecuador-Peru) increase when the associated currents appear to reverse as the surface water temps increase, slam back into the South America and spread out both north and south along the landmass of the continents that prevents the currents from continuing in a east to west fashion. The end of the el nino usually has an affect on the upwelling off the Farralon Islands, which in turn brings about a smorgasbord for aquatic species as noted at the end of the last el nino by the local news reporting agencies. Without the upwelling of nutrient rich cold water, it seems that the food chain just collapses. Wouldn't migratory species also avoid dead zones in the ocean as they make their way to and fro? With the warmer water temps along the landmass, we also have sub tropical species enter the area as well. I can recall during one of the last el ninos when water temps were round 70 degrees off of Half Moon Bay compared to the average water temp runs around 54. During el nino years or sometime when there were low returns of salmon in the area where and when my father fished commercially, sometimes the salmon were void of any baitfish in their stomachs. I always thought this was odd, but never really paid much attention to it as a kid.
I am not associating the upwelling or lack there of to global warming, but the reported el nino effects. What I also found interesting was as a drought fear continued and the water management shifted away from other primary concerns, water temps below dams increased in the rivers as well. Last shad season, shad were hard pressed to enter rivers under discussion as well. The American River in particular had a low flow back in June 07 that was increased because smolts still in the river were affected by the higher than normal water temps. Al Smatski, whom we had fished with over a course of a couple of days, said the powers that be, either F&G biologists or others, had concerns because the scales of smolts were falling off. It struck me as odd at the time; from a curious stand point only, how this problem was being dealt with by numerous agencies as a water management issue to cooperate to drop the river's water temperature. Sure enough, over night flows from Folsom were increased after it was determined that there were sufficient acre-feet behind the dam to sustain the flows if increased. Not only did the water temps drop to safer levels, the shad moved upstream over night as well from their holding areas the day before.
Bob's point about the return levels on the A, it would be interesting know what the numbers/percentages were by the time they made it out of the Golden Gate area on their migration for the corresponding year.
It will be interesting to see how this plays out in the future.
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