On the way from the whole fish to the skinned fillet nearly two thirds of a cod’s original weight is lost. In the case of a herring weighing 200 g hardly more than 100 grams are used to produce matje herring or rollmops, and the yield from redfish is only half of this. For a very large number of fish species the edible share that is ultimately served on the plate as a fillet hardly adds up to more than half of the fish’s original weight. The head alone accounts for a good 20% of a fish’s biomass, and then there are the guts, the fins, the skin and the bones. Nearly all of what finishes up as waste contains a lot of proteins and polyunsaturated fatty acids, besides minerals and trace elements, enzymes, hormones, pigments and aromas. And many of these substances are urgently needed in industry. What in the past was only seen as waste is really a valuable raw material from which substances that are wanted for food production and agriculture, aquaculture, pharmaceutical and cosmetic products can be extracted. Biotechnologists all over the world are thus looking for suitable methods to extract these individual ingredients from “waste”.
At the moment there is no lack of the basic raw material because during the course of fishing, aquaculture and processing techniques enough “fish waste” is produced… One only has to think of the by-catches of fishing vessels that are thrown back into the sea – unused – as discards, or of the unfortunate habit of disposing of the guts and often even the heads of demersal fishes when they are gutted at sea by “disposing” of them overboard. It is estimated that up to 11% of the biomass of all demersal fish catches is never actually landed as a result of “primary processing at sea”.
Aquaculture, too, is a constant source of fish waste since on fish farms fishes are lost nearly every day, be it through technical faults, inappropriate handling, or inadequate farming conditions. Even losses that are caused by disease would be utilisable, as long as the disease was not ISA, VHS, IHN or similar dangerous epidemics that appear in Lists I and II of the EU guideline 91/67. At present, dead fishes are disposed of in an appropriate way, mostly by burning or as silage. Under regular conditions the mortality in most aquaculture facilities is fairly low, however, so that it would be an expensive logistical challenge to collect these raw material quantities and concentrate them somewhere from where they could later on be taken for effective utilisation.
In comparison, the waste volumes that occur at fish processing facilities on land would seem far more profitable. Because fish processing lines are mostly only suitable for one particular fish species the waste that they leaves behind is of very consistent quality: it comes from only one species and can thus be divided up, in the case of cod, for example, into liver, roe, heads, skin, guts and backbone. This makes further processing considerably easier because the separate raw materials are available in larger quantities.
Fertilizers, animal feed and fishmeal
Even today, fish waste is mainly used in a very simple way. For example, it has for centuries been common practice in some regions of the world for unused fishes and fish waste to be used as fertilizer in agriculture. This technique is currently undergoing a revival in the western world in the form of “fish emulsions” and “hydrolysed fish”. Both products are rich in organic nitrogen and phosphor and also contain a variety of other substances that are indispensable for healthy plant growth. Under the influence of the organic and ecological movement some hobby gardeners object to the use of mineral fertilisers in their gardens and prefer to rely on these two products of fish origin. Fish emulsions are produced from gutted fishes that are heat-dried (to kill microorganisms) and ground. The nutrient-rich powder (N : P : K = 5 : 2 : 2) is later soaked in water and poured onto the roots of the plants. Fish hydrolysate consists of ungutted whole fish that are minced and treated with enzymes. To prevent excessive decomposition the resulting fish pulp is also acidified with phosphoric acid. This protects the proteins, vitamins and micronutrients (N : P : K = 4 : 2 : 2)
It was not until the end of the 19th century that the value of fish waste was recognized as a raw material for animal feeds and since then it has also been used in pig and poultry farming, either as so called fish silage or in the form of fishmeal. Fish silage is produced using a relatively simple cold technique which is very similar to the production process used for fish hydrolysate. During silage, too, the proteins are decomposed to a large extent and the fish waste is liquidized. The resulting pulp is stabilised using acids and preservatives to give it a longer shelf-life. In this form it can then be added to animal feed as a source of protein. Fishmeal has gained much more significance, however, as an all-round product. It serves as a protein-rich component in feeds for aquaculture and agriculture, but can also be used as a fertilizer (N : P : K = 10-12 : 6 : 2). For the production of a stable dry fishmeal the fish raw material is first heated and then the fish oil and water it contains is separated using pressure. This leads to the loss of about 70% of the raw material in the form of water and 10% in the form of oil (the oil is subsequently cleaned of suspended matter using centrifugation). What remains is then cooled and ground to fishmeal before being filled into sacks or left loose as bulk ware.
In recent years fishmeal has lost its status as an inexpensive source of protein and is in the meantime traded on the world market for extremely high prices. Processing fish wastes to fishmeal is thus by no means any longer a stopgap but a genuine alternative that often leads to higher profits than merely selling the raw material. This is a good moment at which to clear up the rumour that in aquaculture fishes are fed on fishmeal that is produced from the remains of farmed fishes of the same species. The fear that salmon, for example, might be fed on the remains of the previous salmon generation is completely unfounded. On the one hand the feed industry has voluntarily committed itself to refraining from this kind of feed recycling, and on the other hand the EU directives (EC) No. 811/2003 and (EC) No. 1774/2002 prohibit such practices. Fishmeal that is used as feed in aquaculture comes without exception from wild fishes.
Biodiesel from fish waste
What other options are there for using “fish waste“? One possibility which exists mainly for high-value fish species is the injection of fish “leftovers” into the fillet. During filleting little bits of fish are often left on the backbone. This, too, is high-quality meat, but it is difficult to use because of its small size. However, the little bits can be minced to a mash and then injected into the compact fillets or loins through a hollow needle. This makes it possible to make more complete use of the fish and increase the yield. A lot of consumers will probably, however, still object to the technique because they will see in it a form of deceit.
Another possible use for fish waste is processing it to diesel fuel. This is actually an almost obvious idea since, prior to the rise of petroleum, blubber from marine mammals was used for lighting and heating purposes. In practice, the production of biodiesel from fish is no longer a mere idea for it is already used commercially in numerous regions of the world, for example in Canada, Alaska and Hawaii, Vietnam and Honduras. In Alaska alone, over 80,000 t of fish biodiesel are produced per year. A large part of this is used by the fish industry itself, mainly for drying fishmeal. But cars run on it, too, when it is mixed into regular diesel. The energy content of fish biodiesel is about 6% lower than that of mineral fuel. Modern engines that are geared to thrift, seeking to get the very most out of every drop of fuel, thus sometimes have problems with the weaker eco alternative. Older engines, on the other hand, can mostly be run even on pure fish biodiesel. The electricity generators and vehicles in Denali National Park in Alaska, for example, all run on environmentally friendly fish diesel. The Honduran company Aquafinca Saint Peter Fish on Lake Yojoa operates its vehicle fleet solely with fish biodiesel, and even produces the fuel itself. Daily production of the fuel, which is about one US dollar per gallon cheaper than petrodiesel, is about 3 tonnes.
Numerous ingredients still not known
But fish waste can be used to make more things than just fertilizers, animal feed and biofuel. Scientists are on the lookout for “biologically active” substances in the heads, livers, eyes, bones, skins and other organs of fish because they have health-giving and other positive qualities… antioxidants, for example, which reduce the visible signs of aging, or natural UV protectors that penetrate the skin cells and thus protect them not only on the surface but also within from damage, or vasodilators which expand blood vessels and stimulate blood circulation within the skin. Bioactive substances which stimulate cell division or support the formation of collagen, the most important protein in the skin, are often the focus of research work. One important application for such substances is cosmetic skin care products to which the product developers want to add special (sales-generating) properties. The chances of coming up with something good during screenings or assays are quite high, as tests carried out by the New Zealand National Institute of Water and Atmospheric Research (NIWA) show. Researchers found substances with “bioactive effects” of very different kinds in 146 of 2,700 samples that came from 130 fish species. Before these can be used, however, they have to undergo further tests successfully.
Another promising possibility for using fish wastes is in so-called nutraceuticals. This word is a combination of “nutrition” and “pharmaceutical”. Nutraceuticals are biologically active and physiologically effective substances which are said to have medical and health values, for example for regulating cholesterol and blood sugar level or reducing the risk of cancer. Nutraceuticals can be used both preventively and therapeutically, and they are sometimes even mixed specifically into foods to produce functional food. So far, nutraceuticals have almost solely been taken from land-living organisms, particularly from plants (as in the case of dietary fibres, secondary plant substances, antioxidants, for example). However, there is no plausible reason to assume that they could not also be found in aquatic organisms. Measured by the variety of species that are to be found in the world’s oceans the potential for new discoveries beneath the water surface is even likely to be quite large.
Waste utilisation is a criterion for sustainability
Surimi is another excellent option for making good use of low-quality fish species, by-catches and processing waste. The FAO estimates that two to three million tonnes of raw fish are used worldwide for producing surimi. In the past, Alaska pollack was about the only fish species that could be used for surimi production. Surimi is a very good way of making good use of fish because whereas fillet yield is only about 28%, when processing the fish to surimi up to 82% of the fish can be used. Today, Alaska pollack only accounts for about half of the raw materials for it has been replaced by other fish species, mostly small, bony species, for which there is otherwise not much demand. In the meantime, the progress made in surimi production even allows fish species with dark-coloured flesh and a relatively high fat content, indeed even freshwater fishes like white or freshwater bream to be used.
Not only fishes, but also crustaceans offer profitable usage potential because the shells of crustaceans can be used for extracting chitin and chitosan. This can be particularly worthwhile if shrimps, spiny lobsters and other crustaceans are available in large quantities and are peeled centrally prior to sale. After cellulose, which is the basic component of wood, chitosan is the most frequent renewable natural raw material. The whitish powder, a long-chain sugar compound is produced by deacetylation of the chitin found in the shells of crustaceans. It is used in medical, pharmaceutical and cosmetic products as well as in agriculture and metallurgy. It is said to strengthen the immune system, regulate cholesterol level and accelerate cell renewal. It is also alleged to have antibacterial effects.
Chitosan production is a relatively complicated process in which numerous companies throughout the world have meanwhile specialised. First, the crustacean shells have to be cleaned and dried. After they have been ground and the larger pieces sieved out, the calcium carbonate is dissolved away and the remaining proteins removed. The resulting powder is sieved again, dried and thoroughly cleaned. The result is almost pure chitosan which can be broken down further by enzymes (lysozyme, chitosanase) into water-solvent components which can be absorbed better by the body. Nearly 100 kg chitosan can be produced from 4,500 kg shrimp shells.
Fish is a valuable resource and we can really no longer afford to waste even the tiniest bit of it. This awareness is making it necessary to rethink a lot of the processes used in numerous areas of the fish industry. By-catches have to be reduced further, for example, and discards have to be prohibited. But even that’s not enough: Some things are going to have to change in fish processing, too. Assessment of sustainability can in future no longer be measured only on achieving an optimum fillet yield but will also have to take into account to what extent and how well the resulting fish wastes are used.