Changes in trophic behaviour and trace metal concentrations in wild fish in a tuna-farming environment. The key role of a sound baitfish choice

Abstract

It is essential from both the consumer perspective and the environmental sustainability aspect, to delve more deeply into the specific traits and conditions of fish reared in a pisciculture setting. Among the many factors that affect the latter, one of the most important is the cycling of chemical compounds in intensive rearing systems. For this reason, we analysed the stomach contents and trace metal levels (cadmium, copper, chromium, lead, and zinc) in the muscles of several fish species that aggregate around mid-Adriatic cages of reared Atlantic bluefin tuna (Thunnus thynnus), attracted by the organic leftovers found in farmed waters. The metals were also analysed in samples of reared bluefin tuna, fish species used as tuna feed (baitfish), and their non-farm-associated counterparts. Finally, to better follow the dietary relationships and metal-acquisition pathways, we also analysed the stable isotopes. The results revealed that wild fish consume a great deal of trophic resources deriving from the cages, mainly in the form of baitfish remains. The isotopes suggest the reared tuna consume high levels of herring baitfish (Clupea harengus), there is an increased trophic level for the wild omnivorous bogue (Boops boops), and this decreases for the carnivorous garfish (Belone belone). No significant differences in metal concentrations were detected between farm-associated and non-farm-associated wild fish, with the exception of the zinc levels in bogue, which were in lower concentrations in specimens aggregating around the farm. Most importantly, the concentrations of trace metals in the tuna, baitfish (sardine, Sardina pilchardus, and herring), and wild fish (garfish and bogue) did not exceed the recommended limits established by European legislation. However, the cadmium values in the garfish, bogue, and sardine were close to, but not over the legal limit value. The reasons behind these results and their potential consequences should be further investigated, although there is only a moderate degree of concern with regard to the metal content of fish stocks captured in the area under study.