> > a fish's body has greater density than water, so if you took a given volume of fish and an equal volume of water, the fish flesh weighs more than the water. The only reason that fish float in the first place is that they use a baglike sac filled with gas called a swim bladder, which they can regulate to give themselves "neutral buoyancy" when diving upwards or downwards.
> >
> > Wolf "phish use physics" spirit
>
> I had thought of including the swim bladder argument in my post (I am an aquarist, if that's a word, and have some experience with swim bladder problems with my fish-- Quite amusing sometimes, until they die.)
>

I've heard of fish farmers using a hypodermic syringe to do emergency deflation of swim bladders in valuable fish breeding stocks or in game fish. It can take a fish several days to diffuse enough gas out of an overextended bladder -- so any fish with a swim-bladder problem, which is also a depth-dweller, would be at risk during that entire time. I guess this technique probably wouldn't work with small fish.


> Anyway, since the fish in the original problem was stated to be alive, I made the assumption that its swim bladder would not be empty, giving it a bulk density equal to that of water. (Unless, of course, the fish is weird and empties its swim bladder so that it can sink to the bottom and play "Rock". I have never known a fish to do this, though, no matter how much I encourage them.)
>

If you accidentally put a seawater brine mix in a tank for fresh-water fish, as I did once, you'll see all your fish dive to the bottom and die. :-(


> I suspect, though, that even if the fish were dead, it would not necessarily be more dense than water-- Only one of my fish has ever sunk after dying, and that was one who had clearly had a swim bladder problem before his death. In general, I would stand by my statement that a live fish will have density (bulk density in this case) equal to water, but I will also postulate based on experience that if the fish were to die, it would be more likely to be less dense than water, and reduce the weight of the bucket/water/fish system as compared to the bucket/water system. I do welcome further discussion, however-- I find this subject very interesting! (You may have guessed that by the fact that I have replied more than once and my replies have been, for me, rather long...)
>
> Don

Durn it, Don, that you had to go and confuse the issue with "(average) bulk density" of /live/ fish versus /dead/ fish. Ack!

Okay. Let me state, speaking out of experience as a cook who's cleaned and prepared whole fresh fish of various sizes, that /dead/ gutted fish -- without their insides -- will fall right to the bottom of a pail filled with water. In other words, fish fillets and fish flesh in itself is denser than water. Live fish float at variable depths, as we've already noticed, only because of their swim bladder control. Dead fish float to the top because after death their internal organs start to break down and fill the abdominal cavity with gases, which increases the buoyancy of the dead fish.

Increased buoyancy is not the same as decreased real density, but (and here's the part that confuses me) the *average* bulk density of the fish in the bucket is, in effect, decreased whenever it diffuses gas into its swim bladder to swim upwards. However, if we consider the fish and the water in the bucket to be 1) a miniature closed system, and 2) we know that fish flesh, pound for pound, is heavier than the equivalent volume of water -- then even a small live fish swimming in a given volume of water should have a small, but definite, increase in the weight of the total system. That is, when compared to a fish-less bucket containing the same total volume of contents.

You'd probably need an analytical weighing balance to see the difference, though.

Wolf "thinks the confusion was due to not understanding the effects of buoyancy" spirit