Sea bird foraging in a changing seascape

Sea birds in the Irish Sea are known to target tidal mixing fronts (the interfaces between seasonally stratified and mixed waters) as foraging grounds as these tend to be areas high in nutrients and primary productivity and thus, prey availability. However, little is known about the inter- and intraannual variability of the areas selected as foraging grounds by the sea birds. Here, using foraging locations derived from GPS tags on Manx shearwaters (Puffinus puffinus) and reanalysis oceanographic data and observational data collected from cruises, we evaluate which oceanographic features are targeted by the shearwaters on their foraging trips into the Irish Sea and how the characteristics of these frontal areas vary seasonally and interannually. Comparing a range of different physical measures to describe front locations, we show that the birds generally select interannually persistent marginally stratified (as defined by the potential energy anomaly) areas. Furthermore, we demonstrate that the features which are selected by the birds as foraging grounds vary over the course of summer months. Next, we use projections of future changes in shelf sea oceanography to highlight and discuss the consequences changing shelf sea stratification and physical features in the Irish Sea.