I am a little fish, which swims in the sea. I have no scales, but am smooth and slipp

I am a little fish, which swims in the sea. I have no scales, but am smooth and slippery. I am of a silver white colour, and my length does not exceed six inches. In summer I keep near the surface of the water, and in winter I go down to a considerable depth. I am found in most parts of the ocean, from the Arctic to the Antarctic circle. The great rivers also, which have an outlet into the sea, I