An Overview of Atlantic Bluefin Tuna Farming Sustainability in the Mediterranean with Special Regards to the Republic of Croatia

Abstract

Atlantic bluefin tuna (Thunnus thynnus) is the most important tuna species in Mediterranean tuna fishery and a valuable commodity on the global fish market. Croatia is a pioneer in tuna farming in the Mediterranean and the only country that has the exclusive right to farm wild-caught juvenile tuna (8 to 30 kg). This paper identifies key challenges to the sustainability of current farming and fattening practices, primarily economic and environmental, and possible solutions to overcome these challenges. This paper analyses data on tuna catch and aquaculture production (FAO FishStatJ and EU-Eurostat database) and updates the latest literature on farming practices, production challenges related to biotechnical, economic and environmental issues, the market and current legislation in Croatia, as well as fattening in other Mediterranean countries. Tuna capture-based aquaculture is attractive to investors because it promises high returns, but the sustainability of intensive tuna farming and fattening is questionable and raises many ethical issues. Tuna farming and fattening relies on wild fish for stocking and feeding, and further expansion of tuna farming and fattening is limited by the size of wild tuna and small fish populations. To meet the growing global demand for tuna and to conserve wild tuna stocks, further investments are needed. The knowledge gained in Croatian tuna farming is valuable for future sustainable close-cycled tuna farming in the Mediterranean. Due to its good environmental status, the availability of small pelagic fish, the availability of a highly qualified and well-organised labour force, the good cooperation between producers and researchers, and the application of modern farming technologies, ABFT farmed in Croatia have high quality and a good reputation on the market. The main weakness of Croatian tuna farming is that the entire industry is dependent on the Japanese market, but this can be overcome by the possibility of product diversification for new markets, including the tourism industry.

1. Introduction

Aquaculture is an important source of food, jobs, and income for millions of people. Since the 1960s, shortages of wild-caught fish and a growing demand for high-quality fish protein have spurred aquaculture production, and aquaculture continues to contribute to a 56% increase in the global aquatic food supply [1]. The European Green Deal, Farm-to-Fork Strategy, and strategic guidelines for more sustainable and competitive aquaculture in the EU highlight the importance of aquaculture to a sustainable food system and the importance of farmed fish as a low-carbon footprint source of protein [2]. As demand for fish protein continues to increase and many wild fish stocks are overexploited, aquaculture is playing an important role in feeding the world effectively, equitably, and sustainably and in providing a sustainable solution for rebuilding fish stocks [1,3]. Tuna is a valuable commodity on the global fish market because of its delicious taste and high market price. Tuna are larger, grow more slowly, mature later, and have a shorter spawning period than other warm-water predatory fish [4], making tuna vulnerable to exploitation [5]. Since the 1980s, increased demand for tuna and high market prices have led to unsustainable fisheries and reduced many tuna stocks in the wild [6]. Unlike the tuna steak and canned tuna industry, the tuna sushi-sashimi industry is highly selective and only a few tuna species are marketable on the sushi-sashimi market, including bluefin tuna (BFT): Atlantic bluefin tuna (ABFT) (Thunnus thynnus. Linnaeus, 1758), Pacific bluefin tuna (Thunnus orientalis, Temminck and Schlegel, 1844), and southern bluefin tuna (Thunnus maccoyii, Castelnau, 1872) [7,8]. In the Mediterranean, ABFT is the most important commercial tuna species in both fisheries and tuna aquaculture [6,9]. Due to its high fat content, ABFT fetches a high price on the Japanese market. Farming of ABFT started in 1996 in Spain and Croatia, and Italy, Malta, Spain, and Croatia are leading producers [10]. In the Mediterranean, ABFT rearing is actually the fattening of reproductively mature wild-caught tuna (over 30 kg) in cages for six to ten months, while Croatia has the exclusive right to farm wild-caught immature juvenile tuna (8 to 12 kg) for 1.5 years or even longer (farming). As ABFT farming and fattening rely on wild-caught ABFT for stocking and ABFT feeding requires a large amount of wild-caught small pelagic fish, the expansion of ABFT farming and fattening in the Mediterranean in the late 1990s increased pressure on wild fish populations as well as on the entire ecosystem [6,9,10,11,12,13].

The aim of this paper is to give an overview of the sustainable farming/fattening industry in the Mediterranean, with special attention to Croatian tuna farming. Particular attention is given to the environmental impact of ABFT farming/fattening. The paper consists of five chapters: ABTF fishing in the Mediterranean; ABFT fattening/farming in the Mediterranean; Fattening/farming challenges; Sustainable ABFT farming; and Conclusions.

2. ABTF Fishing in the Mediterranean

Until the 1950s, red lean tuna meat was popular for sushi-sashimi, but Japanese tastes changed and high-fat tuna species, including ABFT, became highly valued [7]. The development of the Japanese sushi-sashimi market in the 1980s made the ABFT fishery extremely profitable [14], especially in the Mediterranean Sea [5]. The increased demand for tuna and the high market price have led to unsustainable fishing (including illegal, unreported, and unregulated (IUU) fishing) and the depletion of tuna stocks [6]. In order to preserve tunas and tuna-like species in the Atlantic Ocean and adjacent seas, the International Commission for the Conservation of Atlantic Tunas (the Commission) establishes mandatory management measures to regulate fishing effort, fishing and farming capacity, Total Allowable Catch (TAC), the minimum size at landing, farming, and time-area closures [15]. There are currently 52 Contracting Parties to the International Convention for the Conservation of Atlantic Tunas [16]. The Commission manages the ABFT as the Eastern Atlantic and Mediterranean BFT stock (E-BFT) and Western BFT stock (W-BFT) with the 45°W meridian as the boundary [5]. W-BFT TAC has been in force since 1981, E-BFT TAC since 1998, while the minimum size limit for ABFT caught in the Atlantic of 6.4 kg (10 kg and 30 kg thereafter) has been in force since 1975 [17]. The 6.4 kg conservation measure was violated for decades in the East Atlantic and Mediterranean fisheries, resulting in the death of millions of fish [18].

The commercial ABFT fishery, including fishing gear, area, and season, has changed since 1996, when ABFT fattening and farming began in the Mediterranean [19]. In the period between 1995 and 2007, E-BFT catches in the Eastern Atlantic and Mediterranean were significantly underreported, and the actual annual catch was estimated to be between 50,000 t and 61,000 t [20]. Since 2008, reported catches have declined and stabilized at TACs levels [21]. In the period between 1991 and 2021 (Figure 1), reported E-BFT catches in the Mediterranean Sea fluctuated between 5790 t (2011) and 52559 t (2007). Reported catches in Croatia fluctuated between 374 t (2012) and 1360 t (1996, when ABFT farming was introduced in Croatia). In 1998, a TAC quota was introduced.

Since most of the catch is used for stocking, the ABFT fishery is dominated by purse seine fishing (63–93% of reported catch) over hand trolling, baitboat, longlines, longline fishing, and trap fishing because this method catches fish alive [9]. According to [13], 99% of the purse seine tuna catch in the Mediterranean is used for fattening. The combination of TACs and size limits has positive effects on E-BFT stock abundance [21]. Since current E-BFT stock status does not require the emergency measures introduced under the Recovery Plan for bluefin tuna in the eastern Atlantic and Mediterranean (Recommendation 17-07 by ICCAT), new Multi-Annual Management Plan for Bluefin Tuna in the Eastern Atlantic and the Mediterranean (the Recommendation 21-08 by ICCAT) entered into force on the 17th of June 2022. The Recommendation 21-08 sets E-BFT TAC at 36,000 t for 2022, and the minimum E-BFT size is 30 kg or 115 cm fork length, while the minimum size for E-BFT caught in the Adriatic Sea for farming purposes is 8 kg or 75 cm fork length, the same as in the Bay of Biscay and some artisanal fisheries [22]. The E-BFT quota for Croatia is set at 950.3 t for 2022 [23].

ABFT Fishing in Croatia

As a European Union (EU) Member country, Croatian fishery is under the Common Fisheries Policy (CFP). The legal framework governing Croatian marine fisheries includes the Marine Fisheries Act (OG 56/10, 127/10, 55/11) and the Act on structural support and market organization in fisheries (OG 153/09, 127/10), as well as special regulations on E-BFT fishing (including purse seine fishing, recreational fishing for trophy tuna, etc.) [24].

The purse seine is the most important fishing gear in Croatian ABFT fishery and the Regulation on fishing opportunities and the allocation of the state quota in 2022 for fishing for bluefin tuna (Thunnus thynnus) (OG 16/2022) set the quota for ABFT purse seine fishing at 833.46 t for 2022 [25]. Purse seine fishing for ABFT is allowed in the Eastern Atlantic and the Mediterranean from 26 May to 1 July, while for fish farmed in the Adriatic Sea, the season may be open from 26 May to 15 July in the Adriatic Sea (FAO fishing area 37.2.1) [22]. The minimum size for ABFT caught in the Adriatic Sea for farming purposes was set at 8 kg or 75 cm fork length based on historical data on the size composition of Croatian ABFT catches. Purse seine tuna fishing in the Croatian Adriatic began in 1929, when only one purse seiner was registered, but by 1954 there were 36 boats [26]. Commercial tuna fishing in Croatia has developed significantly since World War II. The first phase of development, which lasted from 1947 to 1964, ended with the disappearance of traps. In the second phase, the Croatian ABFT fishery has been dominated by purse seine fishing since 1965 [27].

Based on an analysis of the size composition of Croatian (Yugoslav) ABFT catches, [26] found that tuna in the 4–5 kg weight class and the 9–10 kg weight class accounted for 70% of catches in the period from 1947 to 1969, while specimens weighing more than 25 kg were very rare. An analysis of the size composition of ABFT purse seine catches from 1978 to 1981 showed that the 8–15 kg weight class dominated with 80% [27]. According to [28,29], tuna weighing 7–15 kg dominated Croatian catches in the period 1990–1997 with 86%.

3. ABFT Fattening/Farming in the Mediterranean

3.1. History

The commercial rearing of BFT began in Canada in the late 1960s by a Japanese farm and then spread to Spain in 1979 and throughout the Mediterranean region in the 1990s [19]. ABFT rearing operations are classified as [30]:

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fattening, when mature fish (30 kg or more) are reared for a short period of time (3 to 7 months) to achieve a high percentage of fat in the muscle;

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farming, when juvenile fish (8 to 30 kg body weight) are reared for an extended period (up to 2 years) to increase body weight and achieve the minimum size for the Japanese market.

In 1996, commercial ABFT fattening for the Japanese market began in Spain and spread to Malta in 2000, Italy in 2001, Turkey in 2002, and Tunisia in 2003. The main producers are Italy, Malta, Spain, and Croatia [10]. Croatia is a pioneer of ABFT farming in the Mediterranean. In 1996, Croatian fishermen with experience in southern BFT farming in Australia returned to Croatia and began ABFT farming [19]. According to [13], Croatia is allowed to farm juvenile tuna based on historical data on the size composition of Croatian ABFT catches, and the fact that Croatia, a pioneer in tuna farming, produces relatively small quantities of tuna.

3.2. Rearing Technology

In the Mediterranean, ABFT farming and fattening includes catching wild tuna with purse seines, overfeeding tuna with small pelagic fish in floating rearing cages, and exporting it for the sushi-sashimi market in Japan [6,9,10,13].

In ABFT fattening, mature tunas (30 kg or more) caught by purse seiners are transferred into circular (30 to 50 m diameter, 20 m deep) or hexagonal (22 m side length) transport cages and towed by a tugboat at a speed of 1 to 1.5 knots to the rearing site [13,14]. In the western Mediterranean, tuna have been shown to spawn inside transport cages [31]. At the rearing site, fish are placed in circular ring-shaped (50 to 120 m in diameter and 15 to 35 m deep) floating deep-sea net cages with a mesh size of 25 cm for rearing [32]. During the rearing period (of an average six months), fish are fed ad libitum with baitfish one to three times per day [9]. The reported stocking density ranges from 2 to 6.2 kg/m³ [33], and mortality ranges from 3.7 to 15.8% in most Mediterranean farms [13].

In ABFT farming, immature tuna (from 8 to 30 kg) caught by Croatian and Italian purse seiners in the Adriatic Sea [19] are reared for up to three years [34] until they reach a harvest size between 30 and 50 kg [35]. Ref. [36] reported that 2-year-old, 10-kg ABFT juveniles grow to 45–90 kg during a rearing period of 18 to 30 months.

ABFT juveniles are kept in circular floating net cages (30 to 60 m in diameter and 13 to 21 m deep) [9,13]. Tuna is fed fresh or frozen small pelagic fish ad libitum one to six times daily, and the daily feed consumption is about 5% of the biomass in a temperature range of 18 °C to 24 °C [11]. Reported stocking densities range from 1 to 2 kg/m³ [33] and mortality ranges from 3% to 5% [11].

3.3. Technical Requirements for Farming Facilities for Bluefin Tuna (FFBs)

Tuna is a large pelagic fish that needs to swim constantly and has a high oxygen demand. Therefore, tuna must be reared in large, robust cages in exposed oligotrophic coastal waters at least 50 m deep or offshore, with currents greater than 10 cm/s, salinity between 36 and 39 ‰, and dissolved oxygen levels greater than 90% (Croatian Regulation on Criteria for the Establishment of Marine Aquaculture Areas, OG 106/18). If FFB is appropriately sited and managed, the potential negative impacts of FFB on the environment are usually small-scale and temporary.

3.4. Harvesting and Processing

The harvest season is usually December, when prices are high due to the many Japanese festivities [8]. The price of tuna correlates with the quality of the meat, and the quality of the meat is highly dependent on feeding, but also on the slaughter, processing (bleeding and cooling), storage, and shipping of the tuna [10]. To prevent metabolic products from accumulating in the muscle, which impart a bad taste to the meat, tuna must be killed, bled, and cooled quickly [37]. The shotgun method (the tuna is shot in the head from outside the water) is good for killing a large number of tunas in a short time. In the speargun method (the tuna is shot in the head by divers with a speargun), the tuna can be picked out and death occurs immediately, but it takes more time [13]. Further processing includes pithing, which involves using a metal rod to induce brain death, bleeding, which removes metabolic products and allows rapid cooling, the removal of internal organs, and placement in a slurry of ice and seawater for rapid cooling [10].

3.5. Market

The tuna price depends on the size of the tuna (the bigger the better), the fat content (the fatter the better), the colour and texture of the meat, the absence/presence of metabolites in the meat, and the condition (fresh/frozen) of the meat [13]. All cultured ABFT are sold on the Japanese sushi-sashimi market [7,10], but unfortunately the majority of reared ABFT is graded as low to medium quality [6]. The development of ABFT aquaculture in the Mediterranean has increased the supply of ABFT, which has led to a drop in tuna prices. The sushi-sashimi market has also changed, from an exclusive Japanese market to a global market, which has also affected the price of tuna [7]. In the 1980s, the price of ABFT on the Tsukiji market ranged from US$ 100 to US$ 300/kg [7]. ABFT can be sold fresh or frozen. Since 2002, most of the ABFT aquaculture production has been exported frozen as a whole to Japanese freezer ships (so-called free on board, where the tuna is loaded on board and all other costs are supported by the buyer), which has resulted in lower costs and also lower tuna prices [13]. The fresh ABFT is transported to Japan by air [6]. In 2015, the price of fresh lower grade ABFT (headed and gutted) ranged from € 8 to € 17/kg and the price of higher grade ABFT (sashimi) ranged from € 35 to € 80/kg [6]. In 2022, the price of ABFT from Spain ranged from €18 to €37/kg [38]. Croatian farmed tuna is sold mainly on the Japanese sushi-sashimi market and generally has a lower quality and a lower price than large farmed tuna [19], although according to [9], the fat content of farmed juveniles can be very high, which could lead to higher prices on the Japanese market. In 2016, farmed ABFT from Croatia (ToroCro Maguro, Sashimigrade) won the Superior Taste Award [39]. The award-winning Croatian tuna is an example of how rearing tuna from the juvenile stage onwards can be very profitable even with small tuna sizes due to the high fat content.

3.6. ABFT Fattening/Farming Legislation

In 1993, ICCAT launched the Bluefin Statistical Document Program (BFSD Program) for frozen tuna and in 1994 for fresh tuna. In 1999, ICCAT decided to use a special import form for reared BFT, but in fact reared tuna has been labelled since 1996 [19]. According to ICCAT Recommendation 06-07 on Bluefin Tuna Farming, farms have been required to report the size of each harvest operation since 2008. The same applies to the Regional Observer Program (ROP) [15]. The Recommendation 21-08 by ICCAT requires the establishment and maintenance of a record of facilities approved to farm BFT caught in the Convention Area (Farming Facilities for Bluefin Tuna, FFBs). Currently, the ICCAT Record of BFT Farming Facilities contains 69 FFBs in the Mediterranean Sea (ICCAT List of BFT Farming Facilities, 2022). In Croatia, Regulation on marking of breeding installations, monitoring of operations on breeding grounds, and traceability during the breeding of bluefin tuna (Thunnus thynnus) (OG 63/2022) was adopted in June 2022, based on the provisions of the Croatian Law on Aquaculture (OG 130/2017, 111/2018; 144/2020) [24].

3.7. ABFT Aquaculture Production

Data on ABFT farm capacity is readily available in the ICCAT Record of ABFT farms. In 2022, there were 69 ABFT farms with a maximum potential capacity of 71,440 t: Italy, 18, 17,700 t; Malta, 8, 13,800 t; Croatia, 4, 7092 t; Morocco, 4, 6600 t; Turkey, 6, 6440 t; Spain, 11, 6002 t; Tunisia, 6, 5000 t; Cyprus, 3, 3000 t; Greece, 2, 2100 t; Egypt, 1, 1800 t; Libya, 3, 1800 t; Albania, 1, 500 t; and Portugal, 2, 500 t [16]. According to [39], in 2017 there were 54 ABFT farming companies with 62 farms and a maximum potential capacity of 54,000 t, mostly distributed in clusters (Cartagena in Spain, Naples in Italy, and Zadar in Croatia)

While the maximum potential capacity is readily available in the ICCAT Record of ABFT farms, the situation is different for the reported data on ABFT aquaculture production. The total Mediterranean ABFT aquaculture production is difficult to estimate because cage input figures (biomass and ABFT size) are only rough estimates and farmers keep output figures confidential [9]. ABFT aquaculture production data have been irregular and there are contradictions in the FAO database (FishstatJ), the EU EUROSTAT database, and national databases [8,13,19,40,41,42]. Ref. [40] found that less than one-third of Mediterranean ABFT aquaculture production was reported in the FAO database (28,450 t vs. 10,000 t) in 2006. However, the data on ABFT aquaculture production in FishstatJ have been updated for the years starting from 2004, except for the data on ABFT aquaculture production in Libya and Morocco, which are not yet available. Figure 2. shows the data on Mediterranean ABFT aquaculture production (t) in FishstatJ, EU ABFT aquaculture production (t) in EUROSTAT, and Croatian ABFT aquaculture production (t) in the CMA database in the period between 1996 and 2000 [11,15,23,43,44].

In 1996, the first year of ABFT farming production in the Mediterranean, ABFT aquaculture totalled 77 t, and 39 t in Croatia (Figure 2). Croatian ABFT aquaculture reached a peak of 6700 t in 2006, while Mediterranean ABFT aquaculture production has been steadily growing, reaching a peak of 34,385 t in 2020. Croatian ABFT production was 3323 t in 2020 and 4372 t in 2021. Croatian farming contributes about 10% to the ABFT aquaculture production in the Mediterranean region/sea.

According to [42], EUROSTAT data on ABFT aquaculture production were not available for Cyprus, Greece, and Italy, and data for these countries are still missing (Greece from 2008, Cyprus from 2009, and Italy from 2012). The lack of data for Italy is a major shortcoming considering that Italy has the highest potential capacity among all Mediterranean countries (17,700 t in 2022). The figures in FishstatJ are low for Italy (from a minimum of 61 t in 2001 to a maximum of 2577 t in 2007) until 2011 and missing from 2012. Reported figures for Greece and Cyprus are missing in EUROSTAT from 2019.

As in the Mediterranean region as a whole, data on ABFT aquaculture production in Croatia were not harmonized in the FAO database (FishstatJ), in the EUROSTAT database, and in the national database, so the data in FishstatJ and EUROSTAT differ until 2004 (minimum 0 t in 1996 and 1997 and maximum 782 t in 2003). The quality of the statistics contained in the FAO database depends mainly on the accuracy and reliability of the data provided by countries. Although FAO member countries should provide statistics to the government on a regular basis, this is not always the case, either due to erroneous reporting (inconsistencies between data compiled by different institutions), political reasons, or sometimes due to communication problems (e.g., change of responsible official, etc.) [45]. There is also a tendency for most ICCAT member states to under-report their catches to ICCAT and FAO in order to support their national finishing fleets. When they under-report landings, they also under-report catch-based aquaculture production levels to maintain consistency in reporting figures [39].

However, the data on Croatian ABFT aquaculture production in FishstatJ and EUROSTAT have been updated for the years starting from 2004. However, there is a large difference between reported ABFT farm production data and ICCAT data on maximum possible ABFT farm capacity in the Mediterranean Sea. According to [39], the reported ABFT farm production in 2013 was 11648, while ICCAT reported the maximum potential ABFT farm capacity to be 60,809 tonnes. In 2017, the reported ABFT farm production was 24,220 t, while ICCAT put the maximum potential ABFT farm capacity at 53,606. In 2022, there are 4 ICCAT registered FFBs in Croatia with 10 different locations/fields, the tuna farming capacity available to the Republic of Croatia is 7880 t, the total FFBs allocated capacity is 7092 t (90% of the tuna farming capacity available to the Republic of Croatia), (10% or 788 t is not allocated) and the maximum input quantity of wild caught ABFT is 2947 t (Figure 3) according to ICCAT Record of farms (2022).

According to [11], ABFT farming of juvenile fish is feasible but economically inferior to ABFT fattening of adult fish.

4. Fattening/Farming Challenges

The benefits of ABFT aquaculture are numerous, as are the challenges. ABFT is highly adaptable in captivity, tolerates a wide range of temperatures, has a rapid growth rate, is resistant to disease (due to heterothermy and a developed immune system), has a high edible meat to body weight ratio (80% of usable meat), and fetches a high market price [13,46,47,48,49,50,51]. Because ABFT fattening and farming facilities are located in exposed coastal or offshore areas and stocking densities are low (2–4 kg/m³), the environmental impact of the farms is also low.

The main challenge and barrier to sustainable ABFT aquaculture is that both fattening and farming are capture-based and depend on catching wild tuna for stocking and wild small pelagic fish for feeding.

4.1. Stocking

To take the pressure off wild ABFT stocks, it is necessary to close ABFT life cycles in captivity. Since the 2000s, there have been many closed-cycle aquaculture projects of ABFT in the Mediterranean Sea [6,39] encouraged by successfully spawned and reared Pacific bluefin tuna in captivity at Kinki University Fisheries Laboratory (Japan) from 2002 to 2004 [6,39]. About 500,000 Pacific bluefin tuna larvae were produced and by 2016, 900 t hatchery produced tuna were sold annually on the Japanese market [52]. From 2011 to 2015, the Spanish Institute of Oceanography (IEO) succeeded in producing a few thousand 5 g ABFT juveniles annually in captivity, and sold hatchery produced tuna in 2014 and 2015 [53]. This was the first successful closed-cycle ABFT aquaculture project. Unfortunately, the profitable production of ABFT juvenile fish is still far from widespread commercial success. Reasons include a shortage of eggs, low larval and juvenile survival due to early floating and sinking death, sensitivity to environmental conditions, feeding problems, stress, collisions with walls, cannibalism, and, most importantly, the transfer mortality of fingerlings [54]. Most closed-cycle ABFT aquaculture projects (in Spain, Malta, Greece, Croatia, Egypt, and Turkey) have been successful, but not in commercially viable quantities [6]. However, the closed-cycle production of ABFT is difficult to estimate because private hatcheries do not publish data on produced fingerlings [39]. However, much work remains to be done to ensure stable and consistent industrial-scale production of relatively inexpensive juvenile ABFT in hatcheries that can meet the demand of the ABFT aquaculture industry.

4.2. Feeding

Fattened and farmed ABFT require wild-caught small pelagic fish for feeding, and reared ABFT are usually fed ad libitum. Because ABFT have a high energy demand due to high body temperature and constant swimming [55], only 5% of the total energy intake is used for growth [56]. The estimated FCR is very high, ranging from 10 to 20:1, and depends on the temperature (the lower the temperature, the higher the FCR) and the size of the fish (in juveniles it is 10:1 and in large fish it can be 40:1) [9,33,57]. Due to the high FCR, the ABFT requires large quantities of baitfish for feeding. In 2004, 225,000 t of baitfish were used to produce 25,000 t of ABFT in Mediterranean ABFT farms [58,59], while in 2001, 15,000 t of baitfish were used to produce 3045 t of ABFT in Croatian ABFT farms [11]. The large number of baitfish required to feed ABFT increases pressure on wild small pelagic fish stocks and has a significant ecological footprint. To reduce the pressure on forage fish, artificial diets must be produced profitably. The formulation of artificial diets must consider the nutritional needs of ABFT, their acceptability and palatability, and the acceptability of artificially fed tuna meat to buyers/consumers. Although artificial diets have already been produced (with high production costs), farmers do not use them because they fear the negative effects of such diets on the quality of tuna meat [9]. Aquaculture diets in general are in a state of upheaval due to the unavailability of fishmeal and fish oil as the main sources of protein and oils in fish diets. Since artificial diets for ABFT, whether dry or wet, pellet or sausage diet, are likely to depend on fishmeal and fish oil, production will also be constrained to some extent. If more sustainable alternatives are used, this will likely affect the quality of tuna meat [57]. Apart from being environmentally unsustainable as an ABFT diet, baitfish also has a high FCR, a high risk of pollution and disease [60], and is very impractical, contributing over 60% to the operating costs of ABFT aquaculture [6]. Therefore, the formulation of improved artificial diets can significantly reduce ABFT production costs.

4.3. The Impact of ABFT Farms on Water Quality, Sediment and Bentic Communities

Many EU-funded projects have addressed the impacts of marine aquaculture on the Mediterranean marine environment, and the literature is replete with reports on the impacts of aquaculture practices on different elements of the marine environment [61].

Intensive ABFT fattening and farming uses large quantities of whole baitfish and produces large amounts of organic waste (uneaten food, excreta, etc.) that can affect water quality and accumulate in sediment. According to [62], the standard volume of production of ABFT farms in the Mediterranean (in 3 to 6 months of fattening) is 1000 t of the initial biomass, and since biomass increases by 10% per year, the gross waste output corresponds to a whitefish load of 700 t per year in the final biomass production (in 16 to 18 months). Based on the TunaMOD model, according to [63], Adriatic ABFT farms producing 4000 t per year produce 1002 t of total nitrogen per year and 21 t of total phosphorus per year. The physical presence of caging installations and increased sedimentation of organic material from the farm may reduce water transparency and alter the composition, distribution, and diversity of benthic communities, especially of very sensitive Posidonia communities [63,64,65]. Nevertheless, the negative impacts of large amounts of organic waste released into the marine environment are spatially and temporally limited in many Mediterranean ABFT farms, due to good farm site selection, controlled feeding, and the oligotrophy of the Mediterranean Sea [66,67,68,69,70,71,72]. Various pollutants (antibacterial agents, disinfectants, antifouling) can enter the marine environment from ABFT farms [73]. According to [74], polychlorinated biphenyls (PCBs) and organochlorine pesticides were detected (at current regulatory limits) in ABFT tissues from a Croatian ABFT farm and the importance of baitfish quality was emphasised, especially when farming top predators such as tuna.

4.4. The Impact of ABFT Fattening/Farming and on Wild Populations

The physical presence of ABFT farms can alter water quality around the cages (sea currents, turbidity, etc.) and alter the behaviour of wild populations (alter migration paths, change feeding habits, etc.) [75]. Farms attract wild organisms as feeding and breeding sites [76]. Large aggregations of wild fish near the cages attract small-scale artisanal fishers, but since fishing is prohibited near the farms, the farms could serve as functional marine protected areas [77]. The farms also attract seabirds (mainly seagulls), which can affect the microbiological quality of the water [78]. According to [49], there is no evidence of disease transmission between wild and farmed ABFT. However, imported frozen baitfish pose a potential risk for the introduction of exotic diseases into local populations [59]. Genetic interactions between reared and wild ABFT do not pose a risk because the genetic material of ABFT is not altered, but there is a risk of spontaneous spawning [79,80,81]. If reared tuna offspring interbreed with wild individuals, this could alter population dynamics [82] and gene flow [83]. According to [83], the dynamics and structure of ABFT stocks in the Atlantic and Mediterranean are not fully understood and it is not clear whether spawning sites separate the gene pool, especially in the Mediterranean, which offers a large number of spawning sites. The expansion of tuna fisheries has had some positive effects on demersal populations. According to [84], 30 bottom trawlers in Croatia have switched from trawling to tuna farming, reducing fishing pressure on overfished groundfish stocks.

5. Sustainable ABFT Farming

Although ABFT aquaculture is currently questionable in terms of sustainability, ABFT farming is a promising alternative to fishing considering that ABFT is highly adaptable, has strong market awareness, and there is high demand for high-quality tuna meat [10]. ABFT aquaculture is attractive to investors because of the high returns, but it is also important to the general population because of the high export value and employment [6]. It is also a source of income from a tourist point of view, as is the case of the Catalan company Balfegó, which has a catamaran to visit the tuna farms in l’Ametlla de Mar (Tarragona, Spain). Although local communities can benefit from ABFT aquaculture, it often conflicts with other activities in the coastal zone (mainly tourism and recreational activities due to odour and oil pollution). Through adequate farm site selection, conflicts between the different stakeholders can be reduced, and the choice of location can determine the economic success, environmental sustainability, and public acceptance of the ABFT farm. Most Mediterranean countries have established distance limits for ABFT farms and designated allocated zones for aquaculture (AZAs) to reduce the negative impacts of ABFT farms on the marine environment [42].

To meet the increasing global demand for sushi-sashimi ABFT and to conserve wild ABFT stocks, future ABFT aquacultures should focus on cost-effective hatchery-produced juveniles and cost-effective, environmentally friendly artificial diets. According to [13], sustainable ABFT farming is based on feeding hatchery-produced juveniles with efficient commercial diets for three years to raise marketable 30 kg individuals for fresh tuna steaks. Because the cost of research with large species such as ABFT is extremely high [57], large investments are needed to support research and innovation in the transition from ABFT CBA to innovative sustainable ABFT farming. As intensive farming of carnivorous fish such as ABFT raises many ethical dilemmas (food safety, animal welfare, social acceptability, etc.) that affect farmers, consumers, the environment, and the organisms being farmed, ethical evaluation should not only involve scientific analysis but should be holistic and flexible [73].

Sustainability of Croatian ABFT Farming

Croatian ABFT farming is an important and promising sector in the Croatian economy and society. In 2015, the Croatian ABFT industry employed about 500 people and exported products worth US$ 239 million, mainly farmed ABFT for the Japanese market [85]. The further development of ABFT farming could create new jobs and regular income for hundreds of people, especially on depopulated Croatian islands, and provide a high-quality local food supply for responsible tourism [84,86]. Croatia has done much to improve the administrative and legal framework for aquaculture and to implement integrated coastal zone management and marine spatial planning [85]. According to the experience of Croatian farms, the ABFT farming of juveniles is feasible but less efficient than the ABFT fattening of adults. Nevertheless, the experience and knowledge gained in Croatian ABFT farming can be useful for future close-cycled ABFT farming in the Mediterranean region.

In addition to the already mentioned Croatian Law on Aquaculture (OG 130/2017, 111/2018; 144/2020), the following legislation for ABFT farming has been enacted in the Republic of Croatia:

• The Regulation on marking of breeding installations, monitoring of operations on breeding grounds and traceability during the breeding of bluefin tuna (Thunnus thynnus) (OG 63/2022)—this regulation imposes the obligation to notify the plan of FFB, the procedure for the identification of the farming facilities, the conditions and procedure for the placing of tuna in the cages, the procedures for the catching of tuna from the cages, the transfer of tuna within the farm, the implementation of conducting random controls, the verification of the transfer of tuna, the transfer of tuna between farms, the handling of dead tuna during farming and tuna below the minimum size in the catch, and the obligation to keep documentation, regional observers, and scientific research.
• The Regulation on the disposal of tuna farming capacity and permitted input quantities of caught wild tuna (Thunnus thynnus) on farms (OG 22/2021) and Amendments to Regulation (OG 9/2022)—this regulation regulates the ways of disposing of tuna farming capacity and the way of disposing of the allowed input quantity of caught wild tuna.

The legislation is harmonized with the Regulation (EU) 2016/1627 of the European Parliament and of the Council of 14 September 2016 on a multiannual recovery plan for bluefin tuna in the eastern Atlantic and the Mediterranean and repealing Council Regulation (EC) No 302/2009 as well as with the Recommendation 21-08 by ICCAT, and has a positive environmental impact by ensuring the rational management and conservation of ABFT. On the other hand, it restricts to some extent the growth of farming capacity and employment.

The Croatian National Strategic Plan for Aquaculture Development 2014–2020 (the NSPA) has set the goals and priorities for aquaculture development in the period from 2014 to 2020. This strategic document contains an analysis of Croatian mariculture, including ABFT farming, for the period from 2003 to 2012. According to [87], the strengths of Croatian ABFT farming are as follows: a long tradition of farming, favorable environmental conditions, numerous experts contributing to the development of ABFT farming, wide acceptance in local communities, and the availability of natural resources. Almost all ABFT caught by the Croatian purse seine fleet are used in farming cages, and a large part of small pelagic fish caught by the Croatian fleet is used as feed in ABFT farming. The fact that tuna production is almost exclusively for the Japanese market is listed as a strength, but this dependence on the Japanese market could also be a weakness. Weaknesses also include insufficient collaboration between science and the sector, and a lack of applied science and research projects. The major threat to ABFT farming is its dependence on natural resource availability (small pelagic fish and tuna TACs). The opportunities are as follows: contribution to the development of island communities (employment, related activities), better use of farming capacities, the possibility of the further implementation of Integrated Coastal Zone Management, and the possibility of support from EUSAIR (Pillar I, Topic II: scientific cooperation and sustainable fisheries management).

In November 2022, a new National Strategic Plan for Aquaculture Development until 2027 (the NPRA) (OG 133/22) was adopted. The NPRA is harmonized with European Commission Strategic guidelines for a more sustainable and competitive EU aquaculture for the period 2021 to 2030, COM (2021) 236 final. The NPRA includes a SWOT analysis of Croatian ABFT farming for the period from 2015 to 2020. According to [88], the strengths of Croatian ABFT farming are the following: a good reputation on the world market, high product quality, the availability of highly skilled labour, the application of modern farming technologies, the availability of baitfish from the catch, the good organisation of farmers, good cooperation with research institutions, active participation in EU activities, good social acceptance, cooperation with local communities, and the viability of a good environmental status. Weaknesses include the fact that tuna production is dependent on quotas, the entire industry depends on one market, and prices depend on market supply. The main opportunity is the diversification of production for new markets, especially the European and Croatian markets, including the tourism sector. Threats include the likelihood of sudden disease outbreaks, tuna escaping from cages in adverse weather conditions, possible feed restrictions, and rising feed prices.

6. Conclusions

ABFT is the most important commercial tuna species in the Mediterranean. The lucrative Japanese market has encouraged the fattening of adult ABFT (Australian-style) in Spain and other Mediterranean countries and the farming of juveniles in Croatia since 1996. In the Mediterranean, tuna farming and fattening are very profitable, provide full year-round employment, contribute to the preservation of cultural heritage, and can also promote tourism development. Unfortunately, the environmental viability of ABFT farming and fattening is questionable, as both farming and fattening are completely dependent on wild stocks, both for stocking and feeding. Although there has been success in closing the ABFT life cycle in captivity, and environmentally friendly feeds are available, hatchery-produced juveniles and new efficient commercial feeds are still insufficient and costly for commercial use. Further investment in research and innovation is needed to conserve wild stocks and ensure the sustainable development of ABFT farming industry.

Croatian ABFT breeding is less profitable than fattening practices in other Mediterranean countries, but is very important for the Croatian economy and society. Croatian ABFT farming also contributes to the development of a number of accompanying activities, such as fishing for small pelagic fish, and can help reduce the trend of the depopulation of vulnerable island communities. Due to good environmental status, the availability of small pelagic fish, the availability of a highly qualified and well-organised labour force, good cooperation between producers and researchers, and the application of modern farming technologies, ABFT farmed in Croatia have high quality and a good reputation on the market. The main weakness of Croatian tuna farming is that the entire industry is dependent on the Japanese market, but this can be overcome by the possibility of product diversification for new markets, including the tourism industry. The Croatian National Strategic Plan for Aquaculture Development until 2027 emphasises that the full potential of Croatian tuna farming has not yet been exploited. Therefore, further investigation is needed.