Trying to drown a fish is essentially keeping the fish from breathing oxygen. Therefore, the fish can not breathe oxygen, first of all, the amount of oxygen dissolved in the water can be reduced, and secondly can make the fish respiratory system problems.
The first is to raise the temperature of the water.
If you have a goldfish at home, you can throw it into heated water and it will drown in no time. This is also the reason why you cannot use plain boiled water to raise fish. Plain boiled water precipitates a large amount of dissolved oxygen at high temperature, and it is difficult to restore to normal oxygen content at room temperature with time changes, which makes it unsuitable for fish cultivation.
The second method: add salt to the water
If you don’t want to change the temperature of the water, you can change the salinity of the water.Salinity in water also affects the dissolved oxygen in water. The increase of salinity in water is accompanied by the decrease of dissolved oxygen saturation in water, which is one of the reasons why freshwater fish cannot run into sea water casually, because they may suffocate.
Third way: change the pressure
Theoretically, the amount of dissolved oxygen in water can be reduced by changing the air pressure. The lower the air pressure is, the lower the content of dissolved oxygen in water will be. But in real life it is so difficult to operate that lowering the pressure is not recommended to drown a fish. In a natural environment, the air pressure is lower on cloudy days, so the fish will come out to breathe.
The fourth method: don’t let it move
For cartilaginous fish, they can only get enough oxygen by swimming around. So there’s another way to drown a shark, besides changing the oxygen content, is to keep the shark from swimming.
This happens all the time in the ocean, because sharks sometimes get caught in fishing nets in the ocean, and immobile sharks, unable to get enough water into their mouths, end up drowning. That’s why when you catch a shark, you have to put a pump in their mouth and pump water right through their gills to keep the shark from choking.