Introduction
Marine aquaculture production relies primarily on wild-caught fish as feed ingredients such as fishmeal (FM) and fish oil (FO). These key ingredients are generally recognised as highly valuable as they meet nutritional needs for fish growth, being easily digestible and containing high levels of long-chain polyunsaturated fatty acids (PUFA).
The levels of contaminants and nutrients in fish from aquaculture and wild-caught sources are a timely and relevant issue for food safety. Contaminants such as heavy metals and metaloids (metal(oids)), PCBs, DDT, and PAHs pose toxic risks due to bioaccumulation, while nutrient levels like Se, Zn, and Fe vary depending on diet and habitat. After a comparison between aquaculture and wild fish in terms of metal(oids), other contaminants, and nutrients, due to the difference in their environmental conditions, feed and origin, it can be said that higher concentrations of some metal(oids ) were observed in wild fish than in aquaculture as well as in other contaminants. The mean Hg and As concentration in wild fish was 0.06 and 3.26 μg/g, respectively, while in aquaculture fish was 0.038 and 1.23 μg/g. The DDT mean concentration in wild fish was 196.48 ng/g, while in aquaculture fish was 44.64 ng/g. The difference in these concentrations does not only depend on the production system, other factors such as age, physiological state or the degree of environmental pollution can have a considerable effect. Therefore, there is a stronger trend toward replacing FM rather than FO because protein alternatives like insect meal and poultry by-products are more nutritional, accessible, like FM and cost-effective. Another critical motivation for seeking alternative ingredients is the reduction of metal(oids ) in farmed fish. By incorporating alternative protein sources with lower contamination levels, it is possible to enhance the safety and quality of aquaculture products while maintaining sustainable production practices.
Taking all mentioned above into account, the purpose of this research was to explore the effect of feeding with different alternative proteins (compared to fishmeal) on the composition metal(iods) in Sparus aurata fillets at two growing stages (300 and 800 g).
Materials and methods
Three types of feed formulations (n=3, CTRL: fish meal-based diet, PAP: processed animal protein-based diet, and ALT: alternative protein-based diet). Fish sampling took place at two growth stages: when fish reached 300 g and 800 g. The nomenclature for fish was as follows: i ) individuals reached 300 g: CTRL300, PAP300 and ALT300; and, ii) individuals reached 800 g: CTRL800, PAP800 and ALT800. Inductively coupled plasma mass spectrometry (ICP-MS) was used for quantification of metal(oids). 0.5 g of freeze-dried sample was weighed into the Teflon cups of the microwave digester, 7 mL of HNO3 69% ppt quality were added, the tubes were left open for 1 hour pre-digestion and 2 mL of H2O2 30% ppt quality were added. HPLC-ICP-MS (Shimadzu, Japón ) was used to analyse speciation of As : AsIII (inorganic arsenite ), AsV (inorganic arsenate), AsB (arsenobetaine ), AsC (arsenocholine), DMA (dimethylarsinic acid) and MMA (monomethylarsonic acid).
Results and discussion
Regarding metal(oids ), the median and mean values were similar in most cases. Overall, our results align with findings in the literature that highlight the influence of fish size and feed formulation could affect the accumulation of them. As concentrations were higher in 800 g fish than in 300 g fish (1.5 times more in CTRL800, 1.3 times more in PAP800, and 1.6 times more in ALT800), with significant differences observed between formulations. Specifically, CTRL showed the highest concentrations, with FM800 having 3.7 times more As than the other diets, and CTRL300 having 3.3 and 4.0 times more than PAP300 and ALT300, respectively. Among As species, AsB was the unique species detected in samples. Recently, the European Commission asked EFSA for a risk assessment on complex organoarsenic species in food. They are typically found in marine foods and comprise mainly arsenobetaine (AsB), arsenosugars and arsenolipids. Following with the rest of the contaminants and potentially toxic elements, Hg levels were also higher in 800 g fish compared to 300 g fish (3.6 times more in CTRL800, 5.1 times more in PAP800, and 6.8 times more in ALT800). While no significant differences were found between formulations, a trend was observed with FM showing higher levels than PAP and ALT. Cd content was generally higher in 800 g fish (2.8 times more in CTRL800, 3.7 times more in PAP800, and 4.4 times more in ALT800), although the median was lower than the mean in 800 g fish across all formulations. Pb concentrations were higher in 800 g fish compared to 300 g fish, though no significant differences were observed. It is noteworthy that the median and mean values were not similar for PAP800 and ALT800. Overall, 800 g fish had considerably higher Pb concentrations than 300 g fish, with increases of 18.9 times in FM800, 18.6 times in PAP800, and 17.1 times in ALT800).
Conclusions
The differences between formulations, particularly in As and Hg levels, emphasize the need for a careful selection of feed ingredients to minimize metal(oids ) exposure in aquaculture. Further research is needed to better understand the mechanisms governing these compounds metabolism and to evaluate the potential health risks associated with their accumulation in farmed fish.