Light pollution from ships disrupts the activity of breeding Yelkouan shearwaters and may affect their breeding success. Regulating light emission and shipping activity during the breeding season may be required to mitigate the impacts on nocturnal seabirds.
Yelkouan shearwaters are seabirds which nest in deep burrows and caves in limestone cliffs across the Mediterranean Sea, and they approach these cliffs only under the cover of darkness to avoid being killed by natural predators. The shearwaters are considered vulnerable to extinction, and breeding colonies have previously been abandoned due to human activity that involved light pollution.
As daylight fades away, Yelkouan shearwaters return from foraging at sea to enter their breeding colony under the cover of darkness. Photo: Marc Schruoffeneger
Malta is an important breeding area situated in the Central Mediterranean Sea, and globally important colonies of Yelkouan Shearwaters occur along the cliffs of Malta. However, Malta is also densely populated, and seabird colonies are under pressure from a wide range of human activities. One such activity is the transfer of fuel to ships at sea, which requires very brightly lit ships if the activity occurs at night. Two of these ‘mobile filling stations’ exist just offshore important Yelkouan shearwater breeding colonies – but so far it was unclear what effect the light pollution from ships had on Yelkouan shearwaters.
Because the shearwaters are nocturnal and cannot be counted visually when they return to their nest sites, Martin Austad and colleagues from BirdLife Malta, the RSPB and University of Giessen (Germany) captured and tagged adult shearwaters with small electronic chips (similar to those used in pets), and then used a radio monitoring system to measure the number of birds passing in and out of the cave during the spring breeding seasons from 2017 to 2020. The authors also quantified light pollution from nearby ships by measuring ambient light and relating these measurements to ship activity during that time.
Brightly lit ships in front of the yelkouan shearwater colony at Majjistral Nature & History Park, Malta. Photo: Paulo Lago
A new paper published in the Journal of Ornithology now shows that the presence of ships increased the brightness of the colony, with an effect similar to a full moon. This is an obvious concern for birds that rely on darkness to sneak into their burrows undetected by predators. As expected, the number of birds entering the colony was lower when the night was brighter. In the presence of ships and the resultant light pollution, the number of shearwaters entering the cave per hour decreased on average by 18%.
Measurements of how bright the entrance of a shearwater colony on Malta was during the breeding season when there were no ships (small black dots) and when several ships were present (larger coloured dots). In the absence of a moon and ships the cliff is very dark, but ships illuminate the cliff as much as a full moon.
The study suggests that because shearwaters avoid entering the colony when it is not dark enough, they may not be able to feed chicks as regularly as needed if ship traffic artificially illuminates a colony. Ship activity in front of shearwater breeding colonies should therefore be limited to daylight hours or bright nights around full moon. The global shipping industry, and especially ships in the Mediterranean, should adopt existing guidelines to reduce its environmental impact. Each ship can reduce light pollution by installing blinds and shielding lights respectively, and similar measures have been adopted successfully in the Southern Oceans and could be implemented in Europe.