## 1 Introduction  
In the last decade, a sharp increase in the marketing and consumption of fresh fish and fish products has been observed [1; 2]. EFSA [3] and WHO [4] recommend a consumption of 1-2 fish-based meals per week as fish is essential for a complete diet, due to its high content of proteins, free amino acids and other health-enhancing components such as vitamins, minerals and omega-3 fatty acids [5]. At the same time, the high level of water in the tissues (80%), a low percentage of connective tissue in the muscles and the poorly acidic pH of the tissues make the fish easily subjected to microbial attack and decomposition. Fish and other seafood may lead to a number of biological and chemical hazards such as pathogenic bacteria, viruses, biotoxins, heavy metals and biogenic amines [6; 7]. Histamine, (2-(4-imidazol)) ethylamine, CAS Number: 51-45-6) (HIM) is a biogenic amine that can be readily produced by bacterial decarboxylases in scomorbid and other fish with relatively high free histidine levels in their muscles when alive [8]. HIM production in fish is related to the histidine content of the fish, to the presence of bacterial histidine decarboxylase and to environmental conditions. Scombroid fish belonging to the families _Scombriade_ (e.g., tuna and mackerel) and _Scombresosidae_ (e.g., sauty) but also non-scombroid species belonging to families of _Clupeidae_ and _Engranulidae_ (e.g., sardines, anchovies, herrings) are characterized by having relatively high levels of histidine in their flesh (from 1 g kg\({}^{-1}\) in herring to 15 g kg\({}^{-1}\) in tuna) [9]. The formation of HIM in fish tissue is attributed to a range of microorganisms, such as _Hafriia alvei_, _Morganella morganii_, _Morganella psychrotolerans_, _Photobacterium phosphoreum_, and _Klebsiella pneumoniae_[9; 10]. Mesophilic organisms are mostly considered responsible for HIM production at an optimum temperature of 20 to 37 \({}^{\circ}\)C, producing more than 500 mg kg\({}^{-1}\) after 48 h [10]; however, Psychrophilic bacteria (_Photobacterium liiopiscarum_) have been found to produce HIM at levels higher than 500 mg kg\({}^{-1}\) after culture at 5 \({}^{\circ}\)C for 3 days, 10 \({}^{\circ}\)C for 36 h, and 20 \({}^{\circ}\)C for 18 h [11]. Food quality and freshness has been linked to HIM so that it is possible to use this compound as a fish product quality marker together with other indicators [12]. Ingestion of food containing low amounts of HIM can cause an allergy-like syndrome referred to as \"histamine intolerance\" [13]. However, in large doses, the normal metabolic mechanisms are insufficient for the detoxification. A HIM intake of 70-1000 mg per single meal may cause so-called \"scombroid poisoning\" [9], characterized by an incubation period ranging from a few minutes to hours and symptoms such as headache, nasal secretion, hypotension, tachycardia and possible death in sensitive subjects [14]. Safety requirements about HIM presence in seafood and rules for correct sampling were fixed by the European Commission in the Regulation (EC) No. 2073/2005 and subsequent modifications [15]. Sampling must be in nine units per batch, taking into account the inhomogeneous distribution of the toxin in the fish [16]. Therefore, nine independent analyses are needed to assess the compliance of a single batch product. For fishery products obtained from fish species with a high amount of histidine, (particularly the families _Scombriade_, _Clupeidae_, _Engranulidae_, _Coryfenidae_, _Pomatomidae_, and _Scombresosidae_) among the nine sample units analyzed for each batch, the mean value must not exceed 100 mg kg\({}^{-1}\) (m), two may have a value of more than 100 mg kg\({}^{-1}\) (m) but less than 200 mg kg\({}^{-1}\) (M) and no sample unit may have a value exceeding 200 mg kg\({}^{-1}\) (M). Higher HIM levels, not more than twice the above values, are permitted in fishery products which have undergone enzyme ripening treatment in brine. In addition, a further modification of Regulation (EC) No.