Microplastic in Commercial Fish in the Mediterranean Sea, the Red Sea and the Arabian Gulf. Part 1: The Mediterranean Sea

Microplastic has become a ubiquitous environmental pollutant. Microplastic in the oceans has detrimental effects on aquatic organisms. The presence of microplastic in marine fish heightens the chance of finding microplastic in seafood targeted for human consumption. The Mediterranean Sea is known to suffer from significant plastic pollution. It is also one of the most thoroughly studied water bodies in regard to microplastic contamination. The manuscript reviews the available literature of 2015-2021 on the presence of microplastic in commercially important fish species in the Mediterranean Sea. The literature data on microplastic content on beaches, in subtidal sediment, in the sediment from the ocean floor and in surface water of different regions of the Mediterranean Sea is reviewed, also.


Introduction
Microplastics (MP) are small plastic particles of 5 mm or less in size. Either, these particles are synthesized in this small size (primary microplastics) or stem from the fragmentation of larger plastic pieces (secondary microplastics). One of the final resting places of MP is aquatic ecosystems, where there is significant transport of MP from land to water through run-offs [1], via wastewater treatment plants [2] and through air movement and subsequent atmospheric deposits [3] [4]. In addition, there is significant direct plastic input into the sea

Microplastic-Composition, Additives and Adsorbents and Their Potential Effects on Marine Organisms
MP can come in different forms such as pellets, fragments, filaments/fibers and films, and be made of a number of different polymer types such as polyethylene (PE), polypropylene (PP), polyurethane (PU) and polystyrene (PS). Interestingly, from a meta-analysis of 39 studies from different regions, Erni-Cassola et al. [24] determined the make-up of the most abundant polymers discharged into water bodies to be the following: PE (polyethylene, 23%), PP (polypropylene, 13%), PS (polystyrene, 4%) and PP&A (group of polymer types formed by polyesters, PEST; polyamide, PA; and acrylics, 13%). This coincides with the report of de Haan et al. that PE, PP, PS, polyesters are the most dominant polymer types found in the Mediterranean Sea [25]. Polymers carry additives. These can be small organic molecules such as phthalates [26], terephthalates, alkanedioates, trimellitates, trialkyl phosphates and siloxanes [27]. Furthermore, there can inorganic filling materials involved, mostly salts, that provide bulk but influence other properties of plastics as well such as stickiness of the surface of the plastic. In addition, there are colorants that can either be organic dyes or inorganic pigments. Small organic molecules can easily attach to the hydrophobic surface of the MPs, and so MPs can act as carriers of organic contaminants already present in the water body. Many of the organic compounds are known to bio-accumulate along the food chain. Thus, Capriotti et al. detected organic pollutants such as polychlorinated biphenyls (PCBs), organophosphous compounds, and polycyclic aromatic hydrocarbons (PAHs) on the surface of MPs collected in Italian coastal waters of the central Adriatic Sea [28]. K. Karkanorachaki [32]. Some of the additives in microplastics are mild estrogens and could act as hormonal disruptors under certain conditions. Other possible adsorbents such as polychlorinated biphenyls (PCBs) can act as neurotoxins [33] or can exhibit genotoxicity [34]. Neurotoxicity and genotoxicity have also been documented in animals exposed to microplastics under laboratory controlled conditions along with oxidative stress, reduced food consumption, growth, reproduction and survival rates [35] [36] [37]. Metals have also been seen to hitchhike rides on plastic surfaces of MP, with a recent study from the Mediterranean Sea showing aluminum, iron, chromium, zinc, nickel, molybdenum, manganese, lead cobalt, and copper at concentrations of mg/kg plastic and arsenic, vanadium, rubidium, and cadmium at concentrations of µg/kg plastic [38].

Abundance of Microplastic in the Mediterranean Sea
The concentration of microplastic [39] as of plastic [40] in the Mediterranean Sea has been described to be very high compared to other large water bodies, representing as much as 5% -10% of global MP by weight [41]. Floating plastic debris in the Mediterranean Sea has already been investigated scientifically in 1980 [42]. Different models have been forwarded that reflect the input and distribution of MP in the Mediterranean Sea [43] [44]. For their model, Kandoorp et al. [43] estimated that in 2015, there was an input of 2100 -3400 tonnes of MP, and that of plastics released since 2006, about 170 -420 tonnes remain afloat in the surface waters, 49% -63% ended up on coastlines, and 37% -51% have sunk down to the sea floor. With many MP possessing positive buoyancy in salt water and with microbial colonization and biofilm formation on MP changing their density over time [45], this leaves about 1% of total MP in the water column according to the model [44].
Physical studies on the occurrence of MPs along the coast, the water column,   While litter concentration is generally high on the seafloor along the Mediterranean coast in comparison with many other European coastlines, the concentration is particularly high near shipping lanes, mussel farms, the entry of large rivers [78] [96] and near large cities [97], for instance along the south-east French Mediterranean coast [98], where the litter count was up to 913 ± 80 items/km 2 , with 80% of the items being plastic [97]. Constant et al. [4] have calculated the annual MP loads of the rivers Rhône and the much smaller Têt to be 5.92 and 0.09 t, respectively, that are discharged into the Mediterranean. Apart, from the Rhone, large discharges can also be seen from the Po, Seyhan and Nile rivers [99]. In this respect, hydrological data of such river basins such as the Seyhan river will be of value to predict future discharges [100]. Large cities that seem to contribute significantly to plastic input are Izmir, Barcelona, Alexandria and Tel Aviv, in the vicinities of which MP concentrations are especially high [99]. Vlachogianni et al. (2017) [101] found that shoreline and recreational activities accounted for approx. 33.4% -38.5% of the marine litter, in general, in different marine compartments in the Adriatic and Ionian Sea, with up to 23.5% coming directly from the sea, and with 6% (beach), 9% (surface water) and 17% (seafloor) coming from fishing gear. MP concentrations are often higher near the coast, i.e., within 1 km of the coastline than further out, though patches with high concentrations of MP can also be found at great distances from the coast (eg., >50 km) [81]. Further off-shore, ocean currents appear to transport microplastics from shallow to deep water, dispersing MPs from open slopes and entraining microplastics transported downslope by gravity currents in submarine channels linked to the coast [102] [103]. Thermohaline bottom currents influence the distribution of MP on the seafloor, leading to hotspots of up to 3.8 MP/g floor sediment [102]. Kane et al. [102] found lower MP concentrations on the continental shelf and the upper slopes than at a water depth range of 600 to 900 m, near the sea bottom. Thus far, research has not shown that the nature/character of the sea floor, apart from its contours, and the marine habitat it represents plays a significant role on MP accumulation [56]. As the distribution of MP is driven in part by seasonally changing currents and as plastic input varies throughout the year as well, local MP concentrations can have considerable seasonal variations [95]. Nevertheless, by region, certain areas such as the Ligurian Sea (mean value 0.94 MP/m 3 ) were found to have a higher MP concentration than others such as the Sardinian Sea (mean value 0.13 MP/m 3 ) [31].

Microplastic Abundance in Fish Species in the Mediterranean Sea
The authors have found reports for 87 fish species for which plastic ingestion has been recorded, many of which are commercially important. The most affected are demersal (30.4%), pelagic (15.9%), benthopelagic (24.6%), bathypelagic (11.6%), and bathydemersal (5.8%) fish [18]. rates from the Adriatic Sea as well, with 64% of the fish specimen with MP. Also, red striped mullets caught in Adriatic Sea [113], along the coast of Turkey [93] Journal of Water Resource and Protection and also in the Balearic Islands [120] were found frequently with MP content (23.5% -70% of the fish). Other goatfish species that have been studied include the golden grey mullet (Chelon auratus), the goldband mullet (Upuneus moluccensis) and Por's goatfish (Upeneus pori). Most of these were studied by Güven et al. [93] along the Turkish coast, where in 29% -36% of the fish's stomach MP was found. Anastasopolou et al. [113] report a very MP intake by Chelon auratus in the Northern Adriatic Sea (Slovenian Sea) with 95% of the fish specimen studied having ingested MP, with a large number of MP found in the fish. Merluccius merluccius, the European hake, is commercially a very important fish that can be found in the East Atlantic, the Black Sea and the Mediterranean Sea. About 100,000 tonnes were landed by European fisheries in 2010, but the species is thought to be overfished. It is a predatory fish, predating also on pilchard and European anchovy. MP ingestion is relatively high for Merluccius merluccius [121] [122] [123], with the exception of data from two studies in the North Thyrrenian Sea [121] and the East Ionian Sea [113]. Looking at mackerels from the Carangidae family, early data, from 1998, showed little ingestion (1% of the fish studied) in both Mediterranean horse mackerel (Trachurus mediterraneus) and blue jack mackerel (Trachurus picturatus), where the data may reflect the time period [124]. Later investigations showed much higher rates of ingestion, with 10% -48% of the fish from both the East Mediterranean along the Turkish coast [93] and the West Mediterranean Sea (Balearic Islands) [108] showing MP.

Also, the Atlantic horse mackerel (Trachurus trachurus), caught in the South
Adriatic Sea, showed appreciable (24% of the fish) MP ingestion. The Mediterranean horse mackerel is commercially an important fish, both in the Mediterranean and the Black Sea, although regionally the catch has declined due to overfishing [125]. The Atlantic horse mackerel has been classified as a vulnerable species. Looking at the mackerels of the Scombridae family, data is known for the Atlantic chub mackerel (Scomber colias) and the Pacific mackerel (Scomber japonicus) as well as for two tuna fish, the albacore (Thunnus alalunga) and the bluefin tuna (Thunnus thynnus). The annual Thunnus alalunga catch in the Mediterranean is variable, but in 2010 it amounted to 2123 tonnes, with Italy being the main producer at 1109 tonnes [126]. In 2009, F.S. Karakulak et al. studied stomach contents of 218 bluefin tuna (from the Eastern Mediterranean Sea and had found plastic in 37 stomachs [127]. These plastics could be labelled as meso and macroplastics. At the time, the presence of MP was not documented. In 2015-2020, a number of studies were forwarded regarding MP ingestion in these fish, where the percentage of fish with MP was recorded at 12.9% for Thunnus alalunga [128], 32.3% for Thunnus thynnus [128], 66.7% for Scomber colias [123] and 43.2% -57% for Scomber japonicus [93]. In addition to B. boops, a number of seabream and porgy species were the subject of studies in the Adriatic Sea and the East Mediterranean Sea. Most studied is the common pandora (Pagellus erythrinus). The annual worldwide production of the common pandora decreased from about 10,000 tonnes (1981)(1982)(1983)(1984)(1985) to 4500 tonnes [129]. The species is overexploited in several Mediterranean subareas. A study from the North Adriatic Sea and the NE Ionian Sea with altogether 80 specimen showed low MP ingestion (2% -3.3%) [101]. Other studies, partially from the same area, but also from the East Mediterranean showed a higher percentage of fish with MP (22% -50%) [ [134], the thornback ray (Raja clavata, 2 specimen, 0% MP) [134] and the brown ray (Raja miraletus, one ray found with MP content) [138] have been analyzed for MP content.

Conclusions
Overall, the published data on MP content in fish of the Mediterranean Sea is quite heterogeneous. Keeping in mind that most of the data stems from 2013-2020 and that therefore the research area is still rather novel, having been previously focused on meso-and macroplastic ingestion, and that the research methodology has been refined over the years, early data [101] [134] often shows relatively little MP ingestion. Otherwise, there are areas within the Mediterranean where fish during the time of the study showed less MP ingestion. A typical example comes from the species B. boops where only 16.7% of the fish were found with MP in a study from the Balearic islands [108] and 13.0% in a study from South Sardinia [107]. Again, this data represents a "screenshot" of the species in that location at that particular time. Fish in neighboring locations within the same general region may give completely different data ( [104], see above).
There is an indication that deep sea catches show less MP content, though some of the data come from the early years of study (see above). Also, seasonal variations in currents and waste discharges can bring about significant differences in MP content along the water column as isolated events such as ship wrecks [140] or ships losing cargo en route. For litter in general, Vlachogianni et al. [101] [141] found the average number of litter per gut higher in fish from the South Adriatic Sea (2.2 P/gut) than from the northeastern Ionian and North Adriatic Sea (1.0 P/gut). Journal of Water Resource and Protection What danger MP containing fish represent to humans is debatable, as with the gutting of fish most MP content is avoided. However, about 20% of fish landings go into fish meal which then is affected by the MP content in the fish. As fish meal also constitutes the offal of fish, the risk to utilize parts of the fish that hold MP is quite high [142]. E. encrasicolus, S. pilchardus, and T. trachurus are some of the main fish species that contribute to fish meal production, and studies have shown that these species carry MP content, when caught in the Mediterranean Sea.
To curb the increase in MP in the Mediterranean Sea, it is important to limit the entry of plastics in general by a better waste management at the community level, especially in the larger cities on the Mediterranean coast. Certain contributors to primary MP such as microbeads in cosmetics will be phased out in the near future and will no longer be of importance, others such as textile fibers and microtires will remain contributors to MP in general, although microtires specifically have not been recorded in fish to date. More important will be secondary MP resulting from the gradual breakdown of the overall solid plastic waste entering the Mediterranean Sea. Even without the introduction of additional plastic waste, secondary MP will remain with us for an indeterminable time in the future due to the long half-life of plastic materials, much of which will lastly degrade by passing through MP stages.