What is Silage? Understanding Fish Silage and Its Benefits

Every year, the global fishing industry generates millions of tonnes of waste – heads, bones, viscera, skin, and trimmings that are often dumped into landfills or back into the ocean. But what if all of that waste could be turned into a high-quality protein source for animal feed? That’s exactly what fish silage does. It’s a simple, low-cost preservation method that converts fish and fish by-products into a nutrient-rich liquid product, and it’s gaining renewed attention as the world looks for more sustainable ways to manage fishery waste.

What is fish silage? 🔗

Fish silage is a liquid product made from whole fish or parts of fish – such as heads, guts, bones, and skin – that are broken down (liquefied) by the natural enzymes present in the fish itself, with the help of an added acid. According to the FAO, the enzymes break down fish proteins into smaller soluble units, while the acid accelerates enzymatic activity and prevents bacterial spoilage. The result is a stable, brown, soup-like liquid with a malty odour that can be stored for months – even years – without refrigeration.

The concept isn’t new. Research on fish silage began in Sweden in the 1930s, and Denmark started commercial production roughly a decade later. Today, with growing pressure on the fishing industry to reduce waste and find sustainable feed alternatives, fish silage is being re-evaluated worldwide as a practical and economical solution.

How is fish silage made? The ensiling process 🔗

The production of fish silage – called ensiling – is remarkably straightforward and requires minimal equipment. Here’s how it works, step by step:

Step 1: Mincing. The raw material (fish waste or whole fish unfit for human consumption) is ground into small particles using a hammer mill or meat grinder. A screen with 10 mm diameter holes typically produces the right particle size.

Step 2: Acid addition. Immediately after mincing, acid is added – typically about 3.5% by weight of formic acid (85%). The acid lowers the pH to around 4.0 or below, creating an environment where harmful bacteria cannot grow. Thorough mixing is critical because any untreated pockets of fish material will spoil.

Step 3: Liquefaction. Once the acid is mixed in, the natural proteolytic enzymes present in the fish tissue – especially those found in the viscera – begin digesting the proteins. The fish material gradually liquefies over a period of 2 to 10 days, depending on temperature. At 20°C, fresh white fish offal liquefies in about two days; at 10°C, it can take 5-10 days or longer.

Step 4: Storage. The resulting liquid product is stored in acid-resistant containers – plastic-lined steel tanks, polyethylene drums, or bitumen-treated concrete tanks. The silage is stable and can be handled, transported, and blended with other feed ingredients like cereals.

The role of acid in ensiling 🔗

The acid serves two purposes. First, it activates and speeds up the fish’s own digestive enzymes, which are most effective around pH 4 and at temperatures between 35-40°C. Second, it prevents microbial spoilage by maintaining an acidic environment where harmful bacteria cannot thrive. The choice of acid matters:

Mineral acids (sulphuric acid, hydrochloric acid) are cheap but require a very low pH (around 2) to prevent bacterial growth. They also cause more corrosion, and the silage must be neutralised before being used as feed.

Organic acids (formic acid, propionic acid, acetic acid) work at a slightly higher pH, have some natural antibacterial properties, and in the case of formic acid, the silage does not need neutralisation before feeding. However, organic acids are more expensive.

Types of fish silage 🔗

Fish silage is broadly classified into two main types based on how the acidification is achieved.

Acid silage (chemical silage) 🔗

This is the most common type. Inorganic or organic acids are directly added to the minced fish material to lower the pH. Among organic acids, formic acid is widely preferred. According to research published in the Online Journal of Animal and Feed Research, formic acid allows liquefaction at a pH range of 4 to 4.5, thanks to its antiseptic properties. Silage preserved with formic acid has a shelf life of up to one year, even at tropical temperatures.

Biological silage (fermented silage) 🔗

In this method, the fish material is mixed with a carbohydrate source – such as molasses, sugar, or whey – and inoculated with lactic acid bacteria (commonly Lactobacillus plantarum). The bacteria ferment the carbohydrates and produce lactic acid, which naturally lowers the pH and preserves the material. This approach is particularly well-suited for tropical developing countries where carbohydrate substrates like molasses are readily available and affordable.

Biological silage has good nutritional value comparable to acid silage. The key to success is ensuring that the lactic acid bacteria produce enough acid to bring the pH down to around 4.5 and keep it stable throughout storage.

What happens to fish protein during ensiling? 🔗

During the ensiling process, the fish proteins undergo hydrolysis – a chemical breakdown driven by the proteolytic enzymes naturally present in the fish. Large protein molecules are progressively broken down into:

Polypeptides – shorter chains of amino acids
Dipeptides – pairs of amino acids
Free amino acids – individual amino acid molecules

This hydrolysis is what gives fish silage its high digestibility. The proteins are essentially “pre-digested,” making the nutrients much more accessible to animals that consume the silage. Over time, the protein becomes increasingly soluble – the longer the silage is stored, the more complete the breakdown becomes.

However, excessive hydrolysis can be a drawback. Highly soluble amino acids can leach out when the silage is incorporated into feeds, and their assimilation by animals may be reduced. To manage this, some producers heat the silage to inactivate enzymes at a certain point, limiting further protein breakdown.

Nutritional composition of fish silage 🔗

The nutritional profile of fish silage closely mirrors that of the raw material it comes from. A typical analysis of silage made from white fish offal shows approximately 70-81% moisture, 15-17% crude protein, 2-4% ash, and 0.5-13% oil (depending on the fish species used).

On a dry matter basis, fish silage is a concentrated protein source. According to data from Feedipedia, high-oil fish silage contains about 56% crude protein and 31% fat on a dry matter basis, while low-oil silage contains about 46% crude protein and only 4% fat.

Fish silage also contains valuable minerals – including calcium, phosphorus, potassium, sodium, and magnesium – and a full range of essential amino acids such as lysine, methionine, leucine, and isoleucine. Its amino acid profile is comparable to that of fish meal, making it a nutritionally viable substitute.

One important distinction: fish meal contains about 65% protein, while fish silage (in its liquid form) contains only about 15%. This means roughly four times as much silage is needed to deliver the same amount of protein as fish meal.

Applications of fish silage 🔗

Fish silage is a versatile product with several practical uses in agriculture and aquaculture.

Animal feed ingredient 🔗

The primary use of fish silage is as a protein supplement in animal feed. It functions similarly to fish meal and can be fed to a wide range of animals. According to the FAO, the fish silage process transforms waste into a liquid mix of hydrolysed proteins, lipids, minerals, and other nutrients that are easily digestible by both terrestrial and aquatic animals.

Pigs: Pig farming is considered the most suitable outlet for fish silage because it can be easily integrated into liquid feeding systems. Feeding trials have shown that pigs grow at the same rate on silage as on fish meal, with good meat quality and flavour.

Poultry: Dried fish silage has been used to replace soybean meal in broiler diets without any difference in weight gain or feed conversion. In layer diets, replacing up to 50% of fish meal with silage did not affect egg quality or flavour.

Cattle and sheep: Fish silage has been fed to ruminants with good results. Research shows high digestibility for dry matter, crude protein, and phosphorus when fed to sheep. Cow’s milk and butter produced from silage-fed cattle show no off-flavours.

Aquaculture: Fish silage has shown strong results as a feed ingredient for farmed fish. Studies with carp found that silage produced better growth than fish meal. Its nutritional properties are similar to fish meal, but the presence of hydrolysed proteins makes it more digestible.

Organic fertiliser 🔗

Fish silage that doesn’t meet feed-grade quality standards can still be used as a natural fertiliser for crop production. It provides nitrogen, phosphorus, and trace minerals to the soil, making it a valuable input for organic farming systems.

Benefits of fish silage 🔗

Fish silage offers multiple advantages – environmental, economic, and nutritional – that make it an attractive option for the fishing industry.

Environmental benefits 🔗

Fish processing generates enormous quantities of by-products – between 30 and 70 percent of the whole fish by weight. When this waste is dumped into oceans or landfills, it disrupts marine ecosystems, generates methane, and contaminates groundwater. Fish silage converts this waste stream into a useful product, effectively closing the loop in the fish production cycle.

Low cost and simplicity 🔗

Unlike fish meal production, which requires expensive industrial equipment, skilled workers, and generates strong odours, fish silage can be produced with basic tools – a grinder, a mixing container, and plastic storage drums. There is no offensive smell during production, making it suitable for small-scale operations in rural and coastal communities. According to the Brazilian Archives of Biology and Technology, the product has good nutritive quality and can be sufficient for animal feeding, and the procedure is safe, cost-effective, and eco-friendly.

Long shelf life 🔗

Properly acidified fish silage keeps at room temperature for at least two years without spoiling. Silage manufactured with organic acids or fermentable substrates can be stored for years and used on demand. This stability eliminates the need for cold chain infrastructure, which is a major advantage in tropical regions and remote coastal areas.

Reduced feed costs 🔗

Fish meal is one of the most expensive protein ingredients in animal feed, and its price continues to rise as global fish stocks decline. Fish silage provides a cost-effective alternative that can replace a significant portion of fish meal in animal diets. Research suggests that using fish processing waste for silage could reduce the cost of producing animal feed by approximately 15 to 20 percent.

Challenges and limitations 🔗

Despite its advantages, fish silage does come with some practical challenges:

High moisture content: Since fish silage retains all the water present in the original raw material, it is four to five times bulkier than fish meal. This makes transportation more expensive, especially over long distances.

Raw material quality: The quality of the finished silage depends directly on the freshness of the input material. If partially decomposed fish waste is used, the silage may contain elevated levels of histamine, which can be harmful in animal feed.

Limited market awareness: Fish meal is a well-established product with a global market. Fish silage, by comparison, is still relatively unknown in many regions and requires marketing effort and farmer education to increase adoption.

Excessive protein hydrolysis: Over time, the amino acids in stored silage become increasingly soluble. This can lead to leaching losses when the silage is mixed into dry feeds, reducing its nutritional efficiency.

Fish silage vs. fish meal: a quick comparison 🔗

Both fish silage and fish meal serve as high-protein animal feed ingredients, but they differ significantly in production, cost, and logistics. Fish meal requires high capital investment, specialised equipment, skilled labour, and produces strong odours during manufacture. Fish silage, on the other hand, requires minimal investment and can be produced by unskilled workers with no offensive smells. However, fish meal is concentrated, easy to store and ship, and has an established global market – whereas fish silage is bulky, best used locally, and still gaining market recognition.

In areas where there is no nearby fish meal factory, or where the volume of available fish waste is too small to justify industrial processing, fish silage is the more practical and economical choice.

The future of fish silage 🔗

As global fish production has increased – from around 20 million tonnes in 1950 to over 170 million tonnes in recent years – the volume of fishery waste has grown proportionally. At the same time, demand for sustainable, affordable animal protein sources continues to rise. Fish silage sits at the intersection of these two trends.

International organisations like the FAO are actively promoting fish silage technology through feasibility studies in countries including Bangladesh, the Philippines, Thailand, Barbados, and Saint Kitts and Nevis. Some producers are also experimenting with combining fish waste with agricultural by-products to create balanced, complete feed products that maximise resource utilisation across industries.

Advances in processing technology, improved quality control methods, and growing awareness of circular economy principles are all driving renewed interest in fish silage as a scalable, sustainable solution for managing fishery waste.

What do you think? Could fish silage become a mainstream feed ingredient in your region’s livestock or aquaculture industry? What changes in infrastructure or policy would be needed to make large-scale adoption a reality?

References

1. https://www.fao.org/4/x5937e/x5937e01.htm
2. https://www.feedipedia.org/node/203
3. https://ojafr.com/main/attachments/article/167/OJAFR%2013(2)%2079-88,%202023.pdf
4. https://www.redalyc.org/journal/437/43774024029/html/
5. https://www.fao.org/flw-in-fish-value-chains/resources/articles/fish-by-products-utilization-getting-more-benefits-from-fish-processing/en/
6. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/fish-silage
7. https://www.scielo.br/j/babt/a/9pyfgdSsmrhmm47Qg7RPnMH/?lang=en