The weather is cold and its winter here with snow falls and temperatures dropping quickly. The color of the fish is much deeper and more intense as you see on this Japanese caught saba.
Fish are ectothermic, which means their body temperatures are controlled by their environment. It looks like different kinds of fish deal with the problem of cold water in a variety of ways. For instance, in some fish, capillary blood flows in the opposite direction to the water flowing over their gills, which allows them to have counter current exchange, thereby maximizing the amount of heat transfer from the water to the fish.
Also mackerel absorbs dissolved oxygen from the ocean through their gills and ultra thin skin that they could abandon those big, spongy red blood cells. Thinner blood requires less effort to circulate around the body and saving energy is always an advantage, especially when you are trying to survive in colder environment.