Sunday, 23 March 2014

Volcano Sockeye – Oct 7, 2010 - Updated, Mar 23, 2014

Volcano Sockeye – Oct 7, 2010

A volcano blows and Fraser sockeye come back in record numbers. Is there a connection? There could well be. Roberta Hamme, at UVic, and colleagues have just had a new paper out in the Geophysical Research Letters that studies the issue. In 2008, volcano, Kasatochi, in the Alaskan Aleutian Islands blew its top and ash drifted out over a large area of the North Pacific Ocean.

Within a few days the largest bloom of phytoplankton ever observed spread across more than 1000 km of surface water. The connection with sockeye is that they eat plankton. Their food is stimulated by the addition of iron, in this case from the volcanic ash, and plankton begin fixing carbon dioxide from the air and growing and doubling in rapid order.

Of great interest is that the iron only stays in the top layer of water for a few weeks before it starts settling out lower, and plankton levels begin falling. At this stage it is not known whether iron can once again be lifted by strong Aleutian winds blowing the surface water aside, resulting in upwelling, or whether deep currents can carry the vital metal to other areas. But the plumes of plankton photographed from space are convincing. Compared with 2007, when Fraser sockeye were very low in numbers and plankton levels were visibly very low, the 2008 satellite shots show a massive bloom of sockeye food.

And fortunate for the fish and everything else that depends on Fraser sockeye, the ash came in summer. This period is when sockeye put on their greatest weight – high sunlight also increases plankton numbers. DFO managers tend to look at population numbers resulting from spawning and predict outcomes. But the ocean’s ability to support salmon plays a great role in keeping fish alive and fattening them up. In the past, it was thought that most of the fattening was done in near coast waters, in the later years – so an open ocean bloom should not have much effect.

Tim Parsons, at the Institute of Ocean Sciences in Patricia Bay, thinks there is another explanation. 2009 numbers barely exceeded 1 million sockeye, whereas we all know the 2010 numbers were an almost unheard of 34 million. This, of course, is confusing, particularly to the Cohen Commission, currently hard at work trying to figure out the cause of the crash. And, the various, environmental, scientific, fishing and farming interests try to use the change to support the views they hold.

As Parson pointed out in a note to me, the main question is: if the bloom occurred in 2008, why did the next year’s run collapse while the following summer, 2010, produced a huge return? Good question. Here’s his answer: animals generally have lowest growth rates at the beginning and end of their lives. In between, like all teenagers, sockeye eating and growing peak. More survive, too.

The sockeye returning in 2010 would have been in the midpoint of their lives and the most dynamic part of their growth curve. The diatom and crustacean peak occurred at the same time and they ate such food as the main source of their diets. Later in life, though, and this prevailed for the previous 2009 returning sockeye, their peak growth period came before the volcano let fly. Since there was no volcanic iron to spark a plankton cycle, the 2009 sockeye had been eating the smaller, less prevalent zooplankton – means animal-based – that prevailed before the ash. Hence they were not supported well in their time on the open ocean.

So what does this mean? Well, if there is no volcanic explosion there will not be as large a plankton bloom. This implies that next year’s Fraser run may well be smaller than the just-past summer. As the volcano only explodes every now and then, it cannot be counted on to buoy sockeye numbers on an on-going basis. On the other hand, if the iron can be brought back up from the depths as part of the normal upwelling process, the effect may last longer. And, of course, iron is only one piece of the puzzle.

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