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Hello Fish-sci,
Can I ask for your feedback on another issue? It's regarding the effects of
small temperature changes on fish stocks - I know that it does affect them,
but I really wonder to what extent it is causing the major changes that we
are seeing. I found a very interesting Atlantic-Pacific temperature
anomaly. Here's an essay on the topic that is from my website:
"SLIGHT CHANGES IN WATER TEMPERATURES" - What are the Effects on Fish
stocks (especially on the SIZE of fish)?
Most of my research into this question was done by reading Canadian
Department of Fisheries and Oceans documents. I specifically looked at
stock status reports on Atlantic and Pacific stocks, which described the
changing trends, as well as papers describing trends in climate change and
correlations made between the two. Before I started I was familiar with the
east coast theory that unusually cold water temperatures in the area over
the last decade were thought to be inhibiting the recovery of the
groundfish stocks.
How does the unusually cold water cause fish to grow more slowly?
Apparently many species simply don’t eat as much when the water is colder,
maybe they or their appetites become semi-dormant? However it works,
apparently it is true, I believe it has even been demonstrated in
laboratory work. Documents from a few years ago predicted that the northern
cod stock on the Grand Banks would start to recover when the water warmed
up a bit. One problem is: during the last few years the water there has
“warmed up” and has actually been above the long-term mean, but the cod
stock has not started to grow. “Unexpectedly” it continues to decline. So
the appetite supressing effect of cold water is not the major factor. I
think it is a relatively minor factor when compared to the availability of
food. (Incidentally, the magnitude of the temperature drop during the cold
spell was between 1 and 1.5 degrees Celsius, which doesn’t seem like a
great lot to me.)
When I read through the reports on the Pacific stocks the same declining
trends were there that I had seen on the east coast. I was very surprised,
however, at the reason that was given for the slow growth in the west coast
fish. It is thought to be because water temperatures in the area have been
unusually warm (El Nino has been more active than usual over the last
decade), and predictions are that the stocks will begin to rebuild when
water temperatures return to normal (i.e. become a bit cooler). How is it
that warm water causes fish to grow more slowly? When an animal is warmer
its metabolic rate increases so more energy is used that way and less
weight is gained. This is probably as true as the way that cold water
inhibits weight gain, but again I wonder how significant a factor it is in
the overall picture.
I found the contrast between cold Atlantic water causing fish to lose
weight, and warm Pacific water causing the exact same thing...to be very
intriguing. And to cast serious doubt on both theories, in fact it struck
me as being bizarre. But maybe the fish species that live in the Pacific
are intrinsically different from the Atlantic ones, in such a way that
their susceptibility to temperature changes is reversed? Unlikely, but I
suppose it could be possible. The two areas do not appear to have a lot of
species in common...but ordinary herring are found in both. It is too much
to believe that herring on the east coast are in decline because the water
is too cold, while those on the west coast are shrinking because it is too
warm. It is clearly time to look beyond the temperature theory. I think it
is obvious that fish are having a hard time finding enough to eat on both
sides of the continent.
Debbie MacKenzie
http://www.fisherycrisis.com
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