The tuna ranching industry is a global, high-value aquaculture activity. Current feeding practices are impractical, unsustainable and pose serious ecological risks. The daily feeding of copious quantities of fresh/frozen fish results in extremely high feed conversion rates (~28 : 1). Seasonal availability and quality variation plus the lack of locally sourced baitfish highlight the urgent need for a balanced feed. Objectives for the present experiment were to compare performance and quality indicators of Atlantic bluefin tuna (ABFT, Thunnus thynnus) fed herring/mackerel or a compound tuna feed based on sustainable American soy.
A thirteen-week feeding demonstration was conducted in two oceanic net pens moored east of San Pedro del Pinatar, Murcia, Spain (37°49.835’ N 000°39.822’ W). Each cage held ~45 fish, weighing approximately 99.5 kg (average initial individual weight, evaluated both by expert visual ranking and corroborated by the AQ1’s AM 100 fish sizing system). Feed consumption and condition factor were recorded. At harvest, dorsal loins were collected and color, mercury, proximate composition, scombrotoxin levels, oxidative stability index and peroxide values were evaluated in the resulting tuna steaks. In addition, a blind sensory evaluation and a commercial taste assessment were performed on the main sashimi cuts obtained from fish fed either diet.
Steak samples from formula-fed fish had similar lipid contents as those fed baitfish but exhibited improved flesh color, texture and had increased oxidative stability and reduced histamine. Results from sensory and organoleptic evaluations (professional sashimi chefs) indicated that sashimi slices from formula fed fish were similar in flavor but more stable on the counter and superior in color. Importantly, there was a significant reduction in tissue mercury in formula-fed fish as compared to baitfish fed ABFT.
In addition, the formulated diet offered feed management options that are quite compatible with the present operation and equipment available at commercial tuna farms in the Mediterranean Sea. A preliminary economic evaluation (2021 prices) indicated that tuna feed is significantly more cost-effective than imported frozen baitfish. Given increased fishing quotas for ABFT and the severe difficulties in sourcing baitfish experience by Mediterranean tuna farmers, the formulated diet may offer a viable alternative to baitfish feeding. In conclusion, these results indicate that the balanced feed provided suitable nutrition for adult ABFT, enhanced the quality and shelf life of the end product and may enable management strategies to optimize tuna performance, thus increasing farm efficiency and reducing the environmental impact of commercial tuna ranching. This research was supported in part by the United States Soybean Export Council.