Reviving Fish?

[baldwin]

What matters is that water is circulating over the thin membranes of the gill filaments, so the CO2, which has a higher concentration than the surrounding water, can diffuse out. The oxygen, which has a higher concentration in the water than in the fish, diffuses in. Shouldn't really matter much which direction the fish is getting pulled in. I'll get back when I check my notes, I vaguely remember something about the way the blood vessels are arranged for maximum oxygen absorption.

[baldwin]

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.

[FishermanTim]

It might also have something to do with debri getting forced into the gills on the pull back.
I prefer (when possible) to pull the fish forward and drag it in a figure 8.
I also look at the dorsal fin to see when it stands up. To me, that's a sign that the fish is building strength for a burst of energy (to escape).

[baldwin]

You may be right. Gill rakers are bony projections that are on the forward edge of the gill structures, keep stuff from contacting the fragile filaments that do the gas diffusion.

[justplugit]

Originally Posted by FishermanTim

I also look at the dorsal fin to see when it stands up. To me, that's a sign that the fish is building strength for a burst of energy (to escape).

Good point.