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I didn't actually find my notes yet, but do remember something about cross-current ventilation. I don't remember if it's in all fish, or just the scombrids (tuna, mackerel). The blood vessels go in the opposite direction of the water flow through the gills. This is so that the oxygen-poor blood is first exposed to the gill filaments farther back from the mouth, so it contacts the water with the most oxygen removed (though still more oxygen than the blood). As the blood moves forward and extracts more oxygen, it keeps contacting water with a higher and higher oxygen content. This is so that even as the blood absorbs oxygen, it is continuously exposed to water with a higher concentration than in the blood. Thus, it maximizes oxygen diffusion into the bloodstream. I don't see, though, where it might harm the fish by moving it back and forth through the water. It would still facilitate oxygen absorption, just not as efficiently as continuously towing it head-first, as in throwing an albie hard into the water.
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