# Marine Bioactives and Their Application in the Food Industry: A Review
## 2 Marine Sources of Bioactive Molecules
### Marine Fish
#### 2.4.1 Fish Lipids  
Marine lipids are derived from the flesh of fatty fish, the blubber of marine mammals (e.g., seals and whales), and the liver of white lean fish, and they are composed of a mixture of fatty acids with different chain lengths and degrees of unsaturation. Several studies have revealed that the major compound of marine lipids is triacylglycerglycerol (TAG), which is abundant in monounsaturated fatty acids (MUFAs) and PUFAs. These fatty acids occur in different proportions that vary according to their source, the composition of the plankton/feedstuff, and the harvest time [5; 7]. Moreover, marine organisms are the best available natural sources of omega-3 PUFA as they are directly or indirectly linked by food webs with marine phytoplankton. These marine phytoplankton (uni- and multicellular marine plants) are considered primary producers of omega-3 fatty acids in the trophic chain, and they transfer these fatty acids, which can remain in liquid form, even under extreme marine conditions, into fish and marine mammals [85; 118]. The long-chain omega-3 PUFAs are mainly comprised of EPA and DHA, and they are considered one of the most abundant essential fatty acids in fish species. In recent decades, marine oils have gained much attention as a rich source of PUFAs due to their beneficial health effects. For example, pelagic species caught in large quantities are considered the main sources of fish oil. Salmon, herring, tuna, mackerel, and small fish such as anchovy and capelin are considered oily fish species that are widely used for fish oil production. Furthermore, several studies have revealed that fish species belonging to Scombridae, Clupeidae, and Salmonidae families contain the highest amounts of EPA and DHA [118]. For example, anchovy contains 21% EPA and 9% DHA, while sardine contains 16% of EPA and DHA. Typically, crude fish oil is considered to be the direct by-product of the fish meal industry. Recently, krill oil has gained great interest due to its unique composition. Krill oil is abundant in phospholipids, and most of the EPAs and DHAs in krill are bound to these phospholipids [130; 131]. However, the constitutes and configurations of marine oil from fish species and marine mammals are different. For example, EPA and DHA are present abundantly in most marine oils, while docosapentaenoic acid (DPA) is frequently found mainly in the oil of marine mammals [85; 132]. These marine oils are usually extracted from the blubber of marine animals like seals and whales. Moreover, the positional distribution of fatty acids in TAG also varies in fish and marine mammal lipids. Fish oils contain long-chain PUFAs mostly in the sn-2 position of TAG, while marine mammal oils are located primarily at the sn-1 and sn-2 positions [133]. Fish oils are one of the main dietary sources of long-chain PUFAs and contain \(\alpha\)-linolenic acid (ALA) and its longer-chain metabolites, namely EPA and DHA. Hence, consumers' preference for the consumption of fish oil is a growing global trend due to the multiple health benefits associated with the constituents of fish oil. The beneficial effects of omega-3 PUFA, particularly EPA and DHA, are crucial in preventing diseases like cardiovascular, inflammations, hypertension, allergies, and some forms of cancer while enhancing functional development and growth of the central nervous system, which directly influences infant growth and development [134]. Apart from oily fish, fish oil supplements are also considered one of the principal dietary sources of long-chain omega-3 PUFAs. However, the typical 'Western' diet is relatively high in omega-6 PUFAs and low in omega-3 PUFAs [7; 85; 134]. Therefore, alternative methods are needed to overcome this issue and ensure the intake of the required amount of omega-3 PUFAs. In order to achieve this, food-grade fish oils are often introduced to the market as capsules or in liquid forms. Fish liver oil, mainly cod or halibut oil, is generally used as capsules for medical applications [135].