A comparative approach was adopted to assess the effect of different feeding strategies on feed transit and daily oscillations of apparent digestibility coefficients (ADCs) of dry matter, protein, and energy, in two species with different feeding habits, gilthead seabream and Senegalese sole. For this purpose, fish juveniles were consecutively fed with a standard and a marked diet (yttrium oxide) and were sampled at difference post-prandial times. Mathematical models were constructed to predict filling and evacuation of the stomach and intestine in gilthead seabream, and the whole digestive tract in Senegalese sole. Results demonstrated that changes in daily feeding time and frequency, modified the feed transit rate and residence time in the gut as well as the apparent digestibility, with obvious consequences for the utilization of nutrients. Variations in apparent digestibility were also observed along the daily cycle. Besides, the obtained response varied significantly depending on the studied fish species, due to the interspecific differences in the anatomy, digestive physiology, and feeding habits. Allocating the whole daily meal in one ration resulted in fast gut filling rate and low apparent digestibility in both species. In gilthead seabream, feeding protocol did not influence the stomach and intestine evacuation rates. However, in Senegalese sole, frequent meals during the whole daily cycle resulted in a faster gut evacuation. Very frequent diurnal feeding led to highest ADCs in both species; although, such higher digestibility values were not necessarily related to longer feed residence times within the gut in gilthead seabream. The results obtained from this study can be useful to establish optimized feeding protocols that help to reduce the production costs and undigested debris in the effluents.

This research was funded by the Spanish Ministry of Economic Affairs and Competitiveness (MINECO) by projects EFISHDIGEST (AGL2014-52888-R) with FEDER/ERDF contribution granted to M.Y. and WISEFEED funded by the European Union's H2020 programme (Marie Skłodowska-Curie grant No 691150). Neda Gilannejad was supported by a doctoral fellowship (BES-2015-071662) from MINECO (Spain).