Constraints on the foraging behaviour of the threespine stickleback (Gasterosteus aculeatus L.).

As feeding is fundamental to any animal, this thesis examined constraints on foraging behaviour from the point of view of a predatory fish, the threespine stickleback, encountering prey sequentially and simultaneously. Subsequent to prey detection, the fish orientated towards the prey and then decided to attack. The probability of attack reduced with a decrease in fish size, an increase in the size of the prey and the fish's stomach fullness and was dependent on the presence of competitors. During the attack, if the fish hung midwater the probability of successful prey capture was greater. Hanging was more likely to occur with smaller fish and when larger prey were encountered and was also longer in duration when prey were encountered simultaneously. When handling the prey, the fish made a decision to eat within the first few seconds. All of the fish ate to fill their stomachs, with the critical factors involved in prey choice being the size of the prey in relation to the size of the fish's mouth and the stomach fullness of the fish. With an empty stomach, the fish ate whatever prey was encountered. As stomach fullness Increased the fish became selective against large prey with high handling costs, depending on the availability of alternative prey. Those prey selected for had low handling costs and were successfully captured whenever encountered. Prey with a width 0.6 of the fish's jaw width were found to be the best option in terms of costs and benefits to the fish. Selectivity was, however, also a function of the capacity of the fish's stomach. The thesis demonstrates how the behavioural response of the foraging stickleback is dynamic, dependent on a number of factors external to and internal of the fish.