Fish farming, or aquaculture, raises fish in controlled environments rather than catching them from the wild. It now produces the majority of the world’s top aquatic animal species. In 2022, farmed grass carp, Nile tilapia, and silver carp alone accounted for over 16 million tonnes of production globally. The process involves hatching eggs in indoor facilities, growing fish to market size in ponds or tanks or ocean enclosures, and harvesting them for processing.
From Eggs to Fingerlings
Every farmed fish starts in a hatchery. Broodstock, the adult fish selected for breeding, produce eggs that are collected and placed in flow-through incubation tanks. These tanks use rotating paddles to gently circulate water around the eggs, keeping them oxygenated. Depending on water temperature, eggs hatch in about 5 to 8 days.
Newly hatched fish, called sac-fry, still carry a yolk sac that feeds them for the first few days. Over 3 to 5 days they absorb this yolk, darken in color, and begin swimming to the surface looking for food. At that point they’re called swim-up fry, and they’re fed a nutritionally complete manufactured feed for a few days before being moved to nursery ponds. There, they eat daily through the growing season and reach fingerling size (3 to 8 inches) over 5 to 10 months. Once they’re large enough to handle the transition, fingerlings move to their final grow-out environment.
Open Net Pens
The most common system for ocean-raised species like salmon and trout is the open net pen: large mesh enclosures anchored in coastal waters. Fish swim in natural seawater, which flows freely through the nets, supplying oxygen and carrying away waste. This setup is relatively simple and inexpensive to operate because the ocean does most of the water management work.
The tradeoffs are significant, though. Nutrients from fish waste flow directly into surrounding waters, contributing to algal blooms and oxygen depletion. Parasites, especially sea lice in salmon farming, spread easily between pens and to wild fish populations. Diseases can transmit to wild stocks. And escapes are a constant concern. Norway’s 1.35 million tonnes of farmed salmon and trout production generated roughly 160,000 escapees in 2018 alone, and those escaped fish, along with sea lice, represent the most serious threats to wild salmon populations.
Land-Based Recirculating Systems
Recirculating aquaculture systems (RAS) take a fundamentally different approach by raising fish in indoor tanks on land, filtering and reusing the water in a continuous loop. A typical RAS includes fish production tanks connected to drum filters that remove solid waste, biological filters that convert toxic ammonia into less harmful compounds, UV and ozone disinfection units, equipment for stripping carbon dioxide and adding oxygen, and monitoring systems that track temperature, pH, and salinity. Depending on the design, these systems recycle between 30% and 100% of their water.
Because the fish never contact natural waterways, RAS eliminates the risk of spreading nutrients, parasites, diseases, or escaped fish into wild ecosystems. The controlled environment also tends to reduce disease, which means less need for medications. Farmers can raise tropical species like tilapia in cold-climate countries without any threat of introducing non-native species if fish escape. Freshwater RAS facilities have an additional benefit: their waste can be recycled as agricultural fertilizer since it contains no salt.
The main disadvantage is energy. Running pumps, filters, heaters, and disinfection equipment around the clock demands far more electricity than simply placing a net in the ocean. That energy cost is the central challenge for making land-based systems competitive at scale.
Pond Farming
Earthen ponds remain the most widely used system globally, particularly in Asia where species like grass carp, silver carp, and tilapia dominate production. Ponds are filled with fresh water, and farmers manage water quality by controlling feeding rates, adding aeration when oxygen drops, and periodically exchanging water. Some operations raise multiple species together in the same pond, a practice called polyculture, where different fish occupy different feeding niches and make more complete use of available nutrients.
Pond systems fall somewhere between net pens and RAS in terms of environmental control. Farmers have more influence over conditions than in open-ocean pens but less precision than in a fully recirculating indoor system.
What Farmed Fish Eat
Farmed fish eat manufactured pellets designed to meet their full nutritional needs. Fishmeal, made from ground-up wild marine fish, has long been the primary protein source in these feeds because of its high nutritional value and palatability. But as aquaculture has grown, pressure on wild fish stocks used for fishmeal has pushed the industry toward alternatives.
Soy protein concentrate, pea protein, yeast cultures, bacterial protein, and insect meal (particularly from black soldier fly larvae) are all being used to partially or fully replace fishmeal in commercial feeds. Experimental diets have tested replacement levels ranging from 25% to 100% of the fishmeal component. Results vary by species: some fish tolerate high levels of plant or insect protein with no health effects, while others show changes in immune function or growth at higher substitution rates. Rainbow trout, for example, showed modest improvements in antioxidant and immune function when fed diets with 50% insect-based protein. The general trend across the industry is toward feeds that use less fishmeal and more diverse protein sources.
Managing Disease and Parasites
Dense populations of fish in confined spaces create ideal conditions for disease and parasite outbreaks. Sea lice are the most persistent problem in salmon farming. These tiny crustaceans attach to fish skin and feed on mucus and tissue, causing stress, secondary infections, and reduced growth.
Farmers use a combination of approaches to manage sea lice. Chemical bath treatments with approved pesticides are one option. Biological control using “cleaner fish,” species like lumpfish that eat lice directly off salmon, is an increasingly popular alternative. Data from Canadian salmon farms shows that sites using lumpfish reduced their chemical inputs, though a Norwegian analysis of more than 500 farms found that cleaner fish effectiveness varied considerably depending on location and timing. Mechanical methods, including warm-water treatments and physical removal systems, round out the toolkit.
The FDA approves a limited set of drugs for use in aquaculture, with specific requirements for each: the species it can be used on, the disease it treats, the dosage, and a mandatory withdrawal period before the fish can be harvested and sold. That withdrawal period ensures no harmful drug residues remain in the flesh. For example, lobsters treated with the antibiotic oxytetracycline must be held for 30 days after treatment before they can be sold.
Stocking Density and Welfare
How crowded the fish are matters for both welfare and meat quality. Stocking density is measured in kilograms of fish per cubic meter of water. Organic certification programs set explicit limits: the UK’s Soil Association and Germany’s Naturland both cap net-pen salmon density at 10 kilograms per cubic meter. Conventional farms often stock more densely than this, though the specific limits vary by country and species. Proposed USDA organic standards require that density levels account for animal health, well-being, and the natural schooling behavior of each species, without specifying a single number.
Harvesting
When fish reach market size, they’re typically fasted for one to two days to empty their digestive tracts, then collected from ponds, tanks, or pens using nets or pumps. The focus in modern operations is on stunning fish quickly before slaughter to minimize suffering.
The two primary humane stunning methods are percussive and electrical. Percussive stunning involves a rapid blow to the head, either manually with a club or priest (a specialized striking tool), or with a mechanical device like a non-penetrating captive bolt gun. Electrical stunning passes a current through the water or directly through the fish to induce immediate unconsciousness. In-water electrical stunning is the most extensively studied method, followed by dry electrical systems where fish are stunned after being removed from water. Large-scale operations increasingly use automated versions of both approaches to handle high volumes while maintaining consistency.
After stunning, fish are bled, gutted, and chilled. From there they move to processing facilities for filleting, packaging, or further preparation, typically reaching retail within days of harvest.