The effects of season, stress and nutritional state upon plasma glucose and fatty acids were monitored in the sand dab, Limanda limanda (L. ), with a view to examining the possible role of these nutrients in controlling appetite return. A chronic cannulation technique was used to obtain blood samples from animals with minimum disturbance. The annual, mean, 'normal', plasma glucose level (after 72 h food deprivation) was 22.44 ±3.96 mg/100ml (±S. D., n=88). Normal plasma fatty acid levels showed two seasonal peaks. During May to early August, levels ranged from 691.00 to 935.73 geq/1 while a second main peak (859.00 geq/1) was observed in December. These periods correspond to the post-spawning and vitellogenic seasons respectively. Mild stress (1 minute emersion to air) caused a significant hyper- glycaemic condition (maximum plasma glucose level = 40.25mg/100ml) in 1.0 - 2.0 h, followed by a hypoglycaemic state some 9h after treatment. Considerable individual variation in the stress response was observed. Trawling stress was reflected in a 230% increase in plasma glucose above normal levels. Ingestion of 2% body weight meals of Mytilus caused plasma glucose levels to rise to between 34-50 mg/100ml. Feeding of glycogen supplemented (2.4%) fish paste meals gave maximum plasma glucose levels of 78.22 mg/100ml in 12 h. The magnitude of the hyperclycaemia was related to the level of dietary glycogen. A reciprocal relationship was observed between plasma glucose and free fatty acid levels of fish fed 6% body weight meals of M ty ilus. The metabolities remained deviated from normal levels for a longer period during the summer, probably due to the greater glycogen content of the food during this season. The glucose/fatty acid reciprocal pattern was replicated by the intra-arterial infusion of a glucose load (0.5g/kg body weight)., which gave a maximum plasma glucose level of 310.4 mg/100ml.
Short-term food deprivation had no effect on plasma glucose levels while plasma fatty acid levels increased from 340.78 µeq/1 (25 h post- operative) to 477.50 µeq/1 (86 h post-operative). There was no further increase for the next 59 h. It is likely that the post-prandial metabolite patterns of fish fed several meals within a short period would be very different, since the evacuation rate of a meal is increased in Limanda when followed by subsequent food. Gastric evacuation rates increased with an increase in body size.
However, larger fish (120g) still had a longer gastric evacuation time (GET)
(15.6h) than smaller fish (30g, 10.1h). In the 120g fish, GET for a
single meal was reduced to 13.6 h when it was, followed, 3h later, by a
similar sized meal.
The basal respiratory rate for 120g fish was 7.11 ±0.92ml 02/h. This rate increased to 15.8 ±1.2m1 02/h and 18.0 ±1.7ml 02/h after a 3% body weight meal of fish paste and a 6% body weight. meal of Mytilus respectively.
Limanda responded positively to a reduction in the level of dietary
lipid or carbohydrate by increasing its food intake. Calory intake was
maintained at a specific level irrespective of which nutrient was manipulated. Maximally elevated plasma glucose or depressed fatty acid levels did not totally suppress appetite. This was so whether. the plasma nutrient levels were altered naturally (by feeding) or artificially (by glucose infusion). The post-prandial increase in oxygen consumption was equally ineffective for totally inhibiting food intake. In addition, appetite return did not correlate closely with clearance of food from the stomach. It was tentatively concluded that appetite in this species may be principally influenced by available energy reserves.