The use of time-at-depth recorders for estimating depth utilization and diving performance of European shags

We used time-at-depth recorders and radio transmitters to measure dive parameters for European Shags (Phalacrocorax aristotelis). An algorithm for partitioning time-at-depth profiles, obtained from individuals making U-shaped, flat-bottomed dives, into foraging time and travel time is presented, and a Fortran program to run the algorithm is provided. The deepest dive recorded was 61 m, but foraging depth parameters varied considerably between colonies and years. On average, European shags spent 47.6% of their underwater time foraging. The duration of each stage of the dive cycle was a function of water depth. The observed trends were qualitatively similar to predictions of optimality models for air-breathing animals that feed underwater, but more data from individuals foraging at depths approaching the physiological maximum for the species are needed.