In vivo total nitrogen and total phosphorous digestibility in Atlantic Bluefin Tuna (Thunnus thynnus thynnus Linnaeus, 1758) under industrially intensive fattening conditions in Southeast Spain Mediterranean coastal waters

Atlantic Bluefin Tuna (hereafter referred to as tuna) fattening has been developed quite a lot in south-eastern Spanish Mediterranean coastal waters, but previous information about nutrition and environmental impact is scarce. Total nitrogen and total phosphorous digestibility was investigated in tuna under routine operation fattening conditions as a first step to know nutrient utilization and to provide tools for estimation of waste output. Tunas were fed ad libitum once a day with a mixture of mackerel, herring, pilchard and gilt-sardine. Body weight of sampled tuna ranged between 150 and 350 kg. Faeces were collected by dissection of the distal intestine after slaughtering and directly with a manual sieve by scuba divers into the sea cages. Endogenous inert marker was acid insoluble ash. Apparent digestibility coefficient (ADC) for total-N was lower when faeces were collected by dissection of the intestine (DI: 658.2 g kg(-1)) than when by direct collection (DC: 937.4 g kg(-1)). Total-P ADC was lower when faeces were directly collected (DC: 481.4 g kg(-1)) than when collected by intestine dissection (DI: 661.9 g kg(-1)). The nitrogen digestibility in tuna was as high as in other carnivorous fish, while low phosphorous digestibility, as in many fishes, could be related with phosphorous excess in diet. DC results seem to be more consistent and appropriate for waste output estimation. Results demonstrate the need for a more accurate nutritional evaluation and development of formulated artificial diets.