Swimming speeds and prey selection of temporally captive, wild grey seals (Halichoerus grypus) were investigated in relation to prey characteristics in an experimental set-up at the Sea Mammal Research Unit (St Andrews, UK). For breath- hold divers, such as seals, the cost of swimming is a key variable in the management of oxygen stores. Thus it is likely that they modulate their swim speeds in order to maximise time spent feeding at a prey patch. We observed a strong relationship between swimming speed and stroke and glide pattern. Seals decreased their swim speeds by increasing their gliding period and as a result they decreased their oxygen consumption. Results indicated that mean swim speed decreased significantly with increasing distance to the patch, consistent with optimality model predictions. In addition, seals modified their swim speeds in response to changes in the speed and density of their prey. Seals decreased their swim speed as the speed of the prey increased. On the other hand, seals increased their swim speed as the density of the prey increased. Concurrently, bottom durations significantly increased suggesting that seals’ foraging strategies allowed them to most efficiently exploit their environment. Prey selection experiments investigated dietary preferences and the factors affecting their choice to select between different types of food. In the present study, seals maximised some aspects of their energy intake but also displayed some individual prey preferences. These findings indicate the importance of fine-scale observations of foraging behaviour and the value of experimental protocols in developing our understanding of marine mammal foraging behaviour.