Abstract
Abstract. Small global positioning system (GPS) trackers are now routinely used to study the movement and behaviour of birds at sea. If the birds rest on the water they become “drifters of opportunity” and can be used to give information about surface currents. In this paper, we use a small data set from satellite-tracked razorbills (Alca torda) in the Irish Sea to test the potential of this idea for measuring tidal currents. Razorbills regularly rest on the sea overnight and their tracks at this time are consistent with their drifting with the tidal flows and changing direction as the flood turns to ebb and vice versa. Data from 4 years (2011–2014) have been binned in a geographical grid and analysed to give the variation of current over a mean tidal cycle in each grid element. A map of maximum current speed is consistent with a numerical model of the tidal currents in the region. The root mean square difference between observed maximum speed and that predicted by the model is 0.15 m s−1, about 15 % of typical current speeds in the area. The divergence between bird-track speed and model prediction increases in regions of the fastest tidal currents. The method clearly has its limitations, but the results of this study show that tagged birds resting on the sea have potential to provide relatively inexpensive quantitative information about surface tidal currents over an extended geographical area.
Highlights
Passive surface drifters, tracked by shore radio or satellite, have been used to measure currents in the open sea for decades (Booth and Ritchie, 1983; Beardsley et al, 2004; Ohshima et al, 2002; Poulain, 2013)
There is a great deal of scatter, there are predominantly two directions of motion: one at about 0◦, which lasts from about 5 h before high water at Liverpool to about 1 h after high water at Liverpool, and another at 180◦, which is observed for the remainder of the tidal cycle
The prevailing winds in the area are from the south-west and these will tend to speed up the bird during the flood tide and slow it down during the ebb
Summary
Passive surface drifters, tracked by shore radio or satellite, have been used to measure currents in the open sea for decades (Booth and Ritchie, 1983; Beardsley et al, 2004; Ohshima et al, 2002; Poulain, 2013). Tracked drifters are useful for following a continuous current over a long distance. Drifters provide information on tidal currents, but this information is not always used as much as it could be: it is often seen as incidental to the main purpose of the drifter deployment. In places where tidal currents are very fast, it is difficult and expensive to deploy traditional instrumented moorings. A mooring can only provide data at one location, and tidal currents, especially in coastal waters, often vary greatly over short distances. A tracked drifter – or, better, a number of drifters – moving through an area of interest has the potential to provide valuable information about the spatial and temporal distribution of tidal currents in the region
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