Being swallowed live is a common way for wild aquatic animals to die

and it probably involves suffocation and chemical burns in digestive juices over minutes.

This came up for this post, but seemed worth pointing out separately in its own post, so this post is mostly copied from there.

I don’t consider the referenced sources here highly reliable for how exactly prey die when swallowed live, but I wasn't able to find anything better when I checked.

Predation is one of the most common ways for wild aquatic animals to die, perhaps the most common way (Dall et al., 1991 for penaeid shrimp, Hurst, 2007 for fish during the winter, Pauly & Palomares, 1989 for Peruvian anchoveta, Vollset et al., 2023). (Predation seems also to be the largest cause of death among terrestrial vertebrates (Hill et al., 2019)).

Predatory/carnivorous fish typically swallow their prey whole (Lindsay, 1984, Meekan et al., 2018, Luiz, 2019, Yasuda, 1960, Amundsen & Sánchez‐Hernández, 2019, St John, 1999, Gill, 2003, Lundstedt et al., 2004), and so without tearing or chewing. This is despite having teeth. This is not true of all predatory fish, as Lindsay (1984) wrote:

Fish that break up food by means of pharyngeal teeth (cyprinids) or other modifications to the buccal cavity (wrasse, rays) had low chitinase activity in spite of consuming chitin in the diet while those fish that tend to gulp prey whole (salmonids, gadoids, perch, eel, red mullet, gurnards, mackerel) had high activity. This suggests that the primary function of gastric chitinase is to disrupt the chitinous envelope of prey allowing access to the soft inner tissues by the digestive juices. This role would also be functional in those fish destined to be piscivorous as adults but which gulp down relatively large invertebrate prey when young.

The cause of death in live swallowing by fish seems most likely to be suffocation/asphyxiation, i.e. too little oxygen in the prey's blood, due to too little dissolved oxygen in the predator fish’s stomach or due to damage to the prey’s gills from digestive juices (Waterfield, 2021, Reddit AskScience thread, Poe GPT-4o, Poe Web-Search). Other possibilities include digestive processes (stomach acid, enzymes) and mechanical injury, e.g. crushing (own guesses, Poe GPT-4o, Poe Web-Search).

Fish can survive minutes outside of water or without oxygen in water, and another fish’s stomach may have some swallowed oxygenated water. The fisheries scientist Gerald Waterfield (2021) wrote:

My best estimate of the time that the consumed fish stays alive is from about 15 to 25 minutes, after which the fish dies from lack of oxygen. This process starts as soon as the fish enters the predator’s throat. It happens a little slower at lower temperatures. Even if the prey fish were regurgitated a few minutes fewer than this time, it probably would still expire due to brain damage from the restricted oxygen intake and it would be blinded by its eyes having been greatly damaged from stomach acid.

On the other hand, Poe GPT-4o responded that asphyxiation “can cause death within a minute or two” and death “typically occurs within a matter of seconds to a few minutes, primarily due to asphyxiation and physical trauma”, but could not provide direct sources for its claims when prompted. My best guess is that it takes at least minutes, in line with the survival time for fish out of water, and because they probably won’t have been substantially injured until reaching the stomach.

During this time, besides potential suffering from suffocation and fear, they probably suffer from chemical burns and tissue damage from digestive juices. They might lose consciousness and so stop suffering some time before they die, but I don’t know how long before.