School of Water, Energy and Environment

Competition between pairs of stickleback was studied to investigate the effects of fish size and prey size on the feeding behaviour and competitive ability of the fish when single prey items were encountered sequentially. All the fish had a high probability of attacking the prey encountered. A fish with a high probability of being first to handle the prey had a high probability of ingesting it only when the prey was the smaller of the two prey sizes offered. With the larger prey, food gain was predicted not by the first to handle but by the jaw size of the fish. Encounter with a larger prey resulted in each fish attempting to capture it more often. This was due to the larger prey being visible for longer as it had to be manipulated before being swallowed. Even when prey were ingested and hence not visible, the unsuccessful competitor remained close by for approximately 7s. The probability of eating any prey encountered was dependent on the phenotypic properties of the individual fish. A fast reacting fish with large jaws was able to reach its daily energy requirement regardless of the encounter rate with any prey size. Competitive ability was therefore a function of the fish, ability to reach the prey first, the capacity of the jaw apparatus to ingest the prey and prey encounter rate. When the pairs of fish encountered a high proportion of large prey, a generalist feeding strategy was the best option. The results can be applied to current foraging models, in particular to extending the sequential encounter model.