Food consumption patterns, digestion processes and growth performance of whiting (Merlangius merlangus L.) in captivity were investigated to clarify the feeding biology of this scarcely researched species. The study began by investigating the gastric emptying process of whiting fed to satiation with fresh sprats containing radio-opaque BaSO₄ paste. The movement of chyme along the alimentary tract was traced to estimate gastric emptying time by X-radiography. The results showed that fish ingesting increasing meal sizes (up to 8-10% b.w) required much longer gastric emptying times. Preliminary investigation revealed that emptying follows a curve and not a straight line. The estimation of maximum stomach capacity using an isometric relationship seems to underestimate the satiation meals, especially in smaller fish. In the present
study stomach volume (SV) increases allometrically with body weight (W); SV = 0.438W ^0.662. It was found that the power value was constant but changes in the constant denoted differences in packing factors of various prey types (0.39: sprats, 0.61: squid pieces and 0.26: shrimp). Food intake studies also revealed that return of appetite with time was not linear. Indeed, the observations suggested that there were coherent relationships between X-ray studies, return of appetite and gastric emptying. The underlying curves were best described by Andersen's power model (S_t = S_max (1 - S_max ^-0.5 p 0. 5 . t)²) for the gastric emptying process and S_t = S_max - (S_max (1 -S_max ^-0.5 p 0.5 . t)²) for return of appetite of whiting fed with squid pieces, sprats and brown shrimp, where S_t = stomach contents at time t, S_max = satiation meals, p = rate parameter and t
= time after feeding.
In addition, a 'physiological ' power model (meal size: Stimulus a W ^0.33 followed by secretion: Response a W ^0.67) was used as a basis to model the gastric secretions of hydrochloric acid and pepsin in the stomach of whiting. The results showed that soon after ingestion, pepsin and gastric acid were secreted into the gastric lumen and peaked 2 - 3h after feeding. Fish size had a significant effect on the amount of gastric acid and pepsin secreted; digestive power increased
a W ^0.67 as predicted by the 'physiological model'. However there were no clear effects of meal size (stimulus or distension volume DV) but other factors such as muscular contractions (mixing mechanism) were related to DV^0.33 in partial support of the model.
Digestibility (absorption) studies after ingestion of the main prey, sprats was efficient; 60 - 68% of moisture, 80 - 94% of protein, 90 - 97% of lipids, 80. 5% of carbohydrate and 85 - 96% of energy were actively absorbed as food passes along the gut sections. However, estimated values of nutrient absorption in similar gut sections for wild whiting were slightly lower because of unknown mixture of prey species in the intestinal samples. Proximate analysis of the digesta showed that sprats and brown shrimps probably dominated the main diets in the wild with minor contributions from other crustaceans and polychaetes.
Finally, monitoring of whiting fed on artificial feeds from a demand feeder showed that whiting fed well during the autumn-winter (15°C) seasons. Peak feeding activities occurred between 'dawn' and noon' despite continuous 24h lighting. Overall growth performance was poor, suggesting other factors such as dietary formulation or stress was diverting energy from the growth processes.