1. Introduction
Fish production displays an increasing trend and according to FAO the total world catches amounted to 96.4 million tons in 2018, with an increase of 5.4% compared to the previous three years [
1]. In 2020, fisheries and aquaculture production reached 214 million tons and the global consumption increased from an annual average of 9.9 kg per capita in the 1960s to 20.2 kg [
2]. Fish products are an excellent source of nutrients, especially proteins, vitamins, minerals, and long-chain polyunsaturated fatty acids (PUFA), including omega-3 [
3]. Nevertheless, they are very perishable foods because of post-mortem modifications and formation of spoilage compounds, such as alcohols, organic acids, aldehydes and ketones, sulfides, histamine, and other biogenic amines [
4,
5].
The decay of fish is mainly caused by microorganisms, but also enzymatic activity and lipid oxidation are responsible for the decrease of freshness [
6]. Good hygiene and proper storage are essential to maintain the quality and safety of fish throughout the fish supply chain from harvest to consumption. Various handling activities (i.e., selecting, gutting, slicing, packing, etc.) taking place on board vessels may represent sources of contamination due to the possibility of dissemination of bacteria through human hands, trays, buckets, floor, etc. Proper cleaning of equipment and facilities as well as frequent washing of hands are effective to manage quality control of fish during these operations.
Large numbers of bacteria belonging to the genera
Acinetobacter,
Aeromonas,
Bacillus,
Clostridium,
Lactobacillus,
Micrococcus,
Moraxella,
Pseudomonas,
Shewanella,
Photobacterium,
Vibrio, etc. are especially found on the skin, in the gills and the gastrointestinal tracts of fish [
7]. They can spread to the flesh during some operations carried out on board vessels and grow when temperature conditions are favorable. The good hygiene practice after capture is noteworthy as it can affect the successive stages of the fish supply chain. The storage temperature of fish after caught is one of the most important measures for preserving and avoiding the microbial growth as well as the production of dangerous substances particularly histamine, which causes a variety of allergy-like symptoms, such as rashes, flushing, swelling of the face and tongue, sweating, nausea, vomiting, diarrhea, headache, dizziness, palpitation, oral burning, metallic taste, and hypotension. Life-threatening cases have also been reported [
8]. Besides scombroid poisoning, many foodborne outbreaks due to the consumption of fish and shellfish are reported every year around the world and they are caused by pathogenic microorganisms, such as
Salmonella spp.,
Vibrio spp.,
Listeria monocytogenes, viruses (hepatitis A virus, norovirus, etc.), and parasites [
9].
This review focuses on the hygiene measures to be applied on board vessels to ensure fish quality and avoid its contamination with different hazards, such as bacteria, viruses, and parasites, as well as histamine formation.
2. Structural requirements for vessels and correct handling and storage of fish
According to Regulation (EC) 853/2004, vessels used to harvest fish products or to handle or process them after catch must comply with some structural requirements, such as the use of equipment and food contact surfaces of corrosion-resistant material, which must be smooth and easy to clean and disinfect. Vessels must be constructed so as not to cause contamination of the products with bilge-water, sewage, smoke, fuel, oil, grease or other objectionable substances. Further obligations are set for vessels designed and equipped to preserve fishery products for more than 24 h, and for freezer and factory vessels, i.e., any vessel on board which only freezing and operations such as filleting, slicing, skinning, shelling, shucking, mincing, or processing are carried out, respectively (
Figure 1).
The application of good hygiene practice on board vessels is imperative to ensure both quality and safety of fish. The first aspect (i.e., fish quality) is particularly associated with freshness, as fish is very perishable because of many intrinsic factors, such as a high moisture and protein content favoring microbial growth, lipid oxidation, especially for PUFA, and the glycogen amount in muscle tissue. The latter can be influenced by the fishing method, e.g., line-caught fish are stressed during harvesting and show lower glycogen than net-caught fish, so that the production of lactic acid is little, and the final post-mortem pH is higher (6.0-6.7) [
10].
Due to its chemical composition, i.e., high water activity, relatively low acidity and significant presence of non-protein nitrogen compounds, this type of food is an ideal substrate for microbial colonization by many spoilage bacteria, the so-called specific spoilage organisms (SSOs). Poor hygiene practices and improper conditions (e.g., abuse temperature) in harvesting, handling, processing, and storage, can favor the growth of such microorganisms [
11,
12]. When SSOs become dominant in the total microbial population, they produce some metabolites responsible of off-odors and off-flavors. Their precursors are carbohydrates for various organic acids, the amino acids cysteine and methionine for sulfur by-products (e.g., dimethyl sulfide), and other amino acids for ammonia and various carbonylic compounds [
13].
Freshness of fish is very important for both food industry and consumers, as it determines the acceptability of the product. The assessment of freshness is based on an organoleptic exam regarding some aspects, i.e., skin and skin mucus, eyes, gills color and smell, and flesh texture.
Table 1 resumes the criteria established in the Regulation (EC) 2406/1996 for whitefish and bluefish classified in the extra freshness category. High quality fish have a seaweed-like smell and not a fishy odor, gills are red, not brown or grey, skin is shiny and firm to the touch, eyes are convex and bright, and not flat or concave, flesh is firm and not soft or flaccid. The sensory analysis based on such regulatory criteria presents many disappointments, because it does not evaluate the species-specific spoilage characteristics, and there is the possibility of highlighting one criterion among others as discriminative to rise the freshness grade. Moreover, no specification for sampling and examination of fish is provided, so that training and experience from operators are required.
There is another test named Quality Index Method (QIM), which is more specific for the individual fish species and evaluates a higher number of attributes. It gives different points to each attribute, generally from 0 to 3 (0 indicates optimal conditions, while 3 is given when fish is almost spoiled) and the overall quality index is obtained from the total scores [
14]. Many differences exist among fish species to consider when a sensory exam is performed. For instance, fish belonging to the
Gadidae and
Merluccidae families exhibit early soft meat after capture and have high levels of total volatile nitrogen compounds responsible of definite smell; fish of the family
Anguillidae show natural mucus on the skin, and
Pleuronectidae have small eyes resulting difficult to check as freshness parameter. While the Regulation (EC) 2406/1996 considers only two fish categories, i.e., whitefish and bluefish described in
Table 1, the QIM takes in account the specific characteristics of each species and therefore its scheme is more detailed and precise.
The fishing method can affect freshness and its evaluation. For instance, fish, selachians, cephalopods and crustaceans belonging to the extra freshness category must be free from signs of pressure or injuries that can be caused by trawling. Several studies demonstrated the effects of different fishing gear, such as trawls, gillnets, traps, trolling rods, longlines and handlines, on fish quality [
15]. Other factors may influence such criterion, such as the fish species and the stress suffered during catch, the time and temperature of storage, and the amount of ice used to cover it [
16]. By contrast, dehydration of fish can occur if they are left uncovered and exposed to the sun, so that the skin can loss brightness.
Physical damages can be also caused by fishermen when they handle fish on board vessels [
10]. The fish flesh is delicate compared with the muscle tissue of mammals, and it is easily injured. Bruising appears when blood seeps into the flesh and clots, while gaping occurs when layers of muscle tissue separate. These lesions can be caused by slamming fish onto the deck, or throwing them into a fish box, and over-filling it so that fish on the bottom are compressed. They can appear several days after the fish is harvested, so it is important to handle the fish gently from catching to off-loading [
17].
Some operations carried out on board vessels can reduce the microbial load of the product. For instance, dressing can include gilling and gutting, but also heading and finning. Small fish are generally chilled as whole and therefore they are minimally handled, whereas large fish must be stunned and bled, and then dressed as soon as possible. All surfaces in contact with fish should be rinsed to remove blood, slime, and offal, and cleaned with a mixture of seawater and detergent [
17]. All equipment used for catching and processing fish should also be cleaned and sanitized. Finally, fish should be protected by contamination from fuel and oil, chemicals, birds, and flies [
18].
The way that fish is stored (whole, fillet, or gutted) also contributes to its final quality. In most studies, whole chilled and frozen fish present longer shelf life than those preserved as gutted and filleted. However, evisceration can hurdle the microbial spread in muscle tissue as well as the so-called belly bursting or burnt belly, produced by the action of digestive enzymes present in the fish gut [
19]. Such enzymes cause massive gas development due to lipid hydrolysis and oxidation, as well as fish muscle softening from extensive autolysis [
20].
Chilling and/or freezing fish after caught remain the most valid preservation method to apply from on board vessels to consumption. Maintaining the cold chain at a temperature approaching that of melting ice reduces both microbial growth and enzyme activity, but it can damage fish if the chunks of ice are large or if the seawater with ice is too cold and fish can freeze. Cooling should be made with a bed of ice deep enough so that when the ice melts, the fish will not be in contact with the bottom of the fish box. Fish should be arranged in rows, side-by-side but not touching each other, and should be completely covered with ice. About 2 kg of ice are necessary to properly chill 1 kg of fish, and a layer of ice over fish is essential so that they do not dehydrate [
17]. Small ice crystals allow for better contact with fish and reduce physical damages, while larger pieces exert more pressure, injuring the fish tissue [
21].
Superchilling is another process used to increase the shelf life and consists of two stages; the product is firstly cooled to its initial freezing point and a little part of its water content (5%–30%) is frozen. Then, the formed ice absorbs heat from the food interior and for short period of transport or storage, it provides a refrigeration reservoir around the product [
22]. In addition, there is a smaller amount of structural damage caused by ice crystals compared to freezing, and the shelf life can be prolonged from one and a half to four times than after chilling [
23]. Gutted cod superchilled onboard vessels as whole or fillets showed a decrease of microbial activity and protein decomposition in whole fish, and an extension of both freshness (2–4 days) and shelf life (3 days) in fillets [
24].