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
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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