"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.