Fish Farming: Water Quality and Waste

Explained

Keeping the water right in an intensive fish farm

An intensive fish farm holds a great many fish in a small volume of water. That is the whole point, because it produces a large amount of protein from a small space, and it is also the source of every problem in this topic.

Crowded fish use oxygen quickly, produce waste quickly, and pass diseases between themselves easily. Each technique used on a fish farm is an answer to one of those three pressures, so learn them as problems and solutions rather than as a list.

Dissolved oxygen

Fish respire aerobically, and a dense population takes oxygen out of the water faster than it dissolves back in from the air. If the level falls, the fish cannot respire properly, so they grow more slowly and eventually die.

Farms monitor the dissolved oxygen and maintain it, either with aerators that bubble air through the water or by keeping a continuous flow of fresh water through the cages or tanks.

Ammonia and other waste

Fish excrete nitrogenous waste as ammonia, which is toxic once it builds up. In an open river this would be diluted and processed by bacteria; in a tank it accumulates.

It is removed by exchanging the water regularly or by biological filtration, where bacteria convert the ammonia to less harmful nitrates. Those are the same nitrifying bacteria that appear in the nitrogen cycle, doing the same job in a filter.

Uneaten food and faeces

Anything that falls to the bottom is broken down by decomposers, and those bacteria respire, so decomposition uses up more of the oxygen the fish need. It also releases nutrients into the surrounding water, which can cause eutrophication downstream of the farm.

Careful feeding is the main control. Feeding little and often, at the right pellet size, means less food is wasted, which saves money and protects the water at the same time.

Disease and pests

Crowding lets parasites and pathogens spread from fish to fish. Farms respond by keeping the stocking density within limits, treating water or fish with pesticides or antibiotics where necessary, and separating age groups so an infection does not run through the whole stock.

The treatments themselves are a drawback. Chemicals can escape into the surrounding water, and heavy antibiotic use encourages resistant bacteria.

Answering an evaluation question

Questions here usually ask you to balance productivity against environmental impact, so the answer needs both sides and a link between them.

On one side: high yield from a small area, a reliable supply of protein, reduced pressure on wild fish stocks, and controlled conditions that let fish grow faster.

On the other: waste and uneaten food polluting nearby water, chemical and antibiotic use, disease spreading to wild populations, escaped farmed fish interbreeding with wild ones, and the fact that carnivorous farmed species such as salmon are fed on wild caught fish, which shifts the pressure rather than removing it.

Then give a judgement that refers to the situation in the question. Which side matters more depends on where the farm is and what is downstream of it, and saying so with a reason is a legitimate conclusion.