Aspects of the regulation of food intake in the dab, Limanda limanda (L.)
The regulation of food-intake in Limanda limanda was investigated, including (a) the role of the stomach as a limiting factor in fish food intake, (b) the qualities of a diet dab respond to and (c) the dynamics of food intake. Fish fed on squid (4.0 U. g'', 76.2% moisture) ate similar daily rations whether fed three times daily or once per day; when fed every three days they were unable to maintain this intake. Fish fed on pellets (18.8 kJ. g'', 8% moisture) could maintain their average daily food intake for all meal intervals tested. After a satiation meal, food intake broadly increased with deprivation time for at least 96 hours with no clear indication that stomach volume was limiting. However, when the data was re-examined using a return map (where meal m is plotted against meal m-1) there was evidence that feeding was restricted by stomach fullness as the interval between meals exceeded 25 hours. Surprisingly such limitation did not occur at higher feeding frequencies. Three models of food intake were used to simulate food-intake data, in which the role of the stomach as a constraining factor was varied: 1) Food intake was assumed always to be completely limited by stomach volume. 2) Food-intake was assumed always to be driven by a systemic need 3) Food intake was assumed to be chiefly limited by a systemic need, but when this was high, stomach volume would constrain intake. Comparisons of experimental results with these models suggest that when fish are fed frequently, or on a high-energy diet, the stomach volume is probably not limiting, whereas for a low-energy diet, fed infrequently, stomach volume was limiting when systemic need was high. Dab adapted their food intake to diets of different water content (and therefore energy density). They also adapted the distensibility of their stomachs in response to the increased volume eaten; fish fed on pellets having less distensible stomachs than those fed on squid. Thus it is unlikely that stomach volume can limit food intake in the long term, unless food quality and/or meal timing is variable and the fish cannot adapt their stomachs to the diet/feeding frequency. Methodological trials proved that that observed stomach VI volume is a function of the measurement technique, as well as the diet history of the fish, and experiments examining stomach volume should take this into consideration. The question of what aspect of a diet L. limanda adapt to was examined by testing different models using path analysis, a method of inferential modelling of causal relationships, in an attempt to explain how food-intake is regulated. Dab were found to be adapting to both the energy content of the diet and to the individual nutrients. The dynamics driving food intake in groups of dab were investigated using non-linear time series analyses. These proved to be low-dimensional, significantly non-linear, deterministic systems. The data also suggests that such systems are either capable of occasional chaotic behaviour, or are on the edge of chaos i. e. complex dynamical systems. Thus food intake is under the direct control of few (two or three) variables, through which the many known factors that influence food intake must act. Comparisons were made with individuals and groups of Oncorhynchus mykiss, with a brief look at Merlangius merlangus and Dicentrarchus labrax. These results were similar to the dab, and so this dynamical behaviour may be a feature of teleost fish in general. Importantly the fact that individual trout had similar feeding behaviour to groups indicate that the findings were not a function of hierarchical dynamics. The significance of this finding should be that appetite control with these properties allows rapid adjustment of fish according to changes in diet quality.