The durations of more than 4000 dives and recovery periods on the surface of 31 shags. Phalacrocorax aristotelis, were recorded using radio-telemetry, during three breeding seasons. Data were also collected on the depth of water where the birds were diving and the size of lesser sandeels. Ammodytes marinus, fed to young each year. On average, shags foraged in water 30 m deep; mean dive and recovery times were 62·0 ± 1·92 sec and 84·4 ± 5·02 sec. respectively. These times were much longer than those recorded in previous studies and also longer than those predicted from allometric relationships for dive and recovery times derived for cormorants. A large proportion (> 50%)) of dives appeared to involve anaerobic metabolism.
Dive and recovery times were examined in relation to water depth, time of day and position within the diving sequence (dive number). In each year. dive time was positively correlated with water depth: in 1987 dive number also had a negative effect. The relationship between recovery time and dive time was positively accelerated, suggesting that birds were diving anaerobically on longer dives. The percentage of the dive cycle spent underwater therefore decreased significantly as dive time and water depth increased. Average vertical rates of ascent and descent of shags were calculated to be between 1·46 1·87 m sec−1. Estimated prey capture rates were significantly higher in 1989 (average 6±8 fish dive−1) when adults were feeding their young on small, low energy value sandeels than the two previous years (averages 1·4 and 1·9 fish dive−1, respectively) when birds were feeding on larger, higher quality fish. We calculated that, on average, birds spent 8 10 sec (36 48%) longer on the bottom at any given depth in 1989, which suggested that the higher prey capture ratcs were associated with an increase in duration of the bottom phase of the dive.