Diet choice, foraging behaviour and the effect of predators on feeding in the three-spined stickleback (Gasterosteus aculeatus L.)
This thesis describes a study on diet choice, foraging behaviour and the effect of predators on feeding in three-spined sticklebacks (Gasterosteus aculeatus L.) from three sites in Scotland. The profitability values of prey items of various types and sizes were determined in a simple laboratory experiment, by dividing the mean energetic contents of each prey type by the mean handling time for that prey type. Profitability increases with fish size and with prey size (for some but not all prey types). Handling time increases with the amont of food in the stomach. Both fixation and handling times are an increasing function of prey size and a decreasing function of fish size. Both fixation and handling times differ from one prey type to another. The effect of experience on various components of feeding on natural prey in sticklebacks was investigated by testing two groups of fish, one reared with only frozen brine shrimps and the other caught in the wild where a wide range of food was available to them. Feeding latency, fixation and handling times and the number of grasps required before natural prey could be eaten are lower in fish with experience with natural food. However, this reduced feeding efficiency in naive fish disappears within just a few days of experience with natural prey. Differences between populations of sticklebacks exposed in nature to different invertebrate fauna in the efficiency (the net energy gain per unit time) of feeding on zooplankton or benthos were investigated. The results suggest that sticklebacks from areas with abundant zooplankton feed more efficiently on this kind of food than sticklebacks from areas of low zooplankton but high benthos which in turn feed more efficiently on benthos (this difference is not significant). Both groups of fish feed more efficiently on zooplankton compared to benthos but this difference is more marked in the case of fish derived from areas with abundant zooplankton. The role of prey profitability in food choice was investigated by using two different sizes of sticklebacks choosing between prey of different profitabilities in a series of laboratory tests. Fish of both sizes do not always choose the more profitable of two prey items. In the overall comparisons, only in 33.3% and 66.6% (for smaller and larger sized fish respectively), of the cases the more profitable prey type was selected. The role of the various visual stimuli that differentiate the natural prey types (speed and type of movement, colour, shape and size) in food selection was investigated systematically. The results suggest that sticklebacks differentiate among various configuration of each stimulus and that the different kinds of stimuli differ in their importance as determinants of prey choice. The results obtained when sticklebacks were offered a choice between prey of different profitability (see above) arose because the fish choose the prey on the basis of one or more preferred visual cues. In some cases, these cues lead the sticklebacks to the most profitable prey and in others they do not do so. The distribution of potential food was studied on three different types of substrates in Loch Lomond over the summer of 1985. The abundance of different food types varies according to the kind of substrate, even within few yards, and from month to month within a single season. Food selection in sticklebacks was also studied in relation to these variations in prey availability. Sticklebacks select zooplankton rather than benthos, but this selection is influenced by the type of substrate above which the fish are feeding and by time of year. Generally, the selective feeding under natural conditions can be explained in terms of the set of preferred visual cues identified in the laboratory experiments (see above), but in this case these cues lead the sticklebacks to the most profitable prey types (zooplankton). Prey size selection was also investigated by comparing prey size distribution in the diet.