Fish By-Products: Turning Bones, Skin and Guts into Resources

What are fish by-products? The discarded portion is bigger than you think

Supermarket fillets, sashimi, canned fish, kamaboko, dried fish. What we eat is really only the "easy to eat" part of a fish. The rest, namely the head, bones, skin, fins, viscera and scales, is called ara or processing residue in the seafood industry. The more a fish is processed, the more by-product is inevitably created.

The volume is far from small. According to an estimate in the trade magazine Ocean Newsletter (Ocean Policy Research Institute, Sasakawa Peace Foundation), seafood supplied domestically excluding shellfish and seaweed is 1.62 million tonnes (2024 data). With an edible yield of 54%, the inedible parts such as heads, bones, skin, fins and viscera come to about 750,000 tonnes. Note that this is the author's estimate and not official statistics from bodies such as the Fisheries Agency.

Rough weight by body part

The share of each part varies greatly with species, size and how the fish is cut. An academic review of collagen peptides breaks down fisheries by-products as follows: heads 9–12% of total weight, viscera 12–18%, skin 1–3%, bones 9–15% and scales about 5%. Taken together, nearly half of a fish's weight is something other than the edible portion.

What about the world?

The FAO report The State of World Fisheries and Aquaculture (SOFIA) 2022 states that by-products can account for up to about 70% of processed fish, depending on size, species and processing method (as quoted by IFFO, the marine ingredients organisation). In other words, processing seafood for eating inevitably yields large amounts of by-products.

We have also covered by-products from other angles. See What do fish bones, skin and scales become? for collagen, Turning seafood processing residue into a resource for the overall picture in Japan, and Turning bycatch into food.

Where do by-products come from? The source decides the destination

Even among by-products, the nature differs greatly depending on where they come from. Ocean Newsletter divides them into three: from processing plants, from wholesale markets, supermarkets and restaurants, and from households.

By-products from processing plants: large volumes of a single species

Plants making kamaboko, frozen fillets or canned fish handle large amounts of the same species, so the by-product also comes in bulk of a single species. Freshness is easy to maintain, and the standard route is to send it to a nearby fishmeal plant to be made into fishmeal and fish oil. The same article estimates that about 74% of the 750,000 tonnes goes to fishmeal companies.

By-products from markets, retail and restaurants: mixed species

Wholesale markets, supermarket back rooms and restaurants produce small amounts of by-product of mixed species and sizes. Collection takes effort and mixing makes quality hard to stabilize. Still, outlets do exist, such as fishmongers selling by-products to households for fish soup.

Household by-products: mostly burnable waste

Fish bones, skin and heads from home cooking normally enter the municipal incineration route as food waste. The amounts are small each, but multiplied by the number of households they become significant. Some municipalities are working on composting or biogas from food waste.

The biggest outlet is fishmeal and fish oil: by-products as aquaculture feed

The best-established use of by-products is fishmeal, made by drying and grinding, and fish oil, obtained by pressing. These are used as feed for farmed salmon, yellowtail and sea bream, as well as livestock and pets.

How fishmeal is made

The basic process is crushing, cooking (steaming), pressing to separate water and oil, and drying into powder. A Shizuoka Prefectural Research Institute of Fishery and Ocean Technology document on Yaizu explains that residue from seafood processing plants goes through boiling and solid-liquid separation to be turned into feed, food and organic fertilizer.

Fishmeal, fish oil and the share of by-products worldwide

According to IFFO, the amount of fish used for fishmeal and fish oil peaked at over 30 million tonnes in 1994 and then fell to 16.7 million tonnes in 2022. The share of by-products in the raw material varies by year and source: Seafish (a UK seafood industry body) cites FAO 2020 for 25–35%, while IFFO gives 30% or 42% depending on the source. Years and definitions differ, so they cannot be compared directly, but roughly 30–40% of the world's fishmeal and fish oil raw material comes from by-products of fish processed for human food.

Challenges of fishmeal and expectations for by-products

Fishmeal made mainly from wild small fish has been criticized for pressure on fish stocks. Directing the by-products of fish already caught and processed for food into fishmeal is a sensible use of resources, and it can reduce the amount of wild small fish newly caught. However, the ratio of protein to ash (bone minerals) changes with species and part, so ways to keep the quality stable as feed are needed.

Broth, extracts and seasonings: drawing out umami

Fish by-products are a treasure trove of umami. The heads, bones and the meat around the backbone contain inosinic acid, amino acids and peptides, and have long been eaten as fish soup and broth. Doing this at factory scale is the processing into extracts and seasonings.

Heat extraction and enzymatic hydrolysis

There are two basic methods. One is hot-water extraction, boiling and concentrating, close to the idea long used in making bonito flakes and the raw material for dashi packs. The other is enzymatic hydrolysis, adding protein-digesting enzymes to break molecules down, which efficiently extracts umami-rich peptides and amino acids. Traditional fish sauce is, in effect, natural enzymatic hydrolysis: fish viscera enzymes and salt ripen the fish over a long time.

Bonito viscera: not food, but "fishing bait"

A Shizuoka Prefectural Research Institute study (fiscal 2024) introduces using extract from bonito viscera (stomach, intestine and pyloric caeca) from seafood processing as an attractant in fishing bait. In tank tests it showed attraction comparable to the conventional bait, Japanese anchovy. Processors want wider uses and higher value, and this is one example of seeking outlets other than food.

Value beyond taste: functional peptides

Research is under way in Japan and abroad on enzyme-digested fish protein, including components expected to help lower blood pressure. However, this is an area where the research stage and the evidence behind each product vary widely. When looking at functional claims, check the ingredient concerned and what the study actually involved.

Collagen and gelatin: skin and scales add high value

The by-product with the highest potential added value is collagen, found in skin, scales and bones. Academic reviews report that about 70% of fish skin is type I collagen, and that scales are about 70% protein, made up mainly of collagen and keratin.

Three extraction methods

Reviews group the ways of obtaining collagen peptides from fish skin into three: acid or alkali treatment, enzymatic hydrolysis and fermentation. Enzymatic hydrolysis is evaluated as a way to hydrolyze skin without losing nutritional value (this is the view of one review, not an established consensus). Scales contain much calcium, so a decalcification step comes first, and a method using the chemical EDTA has been reported.

Collagen versus gelatin

Gelatin is obtained by partially hydrolyzing collagen, and collagen peptides are the further broken-down form. Fish gelatin is said to cause fewer allergic reactions than gelatin from cattle or pigs and is drawing attention as an alternative in regions with religious dietary restrictions. On the other hand, fish gelatin has a lower melting point, which makes jellies set differently.

Fertilizer, feed and biogas: closing the loop

Fertilizer, feed and energy use are not high in value but can absorb large volumes.

Fertilizer: a source of nitrogen, phosphorus and calcium

Fish by-products contain nitrogen, phosphorus and bone-derived calcium, and have long been returned to farmland as dried fish meal fertilizer or fermented organic fertilizer. As in the Yaizu example, solids from boiling and solid-liquid separation can go to organic fertilizer. It is a loop returning the nutrients taken from the sea to crops on land.

Fish silage: preserving with acid

Adding organic acids to fish by-products so that they self-digest into a liquid feed is called fish silage. It needs no drying equipment and is easy to adopt even on a small scale. Storage improves, and it can be used as a feed ingredient for livestock and farmed fish.

Biogas: turning it into energy

By-products rich in fat and protein are also good material for methane fermentation. If the gas produced is used for power or heat, it can reduce the fossil fuel burned compared with incineration. However, salt and ammonia in fish affect the fermenting microbes, so adjustments such as mixing with other organic matter are essential.

Barriers to using by-products and ways forward

The technology for using by-products is largely established. They have not spread fully because of barriers other than technology.

Barrier 1: freshness declines quickly

The value of by-products is decided by freshness. Once enzymes in the viscera act, protein and fat degrade in a short time. Small processing plants without enough refrigeration or freezing find them hard to use as raw material unless they are processed immediately.

Barrier 2: small volumes make collection costly

In areas far from ports and for small businesses, the daily volume is small, and transport to a distant plant can cost more than it earns. Mixed species also make it hard to direct them to high-value uses.

Barrier 3: rules and quality standards for each use

To be used as feed or food, safety and quality standards must be met. For feed ingredients, for example, contaminant control and rules on which parts may be used are required. Cosmetics and food also need traceability of raw materials.

Ways forward

• Joint processing in the region:Nearby plants pool their by-products and share refrigeration and primary processing
• Cascading use:Use skin for collagen, bones for calcium and the rest for fishmeal and fertilizer, in order of value
• Opening new outlets:Look for demand beyond food, such as bonito viscera extract as fishing bait
• Understanding among consumers:Choose and buy foods and materials derived from by-products

What we can do: choosing to eat the by-products too

Using by-products is not an issue for businesses and research institutes alone. Consumer choices also widen the outlets.

Three actions at home

• When buying fish, choose whole fish or cuts with the bones as well as fillets, and use them up in fish soup or broth
• Try foods made from by-products, such as bone crackers, fish sauce and dashi packs
• Reduce leftovers. Not wasting the flesh or skin leads to less food loss even before it becomes by-product

Supporting shops and producing regions

Regions and companies selling products made from by-products are gradually increasing. When choosing products, read the explanation of the raw materials and efforts. Choosing seafood with sustainable fishery certification such as MSC is also an action that values the resource as a whole.