Just like with other farmed animals, huge numbers of fish are reared on factory farms—only this time, they're underwater. To ensure maximum profit, farmers tightly pack many thousands of fish into underwater cages and tanks. These farms are crowded, barren places where the fish have little to do other than swim in endless circles.

When large numbers of fish are crowded into a farm, as with land animals like chickens and pigs, their welfare suffers. Overcrowding can cause stress, and this can result in fighting and injuries from biting.

Water quality is important for fish welfare. Good water quality can ensure that the fish have adequate oxygen to breathe comfortably and reduces the prevalence of disease. Fish excrete into the water, so when lots of fish are in a small space, waste products such as ammonia can build up to dangerous levels. The resulting poor water quality can significantly impact a fish’s well-being.

Fish also need enough space to swim and behave normally. Being able to spend time in certain areas of the cage so they can adjust to different temperatures and light is essential for higher welfare – and this is restricted in higher stocking densities. It has even been shown that fish may suffer from sunburn if they are forced too close to the surface of the water.

Fish are usually given antibiotics in their feed to prevent the spread of various diseases, which may occur in intensive conditions.

Fish are kept in such huge numbers that diagnosis, separation, and treatment of an individual fish is impossible. However, as with land animals, there are issues when fish are given feed containing antibiotics as a preventative measure, or where all animals in the group are treated even though only a few are sick. Use of antibiotics in this way means that the fish may be more able to survive in overcrowded and dirty conditions; but antibiotics should not be used as a substitute for good animal welfare.

When antibiotics are given to fish in an open environment like a sea cage, antibiotics will leak out from the farm. Some studies looking at sediment beneath sea cages have even found antibiotic resistant genes several kilometres away from the farms. In general, microbes can become resistant to antibiotics faster the more they are used. This risks the future effectiveness of antibiotics – even for humans.

Birds, seals, sea lions, walruses, and otters are predators of farmed fish in sea cages, land-based tanks, or pond farming systems. Also, dolphins and wild fish such as swordfish and bluefin tuna prey on farmed fish. These animals can damage nets, leading to fish escapes, and eat or distress the farmed fish.

Farmers often take harmful or lethal measures to control these predators. One example is the shooting of wild seals by Scottish salmon farmers. Fish farms can apply for a licence to kill seals that are on or near their farms. What’s worse – there’s no restriction to shooting seals during breeding seasons, so some of the seals shot will be pregnant or nursing young pups.

Some farms use tools called acoustic deterrent devices (ADD) to keep aquatic mammals away. These devices use random frequency sweeps and tones to unsettle approaching animals. Other devices use sound pressure at a specific frequency to cause discomfort to animals if they come within close range. These devices may cause long term damage to the hearing of the seals, dolphins, and whales that swim into the areas surrounding farms. Hearing is a critical sense to these animals, who depend on their hearing for navigating, finding food, and communicating underwater, so the results may be devastating.

In some European countries, pond systems have problems with predation by otters and beavers. Both otters and beavers are frequently culled despite their protected status.

Before certain procedures that require fish to be crowded and handled, such as transport or slaughter, fish are starved for long periods of time. Sadly, this practice is common across the entire fish farming industry.

In certain cases, starving for periods of 1-3 days is deemed necessary to not further harm the fish. For example, clearing the gut before transport means that there is less waste excreted into the water while fish are transported, reducing the risk of harm to the fish from poor water quality.

However, fish are regularly starved for far longer than necessary – sometimes even up to 2 weeks. This may be to boost profits by saving money on feed. The fish suffer from hunger and frustration at not being able to search for food, which can result in aggression.

Intensive farms are completely barren and vastly different to the natural habitats that fish would find in the wild. As with other farmed animals, good welfare means giving fish an environment that is similar to conditions in the wild and complex enough to meet their behavioral and mental needs.

Fish suffer from boredom and frustration. Barren environments limit the expression of natural behaviors, resulting in negative impacts on the physical and mental health of the animals, with many experiencing inescapable boredom.

Environmental enrichment involves deliberately increasing environmental complexity to improve welfare. There are a growing number of studies showing various welfare benefits of enrichment for fish. Providing enrichment can lead to reduced aggressive interactions, a reduction in disease and injuries, improved cognitive capacity and exploration, reduced impact from stressors, improved foraging ability, and decreases in larval deformity and mortality.

Enrichments that allow the fish some level of control over their environment can include shelters (e.g. pipes or shells), changing the color of the tank, self-activated fish feeders, or adding a cover to the top to create a shade. There are even some examples of music having a positive effect on fish growth.

We face a global crisis of overfishing. Without massive new marine conservation areas and industry regulation, we may face a future of empty seas without wild fish.

Fish farming is responsible for much of the industrial fishing of our decimated oceans. Many widely-farmed species, such as trout and salmon, hunt other fish in the wild. When these animals are farmed, they are given fish feed that is made of wild-caught fish. Approximately one-quarter of all wild-caught fish are used to make fish feed. This comprises of somewhere between 450 billion and 1 trillion fish. In other words, it can take up to 350 wild caught fish to raise a single farmed salmon, so fish farming actually increases the pressure on wild stocks.

Since these wild fish die without any form of humane slaughter or prior stunning, the welfare cost of the fish feed is massive.

Most fish are caught by huge industrial fishing vessels. They are caught in vast numbers, so no individual capture and slaughter takes places. Many are caught by trawling vessels that scoop up hundreds of thousands of fish in one go, and they are crushed together in the nets causing injury and death. Those that survive the capture are left to suffocate on decks or are gutted alive.

Fish are sentient—they can feel pain and fear, and they can suffer. Therefore, slow, painful slaughter without prior stunning is a horrific ordeal for them.

Many farmed fish are killed using painful, stressful methods, and their suffering can sometime last hours. Some will be left to suffocate to death in tanks of ice slurry.

Wild-caught fish also frequently die in slow and painful ways. Many of the ways of capturing wild fish can cause the fish to be crushed to death, suffer from the pressure change of being pulled from the depths of the ocean, or be dragged along a line with a painful hook in their mouths for over a day. Those who survive the capture are left to suffocate on decks or are gutted alive.