During the lad's stay the 4oz pike, which was making its official public debut, decided to regurgitate its last meal. As if by magic a partly digested small roach appeared on the bed of the fish tank. The young fellow then asked me why the roach had sunk and not floated like he thought it should have done. The incident got me thinking about the subject of fish buoyancy.
Do dead fish float or sink?
When kids ask you what at first appears an easy question, you know you are in trouble because inevitably the answer is subtly complex. Thus it is with dead fish. When we get fish deaths, say following a pollution, some of the carcasses will immediately sink, whilst others will float. After a few days some of these fish that originally sank might rise up again. This is caused by the build-up of gases such as carbon dioxide and methane which accumulate in the abdomen of the fish, giving rise to the ‘belly up' expression. But what about live healthy fish, why don't they either just float or sink?
Float or sink
A fish that is less dense than water will surely float to the surface. A fish denser than water will equally certainly sink to the bed of the fishery. Most of the tissues that make up the body of the fish, such as muscle and bone, are denser than water. So how do fish manage to reach and maintain a state of neutral buoyancy? Evolution, or God through the mechanism of evolution, has come up with two basic strategies to obtain neutral buoyancy: swim bladders and the use of lipids.
Swim bladders, sometimes also known as gas bladders, are organs that are filled with gas, most of which is oxygen with relatively less nitrogen compared to the percentage found in air. Gas, of course, has a very low density as there are big gaps between the molecules. Typically in coarse fish such as roach, carp and bream, the swim bladder occupies around 7% of the volume of the body of the fish.
If a fish swam to a depth of 10 metres below the surface, the pressure would double and the volume of gas in its swim bladder would decrease by around half. Therefore to maintain neutral buoyancy, the fish needs to add more gas to the bladder. It does this by complex physiological mechanisms outside the scope of this blog.
Catching fish in very deep water
When a fish with a swim bladder is hooked in very deep water and brought to the surface quickly, the water pressure decreases and the volume of gas (and therefore the size of the swim bladder itself) increases. It effectively blows up a bit like a balloon. The external symptoms are that the abdomen becomes enlarged and the fish's eyes will start to bulge. This can result in death, although this is not an issue if the catch is destined for the table. If the fish is played slowly there may well be sufficient time for it to be able to naturally adjust the amount of gas that needs to be removed from the swim bladder. Most anglers have fish conservation at the heart of their thinking and they have even developed methods for returning fish safely when fishing in really deep waters.
In the UK, it seems that pike are the most likely fish to suffer from ‘gassing up' and the Pike Anglers Club have produced a useful guidance for anglers on what measures to take to ensure fish safety.