Rainbow trout consumed food in direct proportion to their body weight although the ration increased with temperature. Satiation time varied as 0.031 weight + 0.868 temperature+ 29.145 min. Appetite after deprivation returned in close conjunction with gastric emptying, as judged from serial slaughter studies. Trout trained to operate demand feeders ate less but exhibited feeding rhythms even under conditions of constant illumination. Radiography showed that these rhythms closely paralleled gastric emptying. When offered food diluted with kaolin, the fish compensated by eating more food. This was achieved by increasing feeding frequency caused by a more rapid rate of gastric emptying.

No abstract

The gastric emptying time (G.E.T.) in turbot was investigated using X-radiography and was found to decrease with bemperature. Small fish processed a given ration, expressed as per cent body weight, faster than large fish (G.E.T. was found to be proportional to (fisheight)^'^^^). Large meals in a given fish were processed at a faster rate than small meals. Gastric emptying rate (G.E.R.) was found to be proportional to (meal size g)o.ela at 8" C and (meal size g)0'7ss at 19" C. These exponents are in agreement with a recently proposed model relating G.E.T. and G.E.R. to meal size (Fange & Grove, 1978). Large fish emptied a meal of given absolute size from the stomach at a faster rate (g h-I) than small fish. Experimental meals diluted with kaolin were evacuated in significantly less time than a control diet, suggesting that turbot may adjust feeding rates when food quality varies.

Gastric emptying time in the dab, Limanda Zimanda, has been studied using an X-ray technique. The addition of 25 % barium sulphate to a test meal did not significantly affect the transit time. Lowering the experimental temperature from 16.4 to 8.5" C markedly decreased gastric evacuation time. An increase in ration size led to an increase in the time required to empty the stomach and also to increase the amount of food digested per unit of time. For any given ration size, expressed as per cent body weight, the larger the animal the longer is the time required for evacuation of that meal. We suggest that the food intake per day, as a percentage of live body weight, will be smaller for largerl. Zimanda in the wild.

Gastric emptying time in Scophthalmus maximus, when fed friable artificial pellets based on fishmeal. is composed of two phases: (a) a delay time ( I d ) during which the meal forms a bolus and which shortens with temperature, and (b) an emptying phase (duration tend) which varies with meal size (8, body weight ( W ) and temperature (0 according to: log, tcnd=4.66+0.448 log, S-0.2664 log, W-0.051 7 (where tend is in h, S is in g, W is in g and Tis C). During the emptying phase, stomach contents decrease curvilinearly according to: S,O.448 = S00.448 -0.448 K (where S, & So is in g and I is in h) in which the instantaneous digestion rate, K , varies with fish weight and temperature as: ~= 0 . 0 2 1 ~0 . 2 6 6 4 p 5 1 iFood pellets were prepared which remained separate and did not form a bolus in the stomach; K increased if a given meal size was subdivided to increase surface area. If meal size was increased by ingestion of identical pellets, K decreased. After a satiation meal, appetite in young turbot returns in direct relation to the degree of stomach emptiness. When food is regularly available, young turbot feed steadily at a rate which maintains their stomachs at c. 85% maximum fullness.When trained to use demand feeders, the fish interact a s a group to feed rhythmically. but feeding rate falls 33% to only two-thirds of the previous rate since stomach fullness, and hence digestion rate (g h-I). i s maintained at a lower level. Reduction in dietary energy density below I kCal g -' increases gastric emptying rate and the turbot demonstrate partial compensation by increasing food intake. On energy-rich diets, protein nitrogen and energy assimilation efficiencies remain high (97.5% and 91 YO respectively) irrespective of feeding rate and frequency.